abiotic
(a′ bye ah tick) [Gk. a: not + bios: life] Nonliving. In ecology, the nonliving (physical and chemical) components of the environment of organisms. (Contrast with biotic.)
abscisic acid
(ab sighs′ ik) A plant growth substance with growth-inhibiting action. Causes stomata to close; involved in a plant’s response to salt and drought stress.
abscission
(ab sizh′ un) [L. abscissio: break off] The process by which leaves, petals, and fruits separate from a plant.
absorption
(1) Of light: complete retention (in contrast to reflection or transmission). (2) In the study of nutrition, the process of transporting molecules from the gut lumen into the blood.
absorption spectrum
A graph of light absorption versus wavelength of light; shows how much light is absorbed at each wavelength.
absorptive heterotroph
An organism (usually a fungus) that obtains its food by
absorptive heterotrophy
, i.e., by secreting digestive enzymes into the environment to break down large food molecules, then absorbing the breakdown products.
abyssal zone
(uh biss′ ul) [Gk. abyssos: bottomless] The deepest portions of the oceans; characterized by extremely high pressures, low dissolved oxygen levels, and cold temperatures.
accessory fruit
A fruit of a flowering plant in which the some of the flesh is derived from tissues other than the ovary; examples include apples and pears.
accessory reproductive organs
Reproductive organs other than the gonads.
acclimation
The change in an individual’s phenotype that results from long-term exposure to a new environment. A form of phenotypic plasticity. Also called acclimatization.
acid growth hypothesis
The hypothesis that auxin increases proton pumping, thereby lowering the pH of the cell wall and activating enzymes that loosen polysaccharides. Proposed to explain auxin-induced cell expansion in plants.
acoelomate
An animal that does not have a coelom.
acquired immune deficiency syndrome (AIDS)
A condition caused by human immunodeficiency virus (HIV) in which the body’s T-helper cells are reduced, leaving the victim subject to opportunistic diseases.
ACTH
See adrenocorticotropic hormone.
actin
[Gk. aktis: ray] A protein that makes up the cytoskeletal microfilaments in eukaryotic cells and is one of the two contractile proteins in muscle. (See also myosin.)
actin filaments
Chains of linked actin molecules. Also called thin filaments. (See also myosin filaments.)
action potential
An impulse in a neuron taking the form of a brief, local, high-amplitude depolarization of the cell membrane. (Contrast with resting potential.)
action spectrum
A graph of a biological process versus light wavelength; shows which wavelengths are involved in the process.
activation energy (Ea)
The energy barrier that blocks the tendency for a chemical reaction to occur.
activator
A transcription factor that stimulates transcription when it binds to a gene’s promoter. (Contrast with repressor.)
active site
The region on the surface of an enzyme or ribozyme where the substrate binds, and where catalysis occurs.
active transport
The energy-dependent transport of a substance across a biological membrane against a concentration gradient—that is, from a region of low concentration (of that substance) to one of high concentration. (See also primary active transport, secondary active transport; contrast with facilitated diffusion, passive transport.)
adaptation
(a dap tay′ shun) (1) In evolutionary biology, a particular structure, physiological process, or behavior that makes an organism better able to survive and reproduce. Also, the evolutionary process that leads to the development or persistence of such a trait. (2) In sensory neurophysiology, a sensory cell’s loss of sensitivity as a result of repeated stimulation.
adaptive defenses
In animals, one of two general types of defenses against pathogens. Involves antibody proteins and other proteins that recognize, bind to, and aid in the destruction of specific viruses and bacteria. Present only in vertebrate animals. (Contrast with innate defenses.)
additive growth
Population growth in which a constant number of individuals is added to the population during successive time intervals. (Contrast with multiplicative growth.)
adenine (A)
(a′ den een) A nitrogen-containing base found in nucleic acids, ATP, NAD, and other compounds.
adenohypophysis
See anterior pituitary gland.
adenosine triphosphate
See ATP.
ADH
See antidiuretic hormone.
adhesion
Binding of one substance or structure to another, as in a cell binding to its extracellular matrix.
adiabatic
(ad′ e uh bat′ ik) [Gk. adiabatos: impassable] Referring to a system that exchanges no heat with its surroundings. When such a “heat insulated” system—for example a parcel of air—contracts or expands, its temperature increases or decreases, respectively. This thermodynamic principle applies to the functioning of air conditioners and refrigerators.
adrenocorticotropic hormone (ACTH)
A tropic hormone produced by the anterior pituitary gland that stimulates release of cortisol and certain other hormones from the adrenal cortex.
adventitious roots
(ad ven ti′ shus) [L. adventitius: arriving from outside] Roots originating from the stem at ground level or below; typical of the fibrous root system of monocots.
aerenchyma
In plants, parenchymal tissue containing air spaces.
aerobic
(air oh′ bic) [Gk. aer: air + bios: life] Requiring molecular oxygen, O2. (Contrast with anaerobic.)
aerotolerant anaerobes
Organisms that tolerate the presence of oxygen, but do not use it for metabolism.
afferent neuron
(af′ ur unt) [L. ad: toward + ferre: to carry] See sensory neuron.
aggregate fruit
A fruit of a flowering plant that develops from the merger of several different ovaries that were separate in the flower.
alcohol fermentation
See fermentation.
aleurone layer
In some seeds, a tissue that lies beneath the seed coat and surrounds the endosperm. Secretes digestive enzymes that break down macromolecules stored in the endosperm.
allantois
(al′ lun toh is) [Gk. allant: sausage] An extraembryonic membrane sac that stores the embryo’s nitrogenous wastes; because it contains blood vessels, it provides the means for gas exchange. In eutherian mammals, forms part of the placenta.
allele
(a leel′) [Gk. allos: other] A specific form of a gene at a given locus on a chromosome, among multiple possible forms.
allergic reaction
[Ger. allergie: altered] An overreaction of the immune system to amounts of an antigen that do not affect most people; often involves IgE antibodies.
allometry
(1) In studies of animals of different sizes, nonproportional variation in relation to body size. (2) In studies of growth, a change in the proportions of various parts of an organism relative to one another as a consequence of unequal growth.
allopatric speciation
(al′ lo pat′ rick) [Gk. allos: other + patria: homeland] The formation of two species from one when reproductive isolation occurs because of the interposition of (or crossing of) a physical geographic barrier such as a river. Also called geographic speciation. (Contrast with sympatric speciation.)
allopolyploidy
The possession of more than two chromosome sets that are derived from more than one species.
allosteric regulation
(al lo steer′ ik) [Gk. allos: other + stereos: structure] Regulation of the activity of a protein (usually an enzyme) by the binding of an effector molecule to a site other than the active site.
α (alpha) helix
A prevalent type of secondary protein structure; a right-handed spiral.
alternation of generations
The succession of multicellular haploid and diploid phases in some sexually reproducing organisms, notably plants.
alternative hypothesis
In statistical inference, the hypothesis that contrasts with the null hypothesis; typically the hypothesis of primary interest.
alternative splicing
A process for generating different mature mRNAs from a single gene by splicing together different sets of exons during RNA processing.
altruism
Behavior that benefits another individual at a cost of reduced fitness to the individual performing the behavior.
alveolar sac
A blind sac found at the end of a branch of the lung airways in a mammal; consists of many alveoli
alveolus
(al ve′ o lus) (plural: alveoli) [L. alveus: cavity] A small, baglike cavity, especially a semispherical subpart of the wall of an alveolar sac in the mammalian lung
amensalism
(a men′ sul ism) A “−/0” interaction between individuals of two different species that harms one individual (i.e., it suffers reduced fitness) but does not affect the other. (Contrast with commensalism, mutualism.)
amine hormones
Small hormone molecules synthesized from single amino acids (e.g., thyroxine and epinephrine). (Compare to peptide hormones, steroid hormones.)
amino acid
An organic compound containing both NH2 and COOH groups. Proteins are polymers of amino acids.
ammonotelic
(am moan′ o teel′ ic) [Gk. telos: end] Pertaining to an organism (typically water-breathing aquatic animals) in which ammonia is the principal final product of breakdown of nitrogen-containing compounds (primarily proteins). (Contrast with ureotelic, uricotelic.)
amnion
(am′ nee on) The fluid-filled sac within which the embryos of reptiles (including birds) and mammals develop.
amniote egg
A shelled egg surrounding four extraembryonic membranes and embryo-nourishing yolk. This evolutionary adaptation permitted mammals and reptiles to live and reproduce in drier environments than can most amphibians.
amphipathic
(am′ fi path′ ic) [Gk. amphi: both + pathos: emotion] Of a molecule, having both hydrophilic and hydrophobic regions.
anabolic reaction
(an uh bah′ lik) [Gk. ana: upward + ballein: to throw] A synthetic reaction in which simple molecules are linked to form more complex ones; requires an input of energy and captures it in the chemical bonds that are formed. (Contrast with catabolic reaction.)
anaerobic
(an ur row′ bic) [Gk. an: not + aer: air + bios: life] Occurring without the use of molecular oxygen, O2. (Contrast with aerobic.)
anaerobic glycolysis
Glycolysis taking place in the absence of molecular oxygen, O2. In animals, the most common process of anaerobic ATP production. In this process glucose is converted to lactic acid.
anaphase
(an′ a phase) [Gk. ana: upward] The stage in cell nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologs) occurs.
ancestral trait
The trait originally present in the ancestor of a given group; may be retained or changed in the descendants of that ancestor.
androgen
(an′ dro jen) [Gk. andros: man + genein: to produce] Any of the several masculinizing steroid hormones (most notably testosterone).
aneuploidy
(an′ you ploy dee) A condition in which one or more chromosomes or pieces of chromosomes are either lacking or present in excess.
angiosperms
Flowering plants; one of the two major groups of living seed plants. (See also gymnosperms.)
animal hemisphere
The non-yolky upper portion of some animal eggs. (Contrast with vegetal hemisphere.)
anion
(an′ eye on) [Gk. ana: upward] A negatively charged ion. (Contrast with cation.)
annual
A plant whose life cycle is completed in one growing season. (Contrast with biennial, perennial.)
ANS
See autonomic nervous system.
antagonistic muscle pair
Two muscles, or groups of muscles, that perform coordinated, opposing actions, thus allowing the bones at a joint to move in opposite directions. When one muscle contracts (to close the angle of a joint), its antagonist relaxes. To open the joint, the formerly relaxed muscle contracts and the formerly contracted muscle relaxes.
antenna system
See light-harvesting complex.
anterior
Toward or pertaining to the tip or headward region of the body axis. (Contrast with posterior.)
anterior pituitary gland
The anterior portion of the vertebrate pituitary gland that is composed of nonneural endocrine cells and produces several hormones, including tropic hormones. Also called the adenohypophysis. (See also pituitary gland, posterior pituitary gland.)
anther
(an′ thur) [Gk. anthos: flower] A pollen-bearing portion of the stamen of a flower.
antheridium
(an′ thur id′ ee um) [Gk. antheros: blooming] The multicellular structure that produces the sperm in nonvascular land plants and ferns.
antibody
One of the myriad proteins produced by the immune system that specifically binds to a foreign substance in blood or other tissue fluids and initiates its removal from the body.
anticodon
The three nucleotides in transfer RNA that pair with a complementary triplet (a codon) in messenger RNA.
antidiuretic hormone (ADH)
A hormone that promotes water reabsorption by the kidney. ADH is produced by neurons in the hypothalamus and released from nerve terminals in the posterior pituitary gland. Also called vasopressin in mammals.
antigen
(an′ ti jun) Any substance that stimulates the production of an antibody or antibodies in the body of a vertebrate.
antigen presentation
In cellular immunity, the process in which a cell ingests and digests an antigen, and then exposes fragments of that antigen, bound to a major histocompatibility complex (MHC) molecule, to the outside of the cell, where the antigen can be recognized by T cells.
antigenic determinant
The specific region of an antigen that is recognized and bound by a specific antibody. Also called an epitope.
antiparallel
Pertaining to molecular orientation in which a molecule or parts of a molecule have opposing directions.
antisense RNA
A single-stranded RNA molecule complementary to, and thus targeted against, an mRNA of interest to block its translation.
aphotic zone
The parts of lakes and oceans below the photic zone, where no sunlight penetrates.
apical–basal axis
Longitudinal definition of function in an adult organism or future function in an embryo, as in the root tip (apex).
apical dominance
In plants, inhibition by the apical bud of the growth of axillary buds.
apical hook
A form taken by the stems of many eudicot seedlings that protects the delicate shoot apex while the stem grows through the soil.
apical meristem
The meristem at the tip of a shoot or root; responsible for a plant’s primary growth.
apomixis
(ap oh mix′ is) [Gk. apo: away from + mixis: sexual intercourse] The asexual production of seeds.
apoplast
(ap′ oh plast) In plants, the continuous meshwork of cell walls and extracellular spaces through which material can pass without crossing a plasma membrane. (Contrast with symplast.)
apoptosis
(ap uh toh′ sis) A series of genetically programmed events leading to cell death.
aquaporin
[L. aqua: water + porus: pore] A transport protein in plant and animal cell membranes through which water passes by osmosis. Also called a water channel protein.
archegonium
(ar′ ke go′ nee um) The multicellular structure that produces eggs in nonvascular land plants, ferns, and gymnosperms.
arteriole
A small blood vessel with muscular walls that carries blood from an artery into a capillary bed.
artery
A muscular blood vessel carrying blood away from the heart to other parts of the body. (Contrast with vein.)
ascoma
The fruiting structure of sexually reproducing sac fungi; often a cup-shaped structure.
ascospores
Haploid spores contained with the sexual reproductive structure (the ascus) of sac fungi.
ascus
(ass′ cus) (plural: asci) [Gk. askos: bladder] In sac fungi, the club-shaped sporangium within which spores (ascospores) are produced by meiosis.
asexual reproduction
The formation of new individuals without the union of genetic material from two different parents. Often takes place by budding or fragmentation. The offspring are genetically identical to their parent.
asynchronous muscle
A muscle in which each excitation results in multiple contractions.
atom
[Gk. atomos: indivisible] The smallest unit of a chemical element. Consists of a nucleus and one or more electrons.
atomic number
The number of protons in the nucleus of an atom; also equals the number of electrons around the neutral atom. Determines the chemical properties of the atom.
ATP (adenosine triphosphate)
An energy-storage compound containing adenine, ribose, and three phosphate groups. When it is formed from ADP, useful energy is stored; when it is broken down (to ADP or AMP), energy is released to drive endergonic reactions.
ATP synthase
An integral membrane protein that couples the transport of protons with the formation of ATP.
atrium
(a′ tree um) [L. atrium: central hall] An internal chamber. In the hearts of vertebrates, the thin-walled chamber(s) entered by blood on its way to the ventricle(s).
autoimmune diseases
Diseases (e.g., rheumatoid arthritis) that result from failure of the immune system to distinguish between self and nonself, causing it to attack tissues in the organism’s own body.
autoimmunity
An immune response by an organism to its own molecules or cells.
autonomic effector
An effector other than skeletal muscle; includes smooth and cardiac muscles, exocrine glands, and endocrine glands. Also called an internal effector.
autonomic nervous system (ANS)
The portion of the nervous system that controls autonomic effectors (effectors other than the skeletal muscles). Also called the involuntary nervous system.
autophagy
The programmed destruction of a cell’s components.
autopolyploidy
The possession of more than two entire chromosomes sets that are derived from a single species.
autosome
Any chromosome (in a eukaryote) other than a sex chromosome.
autotroph
(au′ tow trowf′) [Gk. autos: self + trophe: food] An organism that is capable of living exclusively on inorganic materials, water, and an energy source other than the chemical bonds of organic compounds. Some autotrophs (photoautotrophs) use sunlight as their energy source. Others (chemoautotrophs) use oxidation of inorganic compounds. (Contrast with heterotroph.)
auxin
(awk′ sin) [Gk. auxein: to grow] In plants, a substance (the most common being indoleacetic acid) that regulates growth and various aspects of development.
avirulence (Avr) genes
Genes in a pathogen that may trigger defenses in plants. (See gene-for-gene resistance.)
axis
In the study of endocrinology, a system in which several types of endocrine cells act on each other in sequence.
axon
[Gk. axle] The part of a neuron that conducts action potentials away from the cell body.
bacillus
(bah sil′ us) [L: little rod] Any of various rod-shaped bacteria.
bacterial conjugation
See conjugation.
bacteriophage
(bak teer′ ee o fayj) [Gk. bakterion: little rod + phagein: to eat] Any of a group of viruses that infect bacteria. Also called phage.
bacteroids
Nitrogen-fixing organelles that develop from endosymbiotic bacteria.
bar chart
A figure that displays frequency distributions of categorical data using bar lengths to represent relative frequency.
bark
All tissues external to the vascular cambium of a plant.
basal metabolic rate (BMR)
The minimum rate of energy use in an awake (but resting) bird or mammal. For measuring the BMR, the animal is resting, fasting, and in its thermoneutral zone (i.e., its metabolic rate is not elevated for thermoregulation).
base
In nucleic acids, the purine or pyrimidine that is attached to each sugar in the sugar–phosphate backbone.
base pair (bp)
In double-stranded DNA, a pair of nucleotides formed by the complementary base pairing of a purine on one strand and a pyrimidine on the other.
basidioma
(plural: basidiomata) A fruiting structure produced by club fungi.
basidium
(bass id′ ee yum) In club fungi, the characteristic sporangium in which four basidiospores are formed by meiosis and then borne externally before being shed.
basilar membrane
A membrane in the human inner ear that flexes in response to sound waves and thereby activates hair cells; flexes at different locations in response to different pitches of sound.
BD model
A mathematical model that describes how Birth and Death rates affect the size and change in size (dynamics) of ecological populations.
behavioral imprinting
In animal behavior, a rapid form of learning in which an animal learns, during a brief critical period, to make a particular response, which is maintained for life, to some object or other organism.
benthic zone
[Gk. benthos: bottom] The substrate at the bottom of an ocean, lake, stream, or other body of water.
β (beta) pleated sheet
A type of protein secondary structure; results from hydrogen bonding between polypeptide regions running antiparallel to each other.
BIDE model
A mathematical model that describes how birth, immigration (movement of individuals into populations), death, and emigration (movement of individuals out of populations) affect the size and change in size (dynamics) of ecological populations.
biennial
A plant whose life cycle includes vegetative growth in the first year and flowering and senescence in the second year. (Contrast with annual, perennial.)
bilateral symmetry
The condition in which only the right and left sides of an organism, divided by a single plane through the midline, are mirror images of each other.
bilayer
A structure that is two layers in thickness. In biology, most often refers to the phospholipid bilayer of membranes. (See phospholipid bilayer.)
binary fission
Reproduction of a prokaryote by division of a cell into two comparable progeny cells.
binomial nomenclature
A taxonomic naming system in which each species is given two names (a genus name followed by a species name).
biofilm
A community of microorganisms embedded in a polysaccharide matrix, forming a highly resistant coating on almost any moist surface.
biogeochemical cycle
A description of the cyclical pathways by which biological, geological, and chemical processes move a chemical element or molecule through biotic and abiotic compartments of planet Earth.
biogeographic region
A large, continental-scale region of planet Earth, encompassing multiple biomes, that contains a distinct assemblage of species, many of which are phylogenetically related. (Contrast with biome.)
biogeography
The spatial distribution of Earth’s species.
bioinformatics
The use of computers and/or mathematics to analyze complex biological information, such as DNA sequences.
biological clock
A self-contained, metabolic mechanism that keeps track of time.
biological determination
As applied to behavior, the belief that genes completely determine an organism’s behavior.
biological species concept
The definition of a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. (Contrast with lineage species concept, morphological species concept.)
biomass
The mass of a sample of biological tissue, living or dead. The term usually refers to the total mass of a designated group of organisms (or of certain of their tissues), and may be measured as the mass of their intact bodies or of their dried bodies.
biome
(bye′ ome) A distinct abiotic environment that is found at different places on the planet and is inhabited in each of these locations by ecologically-similar organisms with convergent adaptations. (Contrast with biogeographic region.)
bioremediation
The use by humans of other organisms to remove contaminants from the environment.
biosphere
(bye′ oh sphere) The region that supports living organisms on Earth, extending about 23 km from the abyssal depths of the oceans into the stratosphere (an upper portion of the atmosphere).
biota
(bye oh′ tah) All of the organisms—animals, plants, fungi, and microorganisms—found in a given area. (Contrast with flora, fauna.)
biotechnology
The use of cells or living organisms to produce materials useful to humans.
biotic
(bye ah′ tick) [Gk. bios: life] Living. In ecology, the living components of the environment of organisms. (Contrast with abiotic.)
biotic interchange
The mixing of distinct groups of species that were isolated by physical or climatic barriers, after the barrier is removed.
bipedal locomotion
The act of walking or running on two appendages.
blade
The thin, flat portion of a leaf.
blastocyst
(blass′ toe cist) A mammalian blastula. The sphere of cells formed by early cell divisions and composed of an inner cell mass that becomes the embryo and an outer layer of cells called the trophoblast.
blastodisc
(blass′ toe disk) An embryo that forms as a disk of cells on the surface of a large yolk mass; comparable to a blastula, but occurring in animals such as birds and reptiles, in which the massive yolk restricts complete cleavage.
blastomere
Any of the cells produced by the early divisions of a fertilized animal egg.
blastula
(blass′ chu luh) An early stage of the animal embryo; in many species, a hollow sphere of cells surrounding a central cavity, the blastocoel. (Compare with blastodisc.)
blood plasma
The watery solution in which red blood cells and/or other blood cells are suspended. The part of the blood other than cells.
blood pressure
The extent to which the pressure of the blood exceeds the pressure of the surrounding external environment.
BMR
See basal metabolic rate.
body cavity
An internal, fluid-filled space, enclosed by mesoderm in animals.
body plan
The general structure of an animal, the arrangement of its organ systems, and the integrated functioning of its parts.
Bohr model
A model for atomic structure that depicts the atom as largely empty space, with a central nucleus surrounded by electrons in orbits, or electron shells, at various distances from the nucleus.
bone
A rigid component of vertebrate skeletal systems that contains an extracellular matrix of insoluble calcium phosphate crystals as well as collagen fibers.
bottleneck
See population bottleneck.
Bowman’s capsule
An elaboration of the renal tubule, composed of podocytes, that surrounds and collects the filtrate from the glomerulus.
brain
The part of the central nervous system of greatest size.
brassinosteroids
Plant steroid hormones that mediate light effects promoting the elongation of stems and pollen tubes.
breathing
The exchange of O2 and CO2 between the environmental medium next to the gas exchange membranes and the outside world; often occurs by ventilation, but can occur entirely by diffusion. Also called external respiration.
breathing-organ circuit
In a circulatory system, the blood vessels that carry blood to and from the breathing organs. (Contrast with systemic circuit.)
breathing organs
Specialized organs involved in the exchange of O2 and CO2 between an organism and the external environment. Classified as either lungs or gills.
brown adipose tissue (BAT)
A specialized fatty tissue, known only in mammals, that serves as the primary site of nonshivering thermogenesis.
bud
In plants, a small protuberance in the shoot that contains an undeveloped leaf, flower or shoot.
budding
Asexual reproduction in which a more or less complete new organism grows from the body of the parent organism, eventually detaching itself.
bulk flow
Mass movement of a fluid (gas or liquid) driven by a difference of pressure, from a region of high pressure to a region of low pressure.
C horizon
See parent rock.
calorie (cal)
[L. calor: heat] A unit of energy equal to the amount of heat required to raise the temperature of 1 gram of water by 1°C. Physiologists commonly use the kilocalorie (kcal) as a unit of energy (1 kcal = 1,000 calories). Nutritionists also use the kilocalorie, but refer to it as the Calorie (capital C).
Calvin cycle
The stage of photosynthesis in which CO2 reacts with RuBP to form 3PG, 3PG is reduced to a sugar, and RuBP is regenerated, while other products are released to the rest of the plant. Also known as the Calvin–Benson cycle.
Cambrian explosion
The rapid diversification of multicellular life that took place during the Cambrian period.
cAMP (cyclic AMP)
A compound formed from ATP that acts as a second messenger.
capillary
[L. capillaris: hair] In the circulatory system, a microscopic blood vessel with walls composed of just a simple epithelium (called the capillary endothelium), one cell layer thick. Capillaries are the principal sites of exchange between blood and other tissues in animals with a closed circulatory system.
carbohydrates
Organic compounds containing carbon, hydrogen, and oxygen in the ratio 1:2:1 (i.e., with the general formula CnH2nOn). Common examples are sugars, starch, and cellulose.
carbon-fixation reactions
The phase of photosynthesis in which chemical energy captured in the light reactions is used to drive the reduction of CO2 to form carbohydrates.
cardiac cycle
Alternating contraction and relaxation of the heart. In a mammal, contraction of the two atria of the heart, followed by contraction of the two ventricles and then relaxation.
cardiac muscle
A type of muscle tissue that makes up the wall of the heart and is responsible for the beating of the heart. Characterized by branching, electrically coupled cells with single nuclei and a striated (striped) appearance. (Contrast with smooth muscle, skeletal muscle.)
cardiovascular system
[Gk. kardia: heart + L. vasculum: small vessel] The heart, and vessels of the circulatory system. Also called circulatory system.
carnivore
[L. carn: flesh + vorare: to devour] An organism that eats animal tissues. (See secondary consumer; contrast with omnivore, detritivore, herbivore.)
carpel
(kar′ pel) [Gk. karpos: fruit] The organ of the flower that contains one or more ovules.
carrier proteins
Membrane proteins that bind specific molecules and transport them through the membrane.
carrying capacity (K)
The maximum population size or density of a given species that a given unchanging environment can support indefinitely.
cartilage
In vertebrates, a tough connective tissue found in joints, the outer ear, part of the nose, and elsewhere. Forms the entire skeleton in some animal groups, notably sharks and their relatives.
Casparian strip
A band of cell wall containing suberin and lignin, found in the endodermis. Restricts the movement of water across the endodermis.
caspase
One of a group of proteases that catalyze cleavage of target proteins and are active in apoptosis.
catabolic reaction
(kat uh bah′ lik) [Gk. kata: to break down + ballein: to throw] A synthetic reaction in which complex molecules are broken down into simpler ones and energy is released. (Contrast with anabolic reaction.)
catalyst
(kat′ a list) [Gk. kata: to break down] A chemical substance that accelerates a reaction without itself being consumed in the overall course of the reaction. Catalysts lower the activation energy of a reaction. Enzymes are biological catalysts.
catch
A state in which some invertebrate muscles maintain a state of contraction over extended periods of time with minimal energy expenditure.
categorical variables
Variables that take on qualitative categories as values, such as human blood types (A, AB, B, or O).
cation
(cat′ eye on) An ion with one or more positive charges. (Contrast with anion.)
Cdk
See cyclin-dependent kinase.
cDNA library
A collection of complementary DNAs derived from mRNAs of a particular tissue at a particular time in the life cycle of an organism.
cDNA
See complementary DNA.
cell body
In a neuron, portion of the neuron that contains the cell nucleus and most of the neuron’s other organelles.
cell cycle
The stages through which a cell passes between one division and the next. Includes all stages of interphase and mitosis.
cell cycle checkpoints
Points of transition between different phases of the cell cycle, which are regulated by cyclins and cyclin-dependent kinases (Cdk’s).
cell division
The reproduction of a cell to produce two new cells. In eukaryotes, this process involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).
cell fate
The type of cell that an undifferentiated cell in an embryo will become in the adult.
cell junctions
Specialized structures associated with the plasma membranes of epithelial cells. Some contribute to cell adhesion, others to intercellular communication.
cell membrane
Lipid bilayer also containing proteins and other molecules that encloses the cytoplasm of the cell and separates it from the surrounding environment.
cell potency
In multicellular organisms, an undifferentiated cell’s potential to become a cell of a specific type. (See multipotent, pluripotent, totipotent.)
cell signaling
Communication between cells or within cells as they respond to specific cues.
cell theory
States that cells are the basic structural and physiological units of all living organisms, and that all cells come from preexisting cells.
cell wall
A relatively rigid structure that encloses cells of plants, fungi, many protists, and most prokaryotes, and which gives these cells their shape and limits their expansion in hypotonic media.
cellular immune response
Immune system response mediated by T cells and directed against parasites, fungi, intracellular viruses, and foreign tissues (grafts). (Contrast with humoral immune response.)
cellular respiration
The catabolic pathways by which electrons are removed from various molecules and passed through intermediate electron carriers to O2, generating H2O and releasing energy.
central nervous system (CNS)
Large structures composed principally of integrating neurons and associated glial cells, forming the site of most information processing, storage, and retrieval; in vertebrates, the brain and spinal cord. (Contrast with peripheral nervous system.)
centralization
(of the nervous system) Over the course of evolution, the tendency of neurons to become clustered into centralized, integrating organs.
centriole
(sen′ tree ole) A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.
centromere
(sen′ tro meer) [Gk. centron: center + meros: part] The region where sister chromatids join.
centrosome
(sen′ tro soam) The major microtubule organizing center of an animal cell.
cephalization
(sef ah luh zay′ shun) [Gk. kephale: head] The evolutionary trend toward increasing concentration of nervous tissue (e.g., the brain) and sensory organs at the anterior end of the animal.
cerebral cortex
The outer layer of the cerebral hemispheres in the vertebrate brain. Plays key roles in sensory perception, learning, memory, and motor control.
channel protein
An integral membrane protein that forms an aqueous passageway across the membrane in which it is inserted and through which specific solutes may pass.
character
In genetics, an observable feature, such as eye color. (Contrast with trait.)
chemical bond
An attractive force stably linking two atoms.
chemical reaction
The change in the composition or distribution of atoms of a substance with consequent alterations in properties.
chemical synapse
A point of contact between two neurons at which the neurons are separated by only a very narrow space and neurotransmitter molecules released from the presynaptic cell induce changes in ion flux and membrane potential in the postsynaptic cell. (Contrast with electrical synapse.)
chemiosmosis
Formation of ATP in mitochondria and chloroplasts, resulting from a pumping of protons across a membrane (against a gradient of electrical charge and of pH), followed by the return of the protons through a protein channel with ATP synthase activity.
chemoautotroph
An organism that uses carbon dioxide as a carbon source and obtains energy by oxidizing inorganic substances from its environment; also called chemolithotroph. (Contrast with chemoheterotroph, photoautotroph, photoheterotroph.)
chemoheterotroph
An organism that must obtain both carbon and energy from organic substances. (Contrast with chemoautotroph, photoautotroph, photoheterotroph.)
chemolithotroph
[Gk. lithos: stone, rock] See chemoautotroph.
chemoreceptor
A sensory receptor cell that senses specific molecules (such as CO2 or O2 internally or environmental odorant molecules externally).
chi-square goodness-of-fit
A statistical test used to assess whether the frequencies of observations in different categories are consistent with an hypothesized frequency distribution.
chitin
(kye′ tin) [Gk. kiton: tunic] The characteristic tough but flexible organic component of the exoskeleton of arthropods, consisting of a complex, nitrogen-containing polysaccharide. Also found in cell walls of fungi.
chlorophyll
(klor′ o fill) [Gk. kloros: green + phyllon: leaf] Any of several green pigments associated with chloroplasts or with certain bacterial membranes; responsible for trapping light energy for photosynthesis.
chloroplast
[Gk. kloros: green + plast: a particle] An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloroplasts occur only in eukaryotes.
choanocyte
(ko′ an uh site) The collared, flagellated feeding cells of sponges.
chorion
(kor′ ee on) [Gk. khorion: afterbirth] The outermost of the membranes protecting mammal and reptile (including bird) embryos; in mammals it forms part of the placenta.
chromatin
The nucleic acid–protein complex that makes up eukaryotic chromosomes.
chromatin remodeling
A mechanism for epigenetic gene regulation by the alteration of chromatin structure.
chromosomal mutation
Loss of or changes in position/direction of a DNA segment on a chromosome.
chromosome
(krome′ o sowm) [Gk. kroma: color + soma: body] In bacteria and viruses, the DNA molecule that contains most or all of the genetic information of the cell or virus. In eukaryotes, a structure composed of DNA and proteins that bears part of the genetic information of the cell.
cilium
(sil′ ee um) (plural: cilia) [L. eyelash] Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than a flagellum.
circadian clocks
A biological clock that has a free-running cycle of about 24 hours.
circadian rhythm
(sir kade′ ee an) [L. circa: approximately + dies: day] A rhythm of growth or activity that recurs about every 24 hours.
circulatory system
A system consisting of a muscular pump (heart), a fluid (blood), and a series of conduits (e.g., blood vessels) that transports materials around the body by mass flow.
citric acid cycle
In cellular respiration, a set of chemical reactions whereby acetyl CoA is oxidized to carbon dioxide and hydrogen atoms are stored as NADH and FADH2. Also called the Krebs cycle.
clade
[Gk. klados: branch] A monophyletic group made up of an ancestor and all of its descendants.
class
(1) In biological classification: A group of related orders. (2) Generally: a group of things defined by particular attributes.
class I MHC molecules
Cell surface proteins that are present on the surface of all nucleated cells. They present antigens to cytotoxic T (TC) cells.
class II MHC molecules
Cell surface proteins that are present on the surface of macrophages, B cells, and dendritic cells. They present antigens to T-helper (TH) cells.
cleavage
The first few cell divisions of an animal zygote. Cleavage divides the embryo without increasing its mass. (See complete cleavage, incomplete cleavage.)
climate
The long-term average state of the weather at a given place over a yearly cycle. (Contrast with weather.)
climate diagram
A way of summarizing the climate of a given location by superimposing graphs of average monthly temperature and precipitation through a year in a way that indicates periods that are favorable or unfavorable for plant growth.
clonal deletion
Inactivation or destruction of lymphocyte clones that would produce immune reactions against the animal’s own body.
clonal lineages
Asexually reproduced groups of nearly identical organisms.
clonal selection
Mechanism by which exposure to antigen results in the activation of selected T- or B-cell clones, resulting in an immune response.
clone
[Gk. klon: twig, shoot] (1) Genetically identical cells or organisms produced from a common ancestor by asexual means. (2) To produce many identical copies of a DNA sequence by its introduction into, and subsequent asexual reproduction of, a cell or organism.
closed circulatory system
Circulatory system in which the circulating fluid is contained within a continuous system of vessels. (Contrast with open circulatory system.)
closed system
A system that isolated from, and does not interact with, its surroundings. (Contrast with open system.)
coccus
(kock′ us) (plural: cocci) [Gk. kokkos: berry, pit] Any of various spherical or spheroidal bacteria.
cochlea
(kock′ lee uh) [Gk. kokhlos: snail] A spiral tube in the inner ear of vertebrates; it contains the sensory cells involved in hearing.
cocurrent gas exchange
The exchange of gas between blood and water in a
cocurrent system
(a system in which the blood and water flow in the same direction along the gas exchange membrane of the animal; sometimes called a concurrent system).
coding region
Nucleotide sequences in a gene that directly specify amino acids in a protein.
coding strand
One of the two strand of DNA that for a particular gene specifies the amino acids in a protein.
codominance
A condition in which two alleles at a locus produce different phenotypic effects and both effects appear in heterozygotes.
codon
Three nucleotides in messenger RNA that direct the placement of a particular amino acid into a polypeptide chain. (Contrast with anticodon.)
coelom
(see′ loam) [Gk. koiloma: cavity] An animal body cavity completely enclosed by mesoderm.
coelomate
(see′ lo mate) An animal that contains a body cavity, the coelom, that develops within the mesoderm.
coenocyte
(see′ no site) An organism that contains many nuclei within each cell membrane.
coenocytic
(seen′ a sit ik) [Gk. koinos: common + kytos: container] Referring to the condition, found in some fungal hyphae, of “cells” containing many nuclei but enclosed by a single plasma membrane. Results from nuclear division without cytokinesis.
coenzyme A (CoA)
A coenzyme used in various biochemical reactions as a carrier of acyl groups.
cofactor
An inorganic ion that is weakly bound to an enzyme and required for its activity.
cohesion
The tendency of molecules (or any substances) to stick together.
cold-hardening
A process by which plants can acclimate to cooler temperatures; requires repeated exposure to cool temperatures over many days.
coleoptile
A sheath that surrounds and protects the shoot apical meristem and young primary leaves of a grass seedling as they move through the soil.
collagen
[Gk. kolla: glue] A fibrous protein found extensively in bone and connective tissue.
collecting duct
In mammals and birds, a tubule that receives urine produced in the nephrons of the kidney and delivers that fluid to the ureter for excretion. Concentration of the urine occurs in the collecting duct.
collenchyma
(cull eng′ kyma) [Gk. kolla: glue + enchyma: infusion] A type of plant cell, living at functional maturity, which lends flexible support by virtue of primary cell walls thickened at the corners. (Contrast with parenchyma, sclerenchyma.)
commensalism
[L. com: together + mensa: table] A “+/0” interaction between individuals of two different species that benefits one individual (i.e., increases its fitness) but does not affect the other. (Contrast with amensalism, mutualism.)
community
The assemblage of interacting individuals of different species within a particular geographic area.
community function
Any aspect of the interactions that occur within an ecological community that affects the rate of input of energy and matter into the community and its output of transformed matter and energy.
community structure
The attributes used to characterize an ecological community, which usually include the number of species in the community, their identities, and their relative abundances. (See species composition, species diversity.)
companion cell
In angiosperms, a specialized cell found adjacent to a sieve tube element.
comparative genomics
Computer-aided comparison of DNA sequences between different organisms to reveal genes with related functions.
compartment
One of the interacting components of a system. Applied to the global ecosystem, the different forms and physical locations in which chemical elements or molecules may occur.
competition
In ecology, a “−/−” interaction between individuals of the same or different species that causes both individuals to lose fitness.
competitive inhibitor
A nonsubstrate that binds to the active site of an enzyme and thereby inhibits binding of its substrate. (Contrast with noncompetitive inhibitor.)
complementary base pairing
The AT (or AU), TA (or UA), CG, and GC pairing of bases in double-stranded DNA, in transcription, and between tRNA and mRNA.
complementary DNA (cDNA)
DNA formed by reverse transcriptase acting with an RNA template; essential intermediate in the reproduction of retroviruses; used as a tool in recombinant DNA technology; lacks introns.
complete cleavage
Pattern of cleavage that occurs in eggs that have little yolk. Early cleavage furrows divide the egg completely. (See also radial cleavage, spiral cleavage; contrast with incomplete cleavage.)
complete metamorphosis
A change of state during the life cycle of an organism in which the body is almost completely rebuilt to produce an individual with a very different body form. Characteristic of insects such as butterflies, moths, beetles, ants, wasps, and flies.
component
The interacting parts of a biological system.
compound eye
An eye characteristic of arthropods, composed of many individual optical units called ommatidia.
computational model
A system in which the interactions among components are expressed as mathematical functions.
condensation reaction
A chemical reaction in which two molecules become connected by a covalent bond and a molecule of water is released (AH + BOH → AB + H2O.) (Contrast with hydrolysis reaction.)
conditional mutation
A mutation that results in a characteristic phenotype only under certain environmental conditions.
conducting airways
In the lungs of animals, the airways that do not participate in the exchange of respiratory gases between air and blood, but rather simply conduct air in and out of the lungs.
conducting system
In the vertebrate heart, a system of specialized muscle cells by which depolarization rapidly spreads throughout the heart muscle.
cone
(1) In conifers, a reproductive structure consisting of spore-bearing scales extending from a central axis. (Contrast with strobilus.) (2) In the vertebrate retina, a type of photoreceptor cell responsible for color vision.
confidence interval
A numerical interval that is calculated to contain the true value of some parameter of interest at a stated probability level.
conformer
An animal in which an internal condition (e.g., body temperature) adjusts to match an external condition (e.g., environmental temperature). (Contrast with regulators.)
conformity
The state in which an internal condition of an animal (e.g., body temperature) changes to match an external condition (e.g., environmental temperature). (Contrast with regulation.)
conidium
(ko nid′ ee um) (plural: conidia) [Gk. konis: dust] A type of haploid fungal spore borne at the tips of hyphae, not enclosed in sporangia.
conjugation
(kon ju gay′ shun) [L. conjugare: yoke together] (1) A process by which DNA is passed from one cell to another through a conjugation tube, as in bacteria. (2) A nonreproductive sexual process by which Paramecium and other ciliates exchange genetic material.
consensus sequences
Short stretches of DNA that appear, with little variation, in many different genes.
conservation ecology
The subdiscipline of ecology whose goal is to understand the processes of extinction of populations and species, and to devise ways to conserve species and the ecological systems of which they are a part.
constant region
The portion of an immunoglobulin molecule whose amino acid composition determines its class and does not vary among immunoglobulins in that class. (Contrast with variable region.)
constitutive
Always present; produced continually at a constant rate. (Contrast with inducible.)
constitutive genes
Genes that are expressed all the time. (Contrast with inducible genes.)
consumer–resource interactions
In ecology, a “+/−” interaction between individuals of different species in which one individual gains fitness by consuming part or all of another individual, which loses fitness from being consumed. Examples are predation, herbivory, and parasitism.
continental drift
The gradual movements of the world’s continents that have occurred over billions of years.
continuous variables
Variables that can take on a continuous range of values.
contractile vacuole
(kon trak′ tul) A specialized vacuole that collects excess water taken in by osmosis, then contracts to expel the water from the cell.
contraction
The condition in which a muscle produces force.
control mechanism
One of four essential elements of a control system; modifies a controlled variable by using information from sensors to determine which effectors to activate and how intense the activation needs to be.
controlled variable
One of four essential elements of a control system; the characteristic of a biochemical system, cell or animal that is being controlled.
convergent evolution
Independent evolution of similar features from different ancestral traits.
coral bleaching
A phenomenon in which corals lose their endosymbionts and become whitened as a result; leads to weakening, and sometimes death, of the coral.
corepressor
A molecule that binds to a bacterial repressor protein, altering its shape so that it binds to the operator sequence in DNA and prevents its transcription.
Coriolis effect
The apparent deflection of a moving body as viewed from a rotating frame of reference. In the system of the rotating Earth, an object (such as a packet of atmospheric gases) moving toward the Equator will appear to be deflected toward the west from the perspective of an observer on the surface; one moving toward a Pole will appear to be deflected toward the east.
cork
In plants, a protective outermost tissue layer composed of cells with thick walls waterproofed with suberin.
cork cambium
[L. cambiare: to exchange] In plants, a lateral meristem that produces secondary growth, mainly in the form of waxy-walled protective cells, including some of the cells that become bark.
cornea
In a vertebrate, the clear, transparent tissue that covers the forward-facing surface of the eye and allows light to pass through to the retina.
correlation coefficient
A measure of the strength of relationship between two quantitative variables, ranging from −1 (a perfect negative relationship) to 1 (a perfect positive relationship).
corolla
(ko role′ lah) [L. corolla: a small crown] All of the petals of a flower, collectively.
coronary circulatory system
A system of blood vessels that carries blood to and from the heart muscle
corpus luteum
(kor′ pus loo′ tee um) (plural: corpora lutea) [L. yellow body] A hormone-producing (endocrine) structure formed from reorganization of the cells of an ovarian follicle that has undergone ovulation. In mammals, it secretes progesterone, estrogen, and inhibin. If fertilization occurs, it remains active during pregnancy; if not, it degenerates.
corpus luteum of pregnancy
A corpus luteum that functions for an extended period of time during pregnancy. Secretes hormones that are important for maintenance of pregnancy.
corpus luteum of the cycle
A corpus luteum that functions for a short time (e.g., about 10 days in humans) and then undergoes internally programmed degeneration.
cortex
[L. cortex: covering, rind] (1) In plants, the tissue between the epidermis and the vascular tissue of a stem or root. (2) In animals, the outer tissue of certain organs, such as the adrenal gland (adrenal cortex) and the brain (cerebral cortex).
cost–benefit approach
An approach to ecological and evolutionary analysis that assumes an animal has a limited amount of time and energy to devote to its activities, and accordingly activities will occur only if benefits exceed costs. (See also trade-off.)
cotyledon
(kot′ ul lee′ dun) [Gk. kotyledon: hollow space] A “seed leaf.” An embryonic organ that stores and digests reserve materials; may expand when seed germinates.
countercurrent heat exchange
The exchange of heat that occurs between two closely juxtaposed fluid streams that are flowing in opposite directions (e.g. blood flowing in opposite directions in two juxtaposed blood vessels).
countercurrent gas exchange
The exchange of gas between blood and water in a countercurrent system (a system in which the blood and water flow in opposite directions along the gas exchange membrane of the animal).
countercurrent multiplication
The mechanism that increases the concentration of the interstitial fluids in the medulla of the mammalian kidney, permitting a mammal to produce concentrated urine. During countercurrent multiplication, countercurrent flow in the loops of Henle multiplies effects of active ion transport.
covalent bond
Chemical bond based on the sharing of electrons between two atoms.
CpG islands
DNA regions rich in C residues adjacent to G residues. Especially abundant in promoters, these regions are where methylation of cytosine usually occurs.
cross-bridges
During muscle contraction, the links formed when the globular heads of myosin filaments bind to specific sites on actin filaments. The heads of a myosin molecule interact with actin molecules to produce contraction.
crossing over
The mechanism by which linked genes undergo recombination. In general, the term refers to the reciprocal exchange of corresponding segments between two homologous chromatids.
crustose
A growth form of organisms, such as lichens, in which the organism forms a thin, close, tight bond with the surface of a rock, tree, or other object.
cryptochromes
[Gk. kryptos: hidden + kroma: color] Photoreceptors mediating some blue-light effects in plants and animals.
ctene
(teen) [Gk. cteis: comb] In ctenophores, a comblike row of ciliabearing plates. Ctenophores move by beating the cilia on their eight ctenes.
current
(1) Flow of a fluid (gas or liquid), for example in ocean waters or in the atmosphere. (2) The flow of electrical charge.
cuticle
(1) In plants, a waxy layer on the outer body surface that retards water loss. (2) In ecdysozoans, an outer body covering that provides protection and support and is periodically molted.
cyclic electron transport
In photosynthetic light reactions, the flow of electrons that produces ATP but no NADPH or O2.
cyclin
A protein that activates a cyclin-dependent kinase, bringing about transitions in the cell cycle.
cyclin-dependent kinase (Cdk)
A protein kinase whose target proteins are involved in transitions in the cell cycle and which is active only when complexed with additional protein subunits, called cyclins.
cytokine
A regulatory protein made by immune system cells that affects other target cells in the immune system.
cytokinesis
(sy′ toe kine ee′ sis) [Gk. kytos: container + kinein: to move] The division of the cytoplasm of a dividing cell. (Contrast with mitosis.)
cytokinin
(sy′ toe kine′ in) A member of a class of plant growth substances that plays roles in senescence, cell division, and other phenomena.
cytoplasm
The contents of the cell, excluding the nucleus.
cytoplasmic determinants
In animal development, materials in the cytoplasm, the spatial distribution of which may determine such things as embryonic axes and cell determination.
cytoplasmic segregation
The asymmetrical distribution of cytoplasmic determinants in a developing animal embryo.
cytoplasmic streaming
A form of amoeboid movement in which a fluid cytoplasm moves and stretches the organism’s body in new directions.
cytosine (C)
(site′ oh seen) A nitrogen-containing base found in DNA and RNA.
cytoskeleton
The network of microtubules and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.
cytosol
The fluid portion of the cytoplasm, excluding organelles and other solids.
cytotoxic T cells (TC)
Cells of the cellular immune system that recognize and directly eliminate virus-infected cells. (Contrast with T-helper cells.)
data
Quantified observations about a system under study.
daughter chromosomes
During mitosis, the separated chromatids from the beginning of anaphase onward.
decomposer
An organism that obtains the energy and materials it needs to survive, grow, and reproduce from the waste products or dead bodies of other organisms.
defensin
A type of protein made by phagocytes that kills bacteria and enveloped viruses by insertion into their plasma membranes.
deficiency disease
A condition (e.g., scurvy) caused by chronic lack of any essential nutrient.
delayed implantation
Embryonic diapause in placental mammals. A state of arrested embryonic development that occurs after an embryo has arrived in the uterus but before it implants in the uterine wall.
deletion
A mutation resulting from the loss of a continuous segment of a gene or chromosome. Such mutations almost never revert to wild type. (Contrast with duplication, point mutation.)
demethylase
An enzyme that catalyzes the removal of the methyl group from cytosine, reversing DNA methylation.
demography
(Gk. demos: population + graphia: description) The quantitative study of the processes that determine the structure of populations (e.g., the relative proportions of individuals of different sizes, ages, stages, and genders), the size of populations, and the changes in these attributes.
denaturation
Loss of activity of an enzyme or a nucleic acid molecule as a result of structural changes induced by heat or other means.
dendrite
[Gk. dendron: tree] The receptive element of most neurons, which receives synaptic input from other neurons. Usually much branched and relatively short compared with the axon.
denitrification
The chemical process by which “fixed” (biologically-useable) nitrogen is converted back into N2 gas. (Contrast with nitrogen fixation.)
denitrifiers
Bacteria that release nitrogen to the atmosphere as nitrogen gas (N2).
density-dependent
Pertaining to any factor or interaction that varies with density. For example, individual survival or reproductive success often changes as population density changes.
deoxygenation
The release of O2 from a combined state with a respiratory pigment such as hemoglobin.
deoxyribonucleic acid
See DNA.
deoxyribonucleoside triphosphates (dNTPs)
The raw materials for DNA synthesis: deoxyadenosine triphosphate (dATP), deoxythymidine triphosphate (dTTP), deoxycytidine triphosphate (dCTP), and deoxyguanosine triphosphate (dGTP). Also called deoxyribonucleotides.
deoxyribose
A five-carbon sugar found in nucleotides and DNA.
depolarization
A change in the membrane potential across a cell membrane so that the inside of the membrane becomes less negative, or even positive, compared with the outside of the membrane. (Contrast with hyperpolarization.)
derived trait
A trait that differs from the ancestral trait. (Contrast with synapomorphy.)
dermal tissue system
The outer covering of a plant, consisting of epidermis in the young plant and periderm in a plant with extensive secondary growth. (Contrast with ground tissue system, vascular tissue system.)
descent with modification
Darwin’s premise that all species share a common ancestor and have diverged from one another gradually over time.
descriptive statistics
Quantitative measures that describe general patterns in data.
determinate growth
A growth pattern in which the growth of an organism or organ ceases when an adult state is reached; characteristic of most animals and some plant organs. (Contrast with indeterminate growth.)
determination
In development, the process whereby the fate of an embryonic cell or group of cells (e.g., to become epidermal cells or neurons) is set.
detritivore
(di try′ ti vore) [L. detritus: worn away + vorare: to devour] An organism that obtains its energy from the dead bodies or waste products of other organisms. (Contrast with carnivore, herbivore, omnivore.)
development
The process by which a multicellular organism, beginning with a single cell, goes through a series of changes, taking on the successive forms that characterize its life cycle.
diaphragm
(dye′ uh fram) [Gk. diaphrassein: barricade] (1) A sheet of muscle that separates the thoracic and abdominal cavities, found only in mammals; helps drive air flow during breathing. (2) A birth control device in which a sheet of rubber is fitted over the woman’s cervix, blocking the entry of sperm.
diastole
(dye ass′ toll ee) [Gk. dilation] The portion of the cardiac cycle when the heart muscle relaxes. (Contrast with systole.)
dichotomous
(dye cot′ oh mus) [Gk. dichot: split in two; tomia: removed] A branching pattern in which the shoot divides at the apex producing two equivalent branches that subsequently never overlap.
differentiation
The process whereby originally similar cells follow different developmental pathways; the actual expression of determination.
diffusion
Random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present.
digestion
The breakdown (by hydrolysis) of ingested food molecules into smaller chemical components that an animal is capable of distributing to the tissues of its body.
digestive vacuole
In protists, an organelle specialized for digesting food ingested by endocytosis.
dihybrid cross
A mating in which the parents differ with respect to the alleles of two loci of interest.
dikaryon
(di care′ ee ahn) [Gk. di: two + karyon: kernel] A cell or organism carrying two genetically distinguishable nuclei. Common in fungi.
dioecious
(die eesh′ us) [Gk. di: two + oikos: house] Pertaining to organisms in which the two sexes are “housed” in two different individuals, so that eggs and sperm are not produced in the same individuals. Examples: humans, fruit flies, date palms. (Contrast with monoecious.)
diploblastic
Having two cell layers. (Contrast with triploblastic.)
diploid
(dip′ loid) [Gk. diplos: double] Having a chromosome complement consisting of two copies (homologs) of each chromosome. Designated 2n. (Contrast with haploid.)
direct development
Pattern of development in which the young resemble the adults. (Contrast with indirect development.)
directional selection
Selection in which phenotypes at one extreme of the population distribution are favored. (Contrast with disruptive selection, stabilizing selection.)
disaccharide
A carbohydrate made up of two monosaccharides (simple sugars).
discrete variables
Quantitative variables that take on only whole number values.
dispersal corridor
A continuous strip of habitat that connects larger pieces of habitat, thus facilitating movement of organisms between them.
disruptive selection
Selection in which phenotypes at both extremes of the population distribution are favored. (Contrast with stabilizing selection.)
disturbance
Events that cause sudden change in the environment and in ecological systems as a result. Examples include fire, landslides, floods, strong storms, and volcanic eruptions—as well as less obvious events such as the death and fall of a large tree that opens up the forest canopy.
disulfide bridge
The covalent bond between two sulfur atoms (–S—S–) linking two molecules or remote parts of the same molecule.
DNA (deoxyribonucleic acid)
The fundamental hereditary material of all living organisms. In eukaryotes, stored primarily in the cell nucleus. A nucleic acid using deoxyribose rather than ribose.
DNA fingerprint
An individual’s unique pattern of allele sequences, commonly short tandem repeats and single nucleotide polymorphisms.
DNA ligase
Enzyme that unites broken DNA strands during replication and recombination.
DNA methyltransferase
An enzyme that catalyzes the methylation of DNA.
DNA microarray
A small glass or plastic square onto which thousands of single-stranded DNA sequences are fixed so that hybridization of cell-derived RNA or DNA to the target sequences can be performed.
DNA polymerase
Any of a group of enzymes that catalyze the formation of DNA strands from a DNA template.
DNA replication
The creation of a new strand of DNA in which DNA polymerase catalyzes the exact reproduction of an existing (template) strand of DNA.
DNA segregation
The separation of two DNAs formed by replication into two new cells during cell division.
DNA transposons
Mobile genetic elements that move without making an RNA intermediate. (Contrast with retrotransposons.)
dNTPs
See deoxyribonucleoside triphosphates.
domain
(1) An independent structural element within a protein. Encoded by recognizable nucleotide sequences, a domain often folds separately from the rest of the protein. Similar domains can appear in a variety of different proteins across phylogenetic groups (e.g., “homeobox domain”; “calcium-binding domain”). (2) In phylogenetics, the three monophyletic branches of life (Bacteria, Archaea, and Eukarya).
dominance
In genetics, the ability of one allelic form of a gene to determine the phenotype of a heterozygous individual in which the homologous chromosomes carry both it and a different (recessive) allele. (Contrast with recessive.)
dominate
In animal behavior, to have control over resources (e.g., food or mates) as a result of defeating challengers in one-on-one behavioral contests.
dormancy
A condition in which normal activity is suspended, as in some spores, seeds, and buds.
dorsal
[L. dorsum: back] Toward or pertaining to the back or upper surface. (Contrast with ventral.)
double fertilization
In angiosperms, a process in which the nuclei of two sperm fertilize one egg. One sperm’s nucleus combines with the egg nucleus to produce a zygote, while the other combines with the same egg’s two polar nuclei to produce the first cell of the triploid endosperm (the tissue that will nourish the growing plant embryo).
doubling time
The time over which an ecological population doubles in size (number of individuals). Populations growing multiplicatively with a constant value of r have a constant doubling time, no matter what their current size is.
duplication
A mutation in which a segment of a chromosome is duplicated, often by the attachment of a segment lost from its homolog. (Contrast with deletion.)
dynamic
(Gk. dunamikos: powerful) Characterized by activity or change.
ecological efficiency
The transfer of energy from one trophic level to the next, often estimated as the ratio of dry biomass at the higher level to dry biomass at the lower level. Note that this use of “efficiency” as the ratio of energy output to input is similar to the measure of efficiency used for many human-made machines.
ecological system
One or more organisms and the abiotic and biotic environment with which they interact.
ecological transition
A transition from one ecological community to a different community following a disturbance. The transition is not easily reversible, and the new community persists even if original environmental conditions are restored; thus the two ecological communities are considered to be alternative stable states of the system. (Contrast with succession.)
ecology
[Gk. oikos: house] Defined by the German biologist Ernst Haeckel in 1866 as “the entire science of the relations of the organism to its external environment,” by which he (and we) include both the biotic and abiotic environment.
ecosystem
(eek′ oh sis tum) In general, shorthand for “ecological system.” Often used more specifically to refer to an ecological community and its abiotic environmental context.
ecosystem function
Any aspect of the interactions that occur within an ecosystem that affects the rate of input of energy and matter into the ecosystem and its output of transformed matter and energy.
ecosystem services
Ecosystem outputs that benefit humans.
ectoderm
[Gk. ektos: outside + derma: skin] The outermost of the three embryonic germ layers first delineated during gastrulation. Gives rise to the skin, sense organs, and nervous system.
ectomycorrhizae
Mutualistic fungi that cover the roots of plants and assist in the uptake of water and minerals from the soil by the plant.
ectotherm
[Gk. ektos: outside + thermos: heat] An animal in which body temperature matches—and varies with—the external temperature. (See also poikilotherm.)
effector cells
In cellular immunity, B cells and T cells that attack an antigen, either by secreting antibodies that bind to the antigen or by releasing molecules that destroy any cell bearing the antigen.
effectors
One of four essential elements of a control system. A tissue, organ, or cell that performs actions (“carries out orders”) under the direction of the nervous system or another physiological control system (e.g., the endocrine system).
efferent neurons
In a nervous system, nerve cells that carry commands from the central nervous system to physiological and behavioral effectors such as muscles and glands.
egg
In all sexually reproducing organisms, the female gamete (also called an ovum); in birds, reptiles, and some other animals, a structure within which the early embryonic development of a fertilized ovum occurs. (See also amniote egg.)
electrical synapse
A type of synapse at which there is direct continuity between the cytoplasms of the presynaptic and postsynaptic neurons and action potentials spread directly from presynaptic cell to postsynaptic cell. (Contrast with chemical synapse.)
electrocardiogram (ECG or EKG)
A graphic recording of electrical potentials that are measured on the body surface but originate from the heart.
electromagnetic radiation
A self-propagating wave that travels though space and has both electrical and magnetic properties.
electron
A subatomic particle outside the nucleus carrying a negative charge and very little mass.
electron shell
The region surrounding the atomic nucleus at a fixed energy level in which electrons orbit.
electron transport
The passage of electrons through a series of proteins with a release of energy which may be captured in a concentration gradient or in chemical form such as NADH or ATP.
electronegativity
The tendency of an atom to attract electrons when it occurs as part of a compound.
element
A substance that cannot be converted to simpler substances by ordinary chemical means.
elicitor
A substance that induces the synthesis of defensive molecules such as phytoalexins in higher plants.
elongation
(1) In molecular biology, the addition of monomers to make a longer RNA or protein during transcription or translation. (2) Growth of a plant axis or cell primarily in the longitudinal direction.
embryo
[Gk. en: within + bryein: to grow] A young animal, or young plant sporophyte, while it is still contained within a protective structure such as a seed, egg, or uterus.
embryo sac
In angiosperms, the female gametophyte. Found within the ovule, it consists of eight or fewer cells, membrane bounded, but without cellulose walls between them.
embryonic diapause
A programmed state of arrested or profoundly slowed embryonic development. (See also delayed implantation.)
embryonic stem cell (ESC)
A pluripotent cell in the blastocyst.
emigration
Movement of individual organisms out of a population.
3′ end
(3 prime) The end of a DNA or RNA strand that has a free hydroxyl group at the 3′ carbon of the sugar (deoxyribose or ribose).
5′ end
(5 prime) The end of a DNA or RNA strand that has a free phosphate group at the 5′ carbon of the sugar (deoxyribose or ribose).
endocrine cells
Cells that secrete hormones into the blood. (See also endocrine gland.)
endocrine gland
(en′ doh krin) [Gk. endo: within + krinein: to separate] A gland or tissue, lacking ducts, in which endocrine cells secrete hormones into the blood. (Contrast with exocrine gland.)
endocytosis
A process by which liquids or solid particles are taken up by a cell through invagination of the plasma membrane. (Contrast with exocytosis.)
endoderm
[Gk. endo: within + derma: skin] The innermost of the three embryonic germ layers delineated during gastrulation. Gives rise to the digestive and respiratory tracts and structures associated with them.
endodermis
In plants, a specialized cell layer marking the inside of the cortex in roots and some stems. Frequently a barrier to free diffusion of solutes.
endogenous retroviruses
A DNA sequence derived from a retrovirus that persists in and is transmitted as part of the genome of a non-viral organism. Some endogenous retroviruses can become active and produce new viral infections.
endomembrane system
A system of intracellular membranes that exchange material with one another, consisting of the Golgi apparatus, endoplasmic reticulum, and lysosomes when present.
endometrium
The inner tissue layer of the uterus of a mammal. Embryos implant in the endometrium, which later helps form the placenta.
endophytic fungi
Fungi that live within the aboveground portions of plants without causing obvious harm to the host plant.
endoplasmic reticulum (ER)
[Gk. endo: within + L. reticulum: net] A system of membranous tubes and flattened sacs found in the cytoplasm of eukaryotes. Exists in two forms: rough ER, studded with ribosomes; and smooth ER, lacking ribosomes.
endoskeleton
[Gk. endo: within + skleros: hard] An internal skeleton covered by other, soft body tissues. (Contrast with exoskeleton.)
endosome
Membrane-enclosed compartment in a cell that is formed after endocytosis.
endosperm
[Gk. endo: within + sperma: seed] A specialized triploid seed tissue found only in angiosperms; contains stored nutrients for the developing embryo.
endospore
[Gk. endo: within + spora: to sow] In some bacteria, a resting structure that can survive harsh environmental conditions.
endotoxin
A lipopolysaccharide that forms part of the outer membrane of certain Gram-negative bacteria that is released when the bacteria grow or lyse. (Contrast with exotoxin.)
end-product inhibition
See feedback inhibition.
endurance exercise
Exercise that consists of many repetitions of relatively low-intensity muscular actions over long periods of time. Also termed endurance training. (Contrast with resistance exercise.)
energy
The capacity to create and maintain organization. More narrowly defined, the capacity do work or move matter against an opposing force.
enteric division
One of three divisions of the vertebrate autonomic nervous system; composed of neurons located between the smooth muscle layers of the gut.
entropy (S)
(en′ tro pee) [Gk. tropein: to change] A measure of the degree of disorder in any system. Spontaneous reactions in a closed system are always accompanied by an increase in entropy.
environmental sex determination
Mechanism in which the sex of an individual is determined by the temperature it experiences during embryonic development.
enzyme
(en′ zime) [Gk. zyme: to leaven (as in yeast bread)] A catalytic protein that speeds up a biochemical reaction.
enzyme–substrate complex (ES)
An intermediate in an enzyme-catalyzed reaction; consists of the enzyme bound to its substrate(s).
eons
The primary division of geologic time. The history of Earth is divided into four eons: Hadeon, Archean, Proterozoic, and Phanerozoic.
epi-
[Gk. upon, over] A prefix used to designate a structure located on top of another; for example, epidermis, epiphyte.
epiblast
The upper or overlying portion of the avian blastula which is joined to the hypoblast at the margins of the blastodisc.
epidermis
[Gk. epi: over + derma: skin] In plants and animals, the outermost cell layers. (Only one cell layer thick in plants.)
epigenetics
The scientific study of changes in the expression of a gene or set of genes that occur without change in the DNA sequence.
epistasis
Interaction between genes in which the presence of a particular allele of one gene determines how another gene will be expressed.
epithelium
(plural: epithelia) A sheet of cells that lines a body cavity or covers an organ or body surface; one of the major tissue types in multicellular animals.
epitope
See antigenic determinant.
epoch
A subdivision of geologic time that divides geologic periods.
EPSP
See excitatory postsynaptic potential.
equilibrium
The state of a system in which there is no net change through time.
equilibrium potential
The membrane potential at which an ion is at electrochemical equilibrium, i.e., at which the concentration and electrical effects on diffusion of the ion are balanced so there is no net diffusion of the ion across the membrane.
ER
See endoplasmic reticulum.
erythrocyte
(ur rith′ row site) [Gk. erythros: red + kytos: container] See red blood cell.
ES
See enzyme–substrate complex.
ESC
See embryonic stem cell.
essential amino acids
Amino acids that an animal cannot synthesize for itself and must obtain from its food.
essential element
A mineral nutrient required for normal growth and reproduction in plants and animals.
essential fatty acids
Fatty acids that an animal cannot synthesize for itself and must obtain from its food.
essential mineral
See essential nutrient.
essential nutrient
A nutrient (e.g., amino acid, fatty acid, vitamin, mineral) that an animal cannot synthesize for itself and must obtain from its food.
ester linkage
A condensation (water-releasing) reaction in which the carboxyl group of a fatty acid reacts with the hydroxyl group of an alcohol. Lipids, including most membrane lipids, are formed in this way. (Contrast with ether linkage.)
estrogens
Any of several feminizing steroid sex hormones; produced chiefly by the ovaries in mammals, and by the placenta during pregnancy.
estrous cycle
A cycle of behavioral readiness to copulate (called estrus and correlated with ovulation) in most female mammals.
estrus
(es′ trus) [L. oestrus: frenzy] The period of heat, or maximum sexual receptivity, observed in most female mammals at times correlated with ovulation.
ether linkage
The linkage of two hydrocarbons by an oxygen atom (HC—O—CH). Ether linkages are characteristic of the membrane lipids of the Archaea. (Contrast with ester linkage.)
ethylene
One of the plant growth hormones, the gas H2C=CH2. Involved in fruit ripening and other growth and developmental responses.
euchromatin
Diffuse, uncondensed chromatin. Contains active genes that will be transcribed into mRNA. (Contrast with heterochromatin.)
eudicots
Angiosperms with two embryonic cotyledons; one of the two largest clades of angiosperms. (See also monocots.)
eukaryotes
(yew car′ ree oats) [Gk. eu: true + karyon: kernel or nucleus] Organisms whose cells contain their genetic material inside a nucleus. Includes all life other than the viruses, archaea, and bacteria. (Contrast with prokaryotes.)
eusociality
A type of social group in which some adults are infertile and provide services to other, fertile adults.
eutrophication
(yoo trofe′ ik ay′ shun) [Gk. eu: truly + trephein: to flourish] Responses of aquatic ecosystems to excessive nutrient input, which include greatly increased primary productivity and resulting depletion of dissolved oxygen.
evolution
Any gradual change. Most often refers to organic or Darwinian evolution, which is the genetic and resulting phenotypic change in populations of organisms from generation to generation. (Contrast with speciation.)
evolutionary developmental biology (evo-devo)
The study of the interplay between evolutionary and developmental processes, with a focus on the genetic changes that give rise to novel morphology. Key concepts of evo-devo include modularity, genetic toolkits, genetic switches, and heterochrony.
evolutionary reversal
The reappearance of an ancestral trait in a group that had previously acquired a derived trait.
evolutionary theory
The understanding and application of the mechanisms of evolutionary change to biological problems.
excitable cell
A cell that can generate and propagate action potentials because its cell membrane contains voltage-gated channels, notably neurons and muscle cells.
excitation
In the study of muscle, the event in which a nerve impulse arrives at a neuromuscular junction and initiates an action potential in the cell membrane of a muscle cell.
excitation–contraction coupling
In a muscle cell, the process by which electrical excitation of the cell membrane leads to contractile activity by the contractile proteins (actin and myosin) in the cell. Key events occur at the transverse tubules and sarcoplasmic reticulum, and result in release of Ca2+ into the cytoplasm.
excitatory postsynaptic potential (EPSP)
At a chemical synapse (specifically, an excitatory synapse), a graded membrane depolarization produced in the postsynaptic cell by arrival of an impulse in the presynaptic cell. (Contrast with inhibitory postsynaptic potential.)
excitatory synapse
A synapse at which impulses in the presynaptic cell act to depolarize the postsynaptic cell.
exocrine gland
(eks′ oh krin) [Gk. exo: outside + krinein: to separate] Any gland, such as a salivary gland, that secretes to the outside of the body or into the gut. (Contrast with endocrine gland.)
exocytosis
A process by which a vesicle within a cell fuses with the plasma membrane and releases its contents to the outside. (Contrast with endocytosis.)
exon
A portion of a DNA molecule, in eukaryotes, that codes for part of a polypeptide. (Contrast with intron.)
exoskeleton
(eks′ oh skel′ e ton) [Gk. exos: outside + skleros: hard] A hard covering on the outside of the body to which muscles are attached. (Contrast with endoskeleton.)
exotoxin
A highly toxic, usually soluble protein released by living, multiplying bacteria. (Contrast with endotoxin.)
expression vector
A DNA vector, such as a plasmid, that carries a DNA sequence for the expression of an inserted gene into mRNA and protein in a host cell.
expressivity
The degree to which a genotype is expressed in the phenotype; may be affected by the environment.
external environment
The conditions of the surroundings outside an animal’s body.
external fertilization
Fertilization that occurs after release of sperm and eggs into the external environment; typical of aquatic animals. Also called spawning. (Contrast with internal fertilization.)
external respiration
See breathing.
extracellular fluid
The aqueous solution (body fluid) outside cells (in the extracellular compartment). In animals with closed circulatory systems, subdivided into blood plasma and interstitial fluid.
extracellular matrix
A material of heterogeneous composition surrounding cells and performing many functions including adhesion of cells.
extraembryonic membranes
Four membranes that support but are not part of the developing embryos of reptiles, birds, and mammals, defining these groups phylogenetically as amniotes. (See amnion, allantois, chorion, and yolk sac.)
extrarenal salt excretion
Excretion of inorganic ions in concentrated solutions by structures other than the kidneys, such as the gills in marine fish and salt glands in marine birds and sea turtles.
extreme halophiles
A group of euryarchaeotes that live exclusively in very salty environments.
extremophiles
Archaea and bacteria that live and thrive under conditions (e.g., extremely high temperatures) that would kill most organisms.
F-box
A protein domain that facilitates protein–protein interactions necessary for protein degradation. Often involved in the regulation of gene expression (through the breakdown of regulatory proteins such as repressors), especially in plants.
F1
The first filial generation; the immediate progeny of a parental (P) mating.
F2
The second filial generation; the immediate progeny of a mating between members of the F1 generation.
facilitated diffusion
Passive movement through a membrane involving a specific carrier protein; does not proceed against a concentration gradient. (Contrast with active transport, diffusion.)
facultative anaerobe
A prokaryote that can shift its metabolism between anaerobic and aerobic modes depending on the presence or absence of O2. (Alternatively, facultative aerobe.)
facultative parasite
A organism that can parasitize other living organisms but is also capable of growing independently.
fallopian tube
See oviduct.
family
A group of related genera.
fast glycolytic cells
Muscle cells that are highly dependent on ATP production by anaerobic glycolysis and that contract and develop high tension rapidly, but fatigue quickly. Also called fast-twitch cells. (Compare to slow oxidative cells.)
fate map
A diagram of the blastula showing which cells (blastomeres) are “fated” to contribute to specific tissues and organs in the mature body.
fatty acid
A molecule made up of a long nonpolar hydrocarbon chain and a polar carboxyl group. Found in many lipids.
fauna
(faw′ nah) All the animals found in a given area. (Contrast with flora.)
fecundity
The average number of offspring produced by an individual female during a particular life stage, age, or period of time.
feedback inhibition
A mechanism for regulating a metabolic pathway in which the end product of the pathway can bind to and inhibit the enzyme that catalyzes the first committed step in the pathway. Also called end-product inhibition.
fermentation
(fur men tay′ shun) [L. fermentum: yeast] (1) Speaking specifically about energy metabolism, the anaerobic degradation of a substance such as glucose to smaller molecules such as lactic acid or alcohol with the extraction of energy. (2) Speaking generally, metabolic processes that occur in the absence of O2.
fertilization
Union of gametes.
fertilizer
Any of a number of substances added to soil to improve the soil’s capacity to support plant growth. May be organic or inorganic.
fetus
Medical and legal term for the stages of a developing human embryo from about the eighth week of pregnancy (the point at which all major organ systems have formed) to the moment of birth.
fiber
In angiosperms, an elongated, tapering sclerenchyma cell, usually with a thick cell wall, that serves as a support function in xylem. (See also muscle fiber.)
fibrous root system
A root system typical of monocots composed of numerous thin adventitious roots that are all roughly equal in diameter. (Contrast with taproot system.)
Fick’s law of diffusion
See law of diffusion for gases.
filament
In flowers, the part of a stamen that supports the anther.
filter feeder
(1) Speaking generally, an earlier term for a suspension feeder. (2) Speaking specifically, an organism that feeds by means of a straining or filtering device on organisms much smaller than itself that are suspended in water or air.
first filial generation
See F1.
first law of thermodynamics
The principle that energy can be neither created nor destroyed.
fission
See binary fission.
fitness
An individual’s contribution of genes to the next generation, as a consequence of its success in surviving and reproducing.
fixed
The condition in which a particular allele is the only allele present at a genetic locus within a population (i.e., there is no genetic variation at the locus).
fixed action pattern
In the study of animal behavior, a genetically determined behavior that is performed without learning, stereotypic (performed the same way each time), and not modifiable by learning.
flagellum
(fla jell′ um) (plural: flagella) [L. flagellum: whip] Long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes.
flora
(flore′ ah) All of the plants found in a given area. (Contrast with fauna.)
floral meristem
In angiosperms, a meristem that forms the floral organs (sepals, petals, stamens, and carpels).
floral organ identity genes
In angiosperms, genes that determine the fates of floral meristem cells; their expression is triggered by the products of meristem identity genes.
florigen (FT)
A plant hormone involved in the conversion of a vegetative shoot apex to a flower.
flower
The sexual structure of an angiosperm.
fluid mosaic model
A molecular model for the structure of biological membranes consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the plane of the bilayer.
flux
The rate of movement of energy, or of matter (an element or a molecular compound), between two compartments of an ecosystem, or between the Earth system and space.
foliose
Having a leafy growth form.
follicle-stimulating hormone (FSH)
A hormone, produced by the anterior pituitary gland, that helps control secretion of hormones by the gonads (i.e., a gonadotropin).
food web
A diagrammatic representation of which organisms consume which other organisms in an ecological community, i.e., a diagram of trophic relationships.
foot
One of the three main body parts of a mollusk.
forebrain
The region of the vertebrate brain that comprises the cerebrum, thalamus, and hypothalamus.
foregut
The beginning of the digestive tract. Consists of the mouth, esophagus, and stomach, and in some animals, a crop or gizzard.
foregut fermenter
An animal that has a specialized, nonacidic foregut chamber housing a mixed microbial community composed of fermenting microbes that assist with the breakdown of food materials.
fossil fuels
Combustible materials, such as coal, natural gas, petroleum, and peat that are formed by geological processes over long periods of time from the remains of living organisms.
founder effect
Random changes in allele frequencies resulting from establishment of a population by a very small number of individuals.
fragmentation
In ecology, the subdivision of a larger piece of habitat into smaller pieces that are isolated from one another by a different habitat.
frame-shift mutation
The addition or deletion of a single or two adjacent nucleotides in a gene’s sequence. Results in the misreading of mRNA during translation and the production of a nonfunctional protein. (Contrast with missense mutation, nonsense mutation, silent mutation.)
frequency distribution
A figure that displays the frequency of different classes of data.
free energy (G)
Energy that is available for doing useful work, after allowance has been made for the increase or decrease of disorder.
fruit
In angiosperms, a ripened and mature ovary (or group of ovaries) containing the seeds. Sometimes applied to reproductive structures of other groups of plants.
fruticose
Having a shrubby growth form.
FSH
See follicle-stimulating hormone.
functional genomics
The assignment of functional roles to the proteins encoded by genes identified by sequencing entire genomes.
functional group
A characteristic combination of atoms that contribute specific properties when attached to larger molecules.
G cap
A chemically modified GTP added to the 5′ end of mRNA; facilitates binding of mRNA to ribosome and prevents mRNA breakdown. Also called 5′ cap.
G protein
A membrane protein involved in signal transduction; characterized by binding GDP or GTP.
G protein–linked receptors
A class of receptors that change configuration upon ligand binding such that a G protein binding site is exposed on the cytoplasmic domain of the receptor, initiating a signal transduction pathway.
G1
In the cell cycle, the gap between the end of mitosis and the onset of the S phase.
G1–S transition
In the cell cycle, the point at which G1 ends and the S phase begins.
G2
In the cell cycle, the gap between the S (synthesis) phase and the onset of mitosis.
gain of function mutation
A mutation that results in a protein with a new function. (Contrast with loss of function mutation.)
gametangium
(gam uh tan′ gee um) (plural: gametangia) [Gk. gamos: marriage + angeion: vessel] Any plant or fungal structure within which a gamete is formed.
gamete
(gam′ eet) [Gk. gamete/gametes: wife, husband] The mature sexual reproductive cell: the egg or the sperm.
gametogenesis
(ga meet′ oh jen′ e sis) The specialized series of cellular divisions, including meiosis, that leads to the formation of gametes.
gametophyte
(ga meet′ oh fyte) In plants and photosynthetic protists with alternation of generations, the multicellular haploid phase that produces the gametes. (Contrast with sporophyte.)
ganglion
(gang′ glee un) (plural: ganglia) [Gk. tumor] (1) In a vertebrate, a discrete cluster of neuron cell bodies in the peripheral nervous system. (2) In an arthropod, an enlarged part of the ventral nerve cord found in each body segment.
gap genes
In Drosophila (fruit fly) development, segmentation genes that define broad areas along the anterior–posterior axis of the early embryo. Part of a developmental cascade that includes maternal effect genes, pair rule genes, segment polarity genes, and Hox genes.
gas exchange membranes
Thin layers of tissue (one or two cells thick) where respiratory gases move between the internal tissues of an animal and the animal’s environmental medium.
gastrovascular cavity
Serving for both digestion (gastro) and circulation (vascular); in particular, the central cavity of the body of jellyfish and other cnidarians.
gastrula
The end product of gastrulation. At this point the embryo contains the forerunners of the three germ layers that will generate the entire body.
gastrulation
Development of a blastula into a gastrula. In embryonic development, the process by which a blastula is transformed by massive movements of cells, from the exterior to the interior, into a gastrula.
gated channel
A membrane protein that changes its three-dimensional shape, and therefore its ion conductance, in response to a stimulus. When open, it allows specific ions to move across the membrane.
gel electrophoresis
(e lek′ tro fo ree′ sis) [L. electrum: amber + Gk. phorein: to bear] A technique for separating molecules (such as DNA fragments) from one another on the basis of their electric charges and molecular weights by applying an electric field to a gel.
gene
[Gk. genes: to produce] A unit of heredity. Used here as the unit of genetic function which carries the information for a polypeptide or RNA.
gene family
A set of similar genes derived from a single parent gene; need not be on the same chromosomes. The vertebrate globin genes constitute a classic example of a gene family.
gene flow
Exchange of genes between populations through migration of individuals or movements of gametes.
gene-for-gene resistance
In plants, a mechanism of resistance to pathogens in which resistance is triggered by the specific interaction of the products of a pathogen’s Avr genes and a plant’s R genes.
general transcription factors
In eukaryotes, transcription factors that bind to the promoters of most protein-coding genes and are required for their expression. Distinct from transcription factors that have specific regulatory effects only at certain promoters or classes of promoters.
genetic drift
Changes in gene frequencies from generation to generation as a result of random (chance) processes.
genetic linkage
Association between genes on the same chromosome such that they do not show random assortment and seldom recombine; the closer the genes, the lower the frequency of recombination.
genetic screen
A technique for identifying genes involved in a biological process of interest. Involves creating a large collection of randomly mutated organisms and identifying those individuals that are likely to have a defect in the pathway of interest. The mutated gene(s) in those individuals can then be isolated for further study.
genetic structure
The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population.
genetic switches
Mechanisms that control how the genetic toolkit is used, such as promoters and the transcription factors that bind them. The signal cascades that converge on and operate these switches determine when and where genes will be turned on and off.
genetic toolkit
A set of developmental genes and proteins that is common to most animals and is hypothesized to be responsible for the evolution of their differing developmental pathways.
genome
(jee′ nome) The complete DNA sequence for a particular organism or individual.
genomic equivalence
The principle that no information is lost from the nuclei of cells as they pass through the early stages of embryonic development.
genomic imprinting
The form of a gene’s expression is determined by parental source (i.e., whether the gene is inherited from the male or female parent).
genomic library
All of the cloned DNA fragments generated by the breakdown of genomic DNA into smaller segments.
genotype
(jean′ oh type) [Gk. gen: to produce + typos: impression] An exact description of the genetic constitution of an individual, either with respect to a single trait or with respect to a larger set of traits. (Contrast with phenotype.)
genus
(jean′ us) (plural: genera) [Gk. genos: stock, kind] A group of related, similar species recognized by taxonomists with a distinct name used in binomial nomenclature.
geographic range
The entire region in which a species occurs.
geographic speciation
See allopatric speciation.
geological time scale
The standardized scale accepted by geologists to mark the major divisions in the history of the Earth.
germ cells
[L. germen: to beget] Cells in the gonads that multiply mitotically and are capable of undergoing meiosis to produce gametes. (Contrast with somatic cell.)
germline mutation
Mutation in a cell that produces gametes (i.e., a germline cell). (Contrast with somatic mutation.)
germination
Sprouting of a seed or spore.
GFR
See glomerular filtration rate.
gibberellin
(jib er el′ lin) A class of plant growth hormones playing roles in stem elongation, seed germination, flowering of certain plants, etc.
gill
A breathing organ that is evaginated (folded outward from the body) and surrounded by the environmental medium. The type of breathing organ found most commonly in water-breathers. (Contrast with lung.)
glial cells
One of the two major types of cells in the nervous system (the other being neurons); unlike neurons, glial cells are typically not excitable and do not conduct action potentials. Also called glia or neuroglia.
global warming
An increase through time in Earth’s average annual surface temperature.
glomerular filtration rate (GFR)
The rate at which all the nephrons in the kidneys collectively produce primary urine by ultrafiltration of blood plasma.
glomerulus
(glo mare′ yew lus) (plural: glomeruli) [L. glomus: ball] Vascular structure in the kidney where blood filtration takes place, forming primary urine. Each glomerulus consists of a knot of capillaries served by arterioles.
gluconeogenesis
The biochemical synthesis of glucose from other substances, such as amino acids, lactate, and glycerol.
glyceraldehyde 3-phosphate (G3P)
A phosphorylated three-carbon sugar; an intermediate in glycolysis and photosynthetic carbon fixation.
glycerol
(gliss′ er ole) A three-carbon alcohol with three hydroxyl groups; a component of phospholipids and triglycerides.
glycosidic linkage
Bond between carbohydrate (sugar) molecules through an intervening oxygen atom (—O—).
glycolipid
A lipid to which sugars are attached.
glycolysis
(gly kol′ li sis) [Gk. gleukos: sugar + lysis: break apart] The enzymatic breakdown of glucose to pyruvic acid.
glycoprotein
A protein to which sugars are attached.
glyoxysome
(gly ox′ ee soam) An organelle found in plants, in which stored lipids are converted to carbohydrates.
Golgi apparatus
(goal′ jee) A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells; functions in secretion from the cell by exocytosis.
gonad
(go′ nad) [Gk. gone: seed] An organ that produces gametes in animals: either an ovary (female gonad) or testis (male gonad). Certain of the tissues in a gonad secrete hormones.
gonadotropin
A type of tropic hormone that controls hormone secretion by gonadal endocrine cells, and that supports and maintains gonadal tissue, including gamete production. The two gonadotropins in vertebrates are follicle-stimulating hormone and luteinizing hormone, both secreted by the anterior pituitary gland.
gonadotropin-releasing hormone (GnRH)
In vertebrates, hormone produced by the hypothalamus that stimulates the anterior pituitary to secrete (“release”) gonadotropins.
Gondwana
The large southern land mass that existed from the Cambrian (540 mya) to the Jurassic (138 mya). Present-day remnants are South America, Africa, India, Australia, and Antarctica.
GPP
See gross primary productivity.
graded membrane potentials
Small local changes in membrane potential caused by opening or closing of ion channels. They can be of any size (thus “graded”).
grafting
Artificial transplantation of tissue from one organism to another. In horticulture, the transfer of a bud or stem segment from one plant onto the root of another as a form of asexual reproduction.
Gram-negative bacteria
Bacteria that appear red when stained using the Gram-staining technique. These bacteria have an outer membrane outside the relatively thin peptidoglycan layer of the cell wall.
Gram-positive bacteria
Bacteria that appear blue to purple when stained using the Gram-staining technique. These bacteria have an outer cell wall consisting of a thick layer of peptidoglycan.
Gram stain
A differential purple stain useful in characterizing bacteria. The peptidoglycan-rich cell walls of Gram-positive bacteria stain purple; cell walls of Gram-negative bacteria generally stain orange.
gravitropism
[Gk. tropos: to turn] A directed plant growth response to gravity.
gray crescent
In frog development, a band of diffusely pigmented cytoplasm on the side of the egg opposite the site of sperm entry. Arises as a result of cytoplasmic rearrangements that establish the anterior–posterior axis of the zygote.
greenhouse effect
The retention of heat energy near Earth’s surface as a consequence of absorption and reradiation of infrared radiation by certain gases (such as carbon dioxide and methane) in the atmosphere.
greenhouse gases
Gases in the atmosphere, such as carbon dioxide and methane, that absorb infrared radiation and thereby retain heat energy within Earth’s lower atmosphere.
gross primary productivity (GPP)
The rate at which primary producers in a given area (or volume) capture energy from sunlight or other abiotic sources and convert it to chemical energy.
ground meristem
That part of an apical meristem that gives rise to the ground tissue system of the primary plant body.
ground tissue system
Those parts of the plant body not included in the dermal or vascular tissue systems. Ground tissues function in storage, photosynthesis, and support. (Contrast with dermal tissue system, vascular tissue system.)
growth
An increase in the size of the body and its organs by cell division and cell expansion. In ecology, change in size of the population of a species.
growth factor
A chemical signal that stimulates cells to divide.
growth hormone
A peptide hormone released by the anterior pituitary gland that stimulates many processes involved in tissue formation and growth.
guanine (G)
(gwan′ een) A nitrogen-containing base found in DNA, RNA, and GTP.
guard cells
In plants, specialized, paired epidermal cells that surround and control the opening of a stoma (pore).
gut microbiome
Populations of microbes, consisting of many species of bacteria and other heterotrophic microbes, living in the gut lumen of an animal.
gymnosperms
Seed plants with seeds not enclosed in carpels. (See also angiosperms.)
gyre
Large-scale circular ocean currents caused by prevailing winds and Earth’s rotation.
habitat patches
Areas of suitable habitat for a species that are separated by areas of unsuitable habitat.
habitat
The characteristic environment(s) occupied by a species.
habitat structure
The physical structure of a habitat, for example the horizontal and vertical distribution of objects (such as the stems and leaves of plants) in a habitat.
Hadley cells
Large-scale vertical patterns of atmospheric circulation in which warm rises near the Equator, moves toward the poles, and cools and sinks around 30 degrees N or S latitude.
half-life
The time required for half of a sample of a radioactive isotope to decay to its stable, nonradioactive form, or for a drug or other substance to reach half its initial dosage.
halophyte
(hal′ oh fyte) [Gk. halos: salt + phyton: plant] A plant that grows in a saline (salty) environment.
haploid
(hap′ loid) [Gk. haploeides: single] Having a chromosome complement consisting of just one copy of each chromosome; designated 1n or n. (Contrast with diploid.)
haplotype
Linked nucleotide sequences that are usually inherited as a unit (as a “sentence” rather than as individual “words”).
Hardy–Weinberg equilibrium
The unchanging frequencies of alleles and genotypes in a population that is expected with random combination of gametes, in the absence of natural selection, mutation, migration, and genetic drift. The expectations of Hardy–Weinberg equilibrium can be used to calculate the expected frequencies of genotypes given the frequency of alleles, under the stated assumptions.
haustorium
(haw stor′ ee um) (plural: haustoria) [L. haustus: draw up] A specialized hypha or other structure by which fungi and some parasitic plants draw nutrients from a host plant.
heart
In a circulatory system, a muscular pump that drives flow of blood around the body.
heat capacity
The ratio of energy absorbed by a substance the increase in temperature of that substance.
heat of vaporization
The energy that must be supplied to convert a molecule from a liquid to a gas at its boiling point.
heat shock proteins
Chaperone proteins expressed in cells exposed to high or low temperatures or other forms of environmental stress.
helical
Shaped like a screw or spring (helix); this shape occurs in DNA and proteins.
helicase
An enzyme that catalyzes the unwinding of a nucleic acid double helix.
helper T cell
See T-helper cell.
hemiparasite
A parasitic plant that can photosynthesize, but derives water and mineral nutrients from the living body of another plant. (Contrast with holoparasite.)
hemizygous
(hem′ ee zie′ gus) [Gk. hemi: half + zygotos: joined] In a diploid organism, having only one allele for a given trait, typically the case for X-linked genes in male mammals and Z-linked genes in female birds. (Contrast with homozygous, heterozygous.)
hemocyanin
A type of respiratory pigment found in arthropods and mollusks, consisting of a copper-based protein. Undergoes reversible combination with O2 at copper-containing loci.
hemoglobin
(hee′ mo glow bin) [Gk. heaema: blood + L. globus: globe] A type of respiratory pigment consisting of an iron-containing protein. Undergoes reversible combination with O2 at iron-containing loci called heme sites.
herbivore
(ur′ bi vore) [L. herba: plant + vorare: to devour] An animal that consumes plant tissues. See primary consumer. (Contrast with carnivore, detritivore, omnivore.)
herbivory
An interaction between individuals of two different species in which one individual, the herbivore, consumes all or part of the other individual, which is a plant.
heritability
The extent to which the value or expression of a trait of an organism is determined by genes transmitted from its parents.
hetero-
[Gk.: heteros: other, different] A prefix indicating two or more different conditions, structures, or processes. (Contrast with homo-.)
heterochromatin
Densely packed, dark-staining chromatin; any genes it contains are usually not transcribed. (Contrast with euchromatin.)
heterochrony
Alteration in the timing of developmental events, leading to different results in the adult organism.
heterocyst
A large, thick-walled cell type in the filaments of certain cyanobacteria that performs nitrogen fixation.
heteromorphic
(het′ er oh more′ fik) [Gk. heteros: different + morphe: form] Having a different form or appearance, as two heteromorphic life stages of a plant. (Contrast with isomorphic.)
heterosis
See hybrid vigor.
heterosporous
(het′ er os′ por us) Producing two types of spores, one of which gives rise to a female megaspore and the other to a male microspore. (Contrast with homosporous.)
heterotopy
[Gk. different place] Spatial differences in gene expression during development, controlled by developmental regulatory genes and contributing to the evolution of distinctive adult phenotypes.
heterotroph
(het′ er oh trof) [Gk. heteros: different + trophe: feed] An organism that requires preformed organic molecules as sources of energy and chemical building blocks. (Contrast with autotroph.)
heterotypy
[Gk. different kind] Alteration in a developmental regulatory gene itself rather than the expression of the genes it controls. (Contrast with heterotopy.)
heterozygous
(het′ er oh zie′ gus) [Gk. heteros: different + zygotos: joined] In diploid organisms, having different alleles of a given gene on the pair of homologs carrying that gene. (Contrast with homozygous.)
heterozygous carrier
An individual that carries a recessive allele for a phenotype of interest (e.g., a genetic disease); the individual does not show the phenotype, but may have progeny with the phenotype if the other parent also carries the recessive allele.
hibernation
[L. hibernum: winter] In mammals, the state of inactivity of some species during winter, during which body temperature falls low enough to match ambient temperature and metabolic rate is prodoundly depressed.
high-throughput sequencing
Rapid DNA sequencing on a micro scale in which many fragments of DNA are sequenced in parallel.
highly repetitive sequences
Short (less than 100 bp), nontranscribed DNA sequences, repeated thousands of times in tandem arrangements.
hindbrain
The region of the developing vertebrate brain that gives rise to the medulla, pons, and cerebellum.
hindgut
The second part of the intestine; in mammals, the hindgut is also called the large intestine.
hindgut fermenters
An animal that has a specialized hindgut chamber housing mixed communities of fermenting microbes that assist with the breakdown of food materials.
histamine
(hiss′ tah meen) A substance released by damaged tissue, or by mast cells in response to allergens. Histamine increases vascular permeability, leading to edema (swelling).
histogram
A figure that displays frequencies of classes of quantitative data binned by ranges of a particular variable.
histone
Any one of a group of proteins forming the core of a nucleosome, the structural unit of a eukaryotic chromosome.
histone acetyltransferases
Enzymes involved in chromatin remodeling. Add acetyl groups to the tail regions of histone proteins. (Contrast with histone deacetylase.)
histone deacetylase
In chromatin remodeling, an enzyme that removes acetyl groups from the tails of histone proteins. (Contrast with histone acetyltransferases.)
HIV
See human immunodeficiency virus.
holometabolous
Undergoing complete metamorphosis.
holoparasite
A fully parasitic plant (i.e., one that does not perform photosynthesis).
home range
A region inhabited by an individual in which other individuals are not excluded. (Contrast with territory.)
homeobox
180-base-pair segment of DNA found in certain homeotic genes; regulates the expression of other genes and thus controls large-scale developmental processes.
homeostasis
(home′ ee o sta′ sis) [Gk. homos: same + stasis: position] Stability of the internal environment of an individual, such as a constant body temperature, and the physiological or behavioral feedback responses that maintain that stability. In the words of Walter Cannon, who coined the term, “the coordinated physiological processes which maintain most of the [constant] states in the organism.”
homeotherm
An animal that maintains a relatively constant body temperature by physiological means, (e.g., changes in metabolic rate) rather than simply by behavior.
homeotic mutation
Mutation in a homeotic gene that results in the formation of a different organ than that normally made by a region of the embryo.
homo-
[Gk. homos: same] A prefix indicating two or more similar conditions, structures, or processes. (Contrast with hetero-.)
homologous pair
A pair of matching chromosomes made up of a chromosome from each of the two sets of chromosomes in a diploid organism.
homologous recombination
Exchange of segments between two DNA molecules based on sequence similarity between the two molecules. The similar sequences align and crossover. Used to create knockout mutants in mice and other organisms.
homology
(ho mol′ o jee) [Gk. homologia: of one mind; agreement] A similarity between two or more features that is due to inheritance from a common ancestor. The structures are said to be homologous, and each is a homolog of the others.
homoplasy
(home′ uh play zee) [Gk. homos: same + plastikos: shape, mold] The presence in multiple groups of a trait that is not inherited from the common ancestor of those groups. Can result from convergent evolution, evolutionary reversal, or parallel evolution.
homosporous
Producing a single type of spore that gives rise to a single type of gametophyte, bearing both female and male reproductive organs. (Contrast with heterosporous.)
homozygous
(home′ oh zie′ gus) [Gk. homos: same + zygotos: joined] In diploid organisms, having identical alleles of a given gene on both homologous chromosomes. An individual may be a homozygote with respect to one gene and a heterozygote with respect to another. (Contrast with heterozygous.)
horizons
The horizontal layers of a soil profile, including the topsoil (A horizon), subsoil (B horizon) and parent rock or bedrock (C horizon).
hormone
(hore′ mone) [Gk. hormon: to excite, stimulate] A chemical substance produced in minute amounts by endocrine cells and transported in the blood to distant target cells, where it exerts regulatory influences on their function.
Human Genome Project
A publicly and privately funded research effort, successfully completed in 2003, to produce a complete DNA sequence for the entire human genome.
human immunodeficiency virus (HIV)
The retrovirus that causes acquired immune deficiency syndrome (AIDS).
humidic
Referring to terrestrial animals, those that are restricted to humid, water-rich microenvironments; unable to live steadily in the open air.
humoral immune response
The response of the immune system mediated by B cells that produces circulating antibodies active against extracellular bacterial and viral infections. (Contrast with cellular immune response.)
humus
(hew′ mus) The partly decomposed remains of plants and animals on the surface of a soil.
hybrid vigor
The superior fitness of heterozygous offspring as compared with that of their dissimilar homozygous parents. Also called heterosis.
hybrid zone
A region of overlap in the ranges of two closely related species where the species may hybridize.
hydrogen bond
A weak electrostatic bond which arises from the attraction between the slight positive charge on a hydrogen atom and a slight negative charge on a nearby oxygen or nitrogen atom.
hydrological cycle
The movement of water from the oceans to the atmosphere, to the soil, and back to the oceans.
hydrolysis reaction
(high drol′ uh sis) [Gk. hydro: water + lysis: break apart] A chemical reaction that breaks a bond by inserting the components of water (AB + H2O → AH + BOH). (Contrast with condensation reaction.)
hydrophilic
(high dro fill′ ik) [Gk. hydro: water + philia: love] Having an affinity for water. (Contrast with hydrophobic.)
hydrophobic
(high dro foe′ bik) [Gk. hydro: water + phobia: fear] Having no affinity for water. Uncharged and nonpolar groups of atoms are hydrophobic. (Contrast with hydrophilic.)
hydroponic
Pertaining to a method of growing plants with their roots suspended in nutrient solutions instead of soil.
hydrostatic skeleton
Stiffness imparted to the body or part of the body of an animal by high blood pressure inside
hyper-
[Gk. hyper: above, over] Prefix indicating above, higher, more. (Contrast with hypo-.)
hyperaccumulators
Species of plants that store large quantities of heavy metals such as arsenic, cadmium, nickel, aluminum, and zinc.
hyperosmotic
In comparing two solutions, refers to the one having a higher osmotic pressure. The term is relative and has meaning only in comparing two specified solutions. (Compare with hyposmotic.)
hyperosmotic regulator
An aquatic animal that maintains a stable blood osmotic pressure higher than the osmotic pressure of the water in which it lives.
hyperpolarization
A change in the membrane potential across a cell membrane so that the inside of the membrane becomes more negative compared with the outside of the membrane. (Contrast with depolarization.)
hypersensitive response
A defensive response of plants to microbial infection in which phytoalexins and pathogenesis-related proteins are produced and the infected tissue undergoes apoptosis to isolate the pathogen from the rest of the plant.
hypertension
High blood pressure.
hypertonic
Having a greater solute concentration. Said of one solution compared with another. (Contrast with hypotonic, isotonic.)
hypha
(high′ fuh) (plural: hyphae) [Gk. hyphe: web] In the fungi and oomycetes, any single filament.
hypo-
[Gk. hypo: beneath, under] Prefix indicating underneath, below, less. (Contrast with hyper-.)
hypophysis
See pituitary gland.
hyposmotic
In comparing two solutions, refers to the one having a lower osmotic pressure. The term is relative and has meaning only in comparing two specified solutions. The word is a contraction of hypo-osmotic. (Compare with hyperosmotic.)
hyposmotic regulator
An aquatic animal that maintains a stable blood osmotic pressure lower than the osmotic pressure of the water in which it lives.
hypothalamo–hypophysial portal system
In vertebrates, a system of blood vessels that connects capillaries in the hypothalamus to capillaries in the anterior pituitary gland; provides a direct pathway by which hypothalamic hormones can reach specific populations of cells in the anterior pituitary.
hypothalamus
A part of the forebrain lying below the thalamus and relatively near the base of the brain. It plays roles in learning, memory, spatial orientation, and control of water balance, reproduction, and temperature regulation.
hypotonic
Having a lesser solute concentration. Said of one solution in comparing it to another. (Contrast with hypertonic, isotonic.)
igneous
Rocks formed from the cooling and hardening of molten magma.
imbibition
Water uptake by a seed; first step in germination.
immunity
In animals, the ability to avoid disease when invaded by a pathogen by deploying various defense mechanisms.
immunoglobulins
A class of proteins containing a tetramer consisting of four polypeptide chains—two identical light chains and two identical heavy chains—held together by disulfide bonds; active as receptors and effectors in the immune system.
immunological memory
The capacity to more rapidly and massively respond to a second exposure to an antigen than occurred on first exposure.
imperfect flower
A flower lacking either functional stamens or functional carpels. (Contrast with perfect flower.)
implantation
The process by which the early mammalian embryo becomes attached to and embedded in the endometrium (lining of the uterus).
imprinting
See behavioral imprinting.
impulse
See action potential.
in vitro evolution
A method based on natural molecular evolution that uses artificial selection in the laboratory to rapidly produce molecules with novel enzymatic and binding functions.
inclusive fitness
The sum of an individual’s genetic contribution to subsequent generations both via production of its own offspring and via its influence on the survival of relatives who are not direct descendants.
incomplete cleavage
A pattern of cleavage that occurs in many very yolky eggs, in which the cleavage furrows do not penetrate all of the cytoplasm. (Contrast with complete cleavage.)
incomplete dominance
Condition in which the heterozygous phenotype is intermediate between the two homozygous phenotypes.
incomplete metamorphosis
Insect development in which changes between instars are gradual.
independent assortment
See law of independent assortment.
indeterminate growth
A open-ended growth pattern in which an organism or organ continues to grow as long as it lives; characteristic of some animals and of plant shoots and roots. (Contrast with determinate growth.)
indirect development
Development in which the young are morphologically very different than the adult. (Contrast with direct development.)
induced mutation
A mutation resulting from exposure to a mutagen from outside the cell. (Contrast with spontaneous mutation.)
induced ovulation
Ovulation (release of an egg from the ovaries of a female) triggered by the act of copulation. (Contrast with spontaneous ovulation.)
induced pluripotent stem cells (iPS cells)
Multipotent or pluripotent animal stem cells produced from differentiated cells in vitro by the addition of several genes that are expressed.
induced responses
Defensive responses that a plant produces only in the presence of a pathogen, in contrast to constitutive defenses, which are always present.
inducer
(1) A compound that stimulates the synthesis of a protein. (2) In embryonic development, a substance that causes a group of target cells to differentiate in a particular way.
inducible enzyme
An enzyme that is present in variable amounts, depending on the conditions to which a cell, tissue, or organism is exposed. Expression of an inducible enzyme depends on levels of specific inducing agents.
inducible genes
Genes that are expressed only when their products are needed. (Contrast with constitutive genes.)
inducible
Produced only in the presence of a particular compound or under particular circumstances. (Contrast with constitutive.)
induction
In embryonic development, the process by which one cell population influences the fate of another cell population.
inflammation
A nonspecific defense against pathogens; characterized by redness, swelling, pain, and increased temperature.
inflorescence
A structure composed of several to many flowers.
inflorescence meristem
A meristem that produces floral meristems as well as other small leafy structures (bracts).
infrared wavelengths
Wavelengths in the electromagnetic spectrum that are too long to be seen by the human eye (i.e., greater than 0.7 μm).
ingroup
In a phylogenetic study, the group of organisms of primary interest. (Contrast with outgroup.)
inhibiting hormones (IHs)
Hormones, released by neurosecretory cells in the hypothalamus, that inhibit the secretion of hormones by the anterior pituitary gland. Each IH is specific for particular pituitary hormones. Also called release-inhibiting hormones.
inhibitory postsynaptic potential (IPSP)
At a chemical synapse (specifically, an inhibitory synapse), a graded membrane hyperpolarization produced in the postsynaptic cell by arrival of an impulse in the presynaptic cell. (Contrast with excitatory postsynaptic potential.)
inhibitory synapse
A synapse at which impulses in the presynaptic cell act to hyperpolarize the postsynaptic cell, thereby shifting the membrane potential of the postsynaptic cell away from the threshold for action potential production.
initiation complex
In protein translation, a combination of a small ribosomal subunit, an mRNA molecule, and the tRNA charged with the first amino acid coded for by the mRNA; formed at the onset of translation.
innate defenses
In animals, one of two general types of defenses against pathogens. Nonspecific and present in most animals. (Contrast with adaptive defenses.)
inner cell mass
Part of the mammalian blastula (blastocyst), the inner cell mass will give rise to the embryo (via the epiblast) and the yolk sac, allantois, and amnion (via the hypoblast). (Contrast with trophoblast.)
innervate
To provide neural input.
inorganic fertilizer
A chemical or combination of chemicals applied to soil or plants to make up for a plant nutrient deficiency. Often contains the macronutrients nitrogen, phosphorus, and potassium (N-P-K).
instar
(in′ star) An immature stage of an insect between molts.
integral membrane proteins
Proteins that are at least partially embedded in the plasma membrane. (Contrast with peripheral membrane proteins.)
integument
[L. integumentum: covering] A protective surface structure. In gymnosperms and angiosperms, a layer of tissue around the ovule which will become the seed coat.
interference RNA (RNAi)
See RNA interference.
interferon
A glycoprotein produced by virus-infected animal cells; increases the resistance of neighboring cells to the virus.
intermediate filaments
Components of the cytoskeleton whose diameters fall between those of the larger microtubules and those of the smaller microfilaments.
intermediate mesoderm
The mesoderm that forms urinary and reproductive tissues.
internal effector
See autonomic effector.
internal environment
In multicellular organisms, the extracellular body fluids (interstitial fluids or tissue fluids), which bathe the cells of the body and therefore constitute the immediate environment of the cells.
internal fertilization
Fertilization inside the body of the female that occurs because sperm are deposited into the female reproductive tract and encounter ova there (Contrast with external fertilization.)
internal membrane
An intracellular membrane.
interneuron
A neuron that communicates information between two other neurons.
interphase
In the cell cycle, the period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. During interphase the cell is most active in transcribing and translating genetic information.
interspecific competition
A mutually detrimental (−/−) interaction between individuals of two different species that causes both to suffer reduced fitness. (Contrast with intraspecific competition.)
interspecific interactions
An interaction between individuals of different species that affects the per capita birth and/or death rate (or equivalently the fitness) of at least some of the individuals involved.
interstitial cells
Testosterone-secreting cells located in the tissue between the seminiferous tubules of the vertebrate testis. Also called Leydig cells.
interstitial fluid
Extracellular fluid that is found bathing the cells in tissues throughout the body. Also called tissue fluid.
intertidal zone
The nearshore region of oceans that lies between the level of the high and low tides, so that it is alternately inundated with water and exposed to air.
intracellular fluid
The aqueous solution inside a cell.
intraspecific competition
A mutually detrimental (−/−) interaction between individuals of the same species that causes both to suffer reduced fitness. (Contrast with interspecific competition.)
intron
Portion of a of a gene within the coding region that is transcribed into pre-mRNA but is spliced out prior to translation. (Contrast with exon.)
invasive species
A species that increases in abundance and spreads widely, often to the detriment of other species, when it is introduced to a new location.
invasiveness
The ability of a pathogen to multiply in a host’s body. (Contrast with toxigenicity).
inversion
A rare 180° reversal of the order of genes within a segment of a chromosome.
involuntary nervous system
See autonomic nervous system.
ion
(eye′ on) [Gk. ion: wanderer] An electrically charged particle that forms when an atom gains or loses one or more electrons.
ion channel
An integral membrane protein that allows ions to diffuse across the membrane in which it is embedded.
ion exchange
A process by which protons produced by a plant root displace mineral cations from clay particles in the surrounding soil.
ionic attraction
An electrostatic attraction between positively and negatively ions.
ionotropic receptor
A receptor protein that is a ligand-gated ion channel and that therefore directly alters membrane ion permeability when it combines with its ligand.
ionotropic receptor cell
A sensory receptor cell that typically has an ionotropic receptor protein that is a stimulus-gated Na+ channel. See ionotropic receptor.
iPS cells
See induced pluripotent stem cells.
iris
(eye′ ris) [Gk. iris: rainbow] The round, pigmented membrane that surrounds the pupil of the eye and adjusts its aperture to regulate the amount of light entering the eye.
island biogeography
Patterns in the spatial distribution of species among oceanic islands or “islands” of one kind of habitat surrounded by an “ocean” of a different habitat.
iso-
[Gk. iso: equal] Prefix used for two separate entities that share some element of identity.
isomorphic
(eye so more′ fik) [Gk. isos: equal + morphe: form] Having the same form or appearance, as when the haploid and diploid life stages of an organism appear identical. (Contrast with heteromorphic.)
isosmotic
Having the same osmotic pressure; said of a solution in comparison to another solution. The term is relative and has meaning only in comparing two specified solutions.
isotonic
Having the same solute concentration; said of two solutions. (Contrast with hypertonic, hypotonic.)
isotope
(eye′ so tope) [Gk. isos: equal + topos: place] Isotopes of a given chemical element have the same number of protons in their nuclei (and thus are in the same position on the periodic table), but differ in the number of neutrons.
iteroparity
[L. itero: to repeat + pario: to beget] A type of reproductive life history in which individuals are physiologically capable of reproducing multiple times in a lifetime. (Contrast with semelparity.)
iteroparous
Characterized by iteroparity.
jasmonate
Also called jasmonic acid, a plant hormone involved in triggering responses to pathogen attack as well as other processes.
jasmonic acid
See jasmonate.
jelly coat
The outer protective layer of a sea urchin egg, which triggers an acrosomal reaction in sperm.
joint
In skeletal systems, a junction between two or more bones.
joule (J)
A metric unit for measuring energy: 1 J = 0.24 calorie; 1 calorie = 4.2 joule.
juxtacrine signal
A type of cell signal that requires that the signaling and responding cells are in direct contact. Usually involves interaction between signaling molecules bound to the surfaces of the two cells.
karyogamy
The fusion of nuclei of two cells. (Contrast with plasmogamy.)
karyotype
The number, forms, and types of chromosomes in a cell.
kidney
An organ that regulates the composition and volume of the blood plasma by producing and eliminating from the body an aqueous solution (urine) derived from the blood plasma or other extracellular fluids.
kilocalorie (kcal)
See calorie.
kinase
See protein kinase.
kinetic energy
(kuh-net′ ik) [Gk. kinetos: moving] The energy associated with movement. (Contrast with potential energy.)
kinetochore
(kuh net′ oh core) Specialized structure on a centromere to which microtubules attach.
kingdom
A group of related phyla.
Koch’s postulates
A set of rules for establishing that a particular microorganism causes a particular disease.
Krebs cycle
See citric acid cycle.
lactic acid fermentation
Anaerobic series of reactions that convert glucose to lactic acid, in some bacteria and animal cells.
lactifers
Plant cell or vessel that contains latex.
lagging strand
In DNA replication, the daughter strand that is synthesized in discontinuous stretches. See Okazaki fragments.
landscape
An ecological system consisting of multiple ecological communities within a geographical area larger than the area occupied by a single community.
lateral gene transfer
The transfer of genes from one species to another, common among bacteria and archaea.
lateral meristem
Either of the two meristems, the vascular cambium and the cork cambium, that give rise to a plant’s secondary growth.
lateral plate mesoderm
One of three regions of mesoderm on either side of the notochord; encloses the coelom and gives rise to the heart, muscles of the digestive system, muscles of the outer body wall, and other tissues.
lateral root
A root extending outward from the taproot in a taproot system; typical of eudicots.
Laurasia
The northernmost of the two large continents produced by the breakup of Pangaea.
law of diffusion for gases
An equation that describes the factors that determine the rate of diffusion of gas molecules within a gas phase, liquid phase, or across the boundary between a gas phase and liquid phase; often called Fick’s law.
law of independent assortment
During meiosis, the random separation of genes carried on nonhomologous chromosomes into gametes so that inheritance of these genes is random. This principle was articulated by Mendel as his second law
law of segregation
In genetics, the separation of alleles, or of homologous chromosomes, from each other during meiosis so that each of the haploid daughter nuclei produced contains one or the other member of the pair found in the diploid parent cell, but never both. This principle was articulated by Mendel as his first law.
laws of thermodynamics
[Gk. thermos: heat + dynamis: power] Laws derived from studies of the physical properties of energy and the ways energy interacts with matter. (See also first law of thermodynamics, second law of thermodynamics.)
leaching
In soils, a process by which mineral nutrients in upper soil horizons are dissolved in water and carried to deeper horizons, where they are unavailable to plant roots.
leading strand
In DNA replication, the daughter strand that is synthesized continuously. (Contrast with lagging strand.)
leaf
(plural: leaves) In plants, the chief organ of photosynthesis.
learning
The process of modifying an individual’s behavior based on prior experiences of the individual.
leghemoglobin
In nitrogen-fixing plants, an oxygen-carrying protein in the cytoplasm of nodule cells that transports enough oxygen to the nitrogen-fixing bacteria to support their respiration, while keeping free oxygen concentrations low enough to protect nitrogenase.
lens
In an eye, a structure composed of transparent proteins that focuses images on the retina or other light-sensing structures.
leukocytes
See white blood cells.
Leydig cells
See interstitial cells.
LH
See luteinizing hormone.
lichen
(lie′ kun) An organism resulting from the symbiotic association of a fungus and either a cyanobacterium or a unicellular alga.
life expectancy
The average time from birth to death of an individual organism.
life history
The sequence of key events, such as growth and development, reproduction, and death, that occur during the life of the average individual of a given species.
life-history tradeoffs
Negative relationships among growth, reproduction, and survival that occur because an individual cannot simultaneously allocate resources to such competing life functions as resource acquisition, maintenance, growth, defense, or reproduction.
ligand
(lig′ and) Any molecule that binds to a receptor site of another (usually larger) molecule.
ligand-gated channel
A channel protein that can open and close and that opens to allow diffusion of a solute as a result of binding by a neurotransmitter (or other specific signaling molecule) to a receptor site on the channel protein.
light reactions
The initial phase of photosynthesis, in which light energy is converted into chemical energy.
light-harvesting complex
In photosynthesis, a group of different molecules that cooperate to absorb light energy and transfer it to a reaction center. Also called antenna system.
lignin
A complex, hydrophobic polyphenolic polymer in plant cell walls that crosslinks other wall polymers, strengthening the walls, especially in wood.
limiting resource
A resource that constrains the fitness of individuals at a given time and place because it is in short supply relative to demand.
limnetic zone
[Gk. limn : pool] The “open water” zone of freshwater lakes.
lineage
A series of populations, species, or genes descended from a single ancestor over evolutionary time.
lineage species concept
The definition of a species as a branch on the tree of life, which has a history that starts at a speciation event and ends either at extinction or at another speciation event. (Contrast with biological species concept, morphological species concept.)
linear regression
A statistical method of fitting a straight line to describe the relationship between two variables in a scatter plot.
linkage
See genetic linkage.
lipid
(lip′ id) [Gk. lipos: fat] Nonpolar, hydrophobic molecules that include fats, oils, waxes, steroids, and the phospholipids that make up biological membranes.
lipid bilayer
See phospholipid bilayer.
littoral zone
The nearshore regions of lakes and oceans. In oceans this includes the intertidal zone.
loam
A type of soil consisting of a mixture of sand, silt, clay, and organic matter. One of the best soil types for agriculture.
locus
(low′ kus) (plural: loci, low′ sigh) In genetics, a specific location on a chromosome. May be considered synonymous with gene.
long-day plant (LDP)
A plant that requires long days (actually, short nights) in order to flower. (Compare to short-day plant.)
loop of Henle
(hen′ lee) Long, hairpin loop in a mammalian nephron. The loop runs from the cortex of the kidney down into the medulla of the kidney and back to the cortex; creates a concentration gradient in the interstitial fluids in the medulla. Some nephrons in birds kidneys also have loops of Henle.
lophophore
A U-shaped fold of the body wall with hollow, ciliated tentacles that encircles the mouth of animals in several different groups. Used for filtering prey from the surrounding water.
loss of function mutation
A mutation that results in the loss of a functional protein. (Contrast with gain of function mutation.)
lung
A breathing organ that is invaginated (folded inward) into the body and contains the environmental medium. The most common type of breathing organ in terrestrial animals. (Contrast with gill.)
lung surfactant
A thin layer of lipids and proteins that coats the inside of a lung; decreases surface tension at the surfaces of the lung epithelium, which reduces the amount of work necessary to inflate the lung.
luteinizing hormone (LH)
A hormone, produced by the anterior pituitary gland, that helps control secretion of hormones by the gonads (i.e., a gonadotropin).
lymphatic system
An elaborate system of vessels, separate from the blood vascular system, that picks up excess interstitial fluid (lymph) and returns it to the blood.
lymphocyte
One of the two major classes of white blood cells; includes T cells, B cells, and other cell types important in the immune system.
lysogeny
A form of viral replication in which the virus becomes incorporated into the host chromosome and remains inactive. Also called a lysogenic cycle. (Contrast with lytic cycle.)
lysosome
(lie′ so soam) [Gk. lysis: break away + soma: body] A membrane-enclosed organelle originating from the Golgi apparatus and containing hydrolytic enzymes. (Contrast with secondary lysosome.)
lysozyme
(lie′ so zyme) An enzyme in saliva, tears, and nasal secretions that hydrolyzes bacterial cell walls.
lytic cycle
A viral reproductive cycle in which the virus takes over a host cell’s synthetic machinery to replicate itself, then bursts (lyses) the host cell, releasing the new viruses. (Contrast with lysogeny.)
M phase
The portion of the cell cycle in which mitosis takes place.
macromolecule
A giant (molecular weight > 1,000) polymeric molecule. The macromolecules are the proteins, polysaccharides, and nucleic acids.
macronutrient
In plants, a mineral element required in concentrations of at least 1 milligram per gram of plant dry matter; in animals, a mineral element required in large amounts. (Contrast with micronutrient.)
maintenance methyltransferase
An enzyme that transfers methyl groups to DNA after DNA replication.
major histocompatibility complex (MHC)
A complex of linked genes, with multiple alleles, that control a number of cell surface antigens that identify self and can lead to graft rejection.
malnutrition
A condition caused by lack of adequate amounts of an essential nutrient.
Malpighian tubules
(mal pee′ gy un) Fine tubules that initiate urine formation in insects. They empty their product into the gut at the junction of the midgut and hindgut.
mantle
(1) In mollusks, a fold of tissue that covers the organs of the visceral mass and secretes the hard shell that is typical of many mollusks. (2) In geology, the Earth’s crust below the solid lithospheric plates.
mass extinction
A period of evolutionary history during which rates of extinction are much higher than during intervening times.
mass number
The sum of the number of protons and neutrons in an atom’s nucleus.
mast cells
Cells, typically found in connective tissue, that release histamine in response to tissue damage.
maternal effect genes
Genes coding for morphogens that determine the polarity of the egg and larva in fruit flies. Part of a developmental cascade that includes gap genes, pair rule genes, segment polarity genes, and Hox genes.
mating type
A particular strain of a species that is incapable of sexual reproduction with another member of the same strain but capable of sexual reproduction with members of other strains of the same species.
maximum likelihood
A statistical method of determining which of two or more hypotheses (such as phylogenetic trees) best fit the observed data, given an explicit model of how the data were generated.
mean
The sum of all values in a sample divided by the number of observations in the sample.
measures of center
Quantities that describe various aspects of the center of a group of observations.
measures of dispersion
Measures that quantify the dispersion of observations in a sample of observations.
mechanoreceptor
A sensory receptor cell that is sensitive to physical movement or physical distortion and generates action potentials in response.
median
The value at which there are equal numbers of larger and smaller observations in a sample.
medusa
(plural: medusae) In cnidarians, a free-swimming, sexual life cycle stage shaped like a bell or an umbrella.
megagametophyte
In heterosporous plants, the female gametophyte; produces eggs. (Contrast with microgametophyte.)
megaphyll
The generally large leaf of a fern, horsetail, or seed plant, with several to many veins. (Contrast with microphyll.)
megasporangia
The structures on a heterosporous plant that produce a few large megaspores (which develop into female gametophytes).
megaspore
[Gk. megas: large + spora: to sow] In plants, a haploid spore that produces a female gametophyte.
megastrobilus
In conifers, the female (seed-bearing) cone. (Contrast with microstrobilus.)
meiosis
(my oh′ sis) [Gk. meiosis: diminution] Division of a diploid nucleus to produce four haploid daughter cells. The process consists of two successive nuclear divisions with only one cycle of chromosome replication. In meiosis I, homologous chromosomes separate but retain their chromatids. The second division meiosis II, is similar to mitosis, in which chromatids separate.
membrane potential
The difference in electrical charge (voltage) between the inside and the outside of a cell membrane, typically expressed in millivolts. The name derives from the fact that potential difference is a synonym for voltage.
memory cells
In the immune system, long-lived lymphocytes produced after exposure to antigen. They persist in the body and are able to mount a rapid response to subsequent exposures to the antigen.
Mendel’s laws
See law of independent assortment, law of segregation.
menstrual cycle
The cycle in which oocytes mature and are ovulated periodically in females of primates, including humans; one phase of each cycle is characterized by menstruation, the shedding of the uterine lining in a blood-tinged discharge from the vagina.
menstruation
In humans and other primates, a process by which the lining of the uterus (the endometrium) breaks down (after a failure of pregnancy to occur), and the sloughed-off tissue, including blood, flows from the body.
meristem
[Gk. meristos: divided] Plant tissue made up of undifferentiated actively dividing cells.
meristem culture
A method for the asexual propagation of plants, in which pieces of shoot apical meristem are cultured to produce plantlets.
meristem identity genes
In angiosperms, a group of genes whose expression initiates flower formation, probably by switching meristem cells from a vegetative to a reproductive fate.
mesoderm
[Gk. mesos: middle + derma: skin] The middle of the three embryonic germ layers first delineated during gastrulation. Gives rise to the skeleton, circulatory system, muscles, excretory system, and most of the reproductive system.
mesoglea
(mez′ uh glee uh) [Gk. mesos: middle + gloia, glue] A thick, gelatinous noncellular layer that separates the two cellular tissue layers of ctenophores, cnidarians, and scyphozoans.
mesophyll
(mez′ uh fill) [Gk. mesos: middle + phyllon: leaf] Chloroplast-containing, photosynthetic cells in the interior of leaves.
messenger RNA (mRNA)
Transcript of a region of one of the strands of DNA; carries information (as a sequence of codons) for the synthesis of one or more proteins.
meta-
[Gk.: between, along with, beyond] Prefix denoting a change or a shift to a new form or level; for example, as used in metamorphosis.
metabolic pathway
A series of enzyme-catalyzed reactions so arranged that the product of one reaction is the substrate of the next.
metabolic rate (MR)
An animal’s rate of energy consumption; the rate at which it converts chemical-bond energy to heat and external work.
metabolic water
Water that is formed by oxidation of food during metabolism. For example, when glucose is oxidized, one of the products is H2O that did not previously exist; this H2O is metabolic water. Also called oxidation water.
metabolism
(meh tab′ a lizm) [Gk. metabole: change] The sum total of the chemical reactions that occur in an organism, or some subset of that total (as in respiratory metabolism).
metabolome
The quantitative description of all the small molecules in a cell or organism.
metabolomics
The study of the metabolome as it relates to the physiological state of a cell or organism.
metabotropic receptor
A receptor protein that is not an ion channel. Typically a G protein–linked receptor protein. (See G protein–linked receptors.)
metabotropic receptor cell
A sensory receptor cell in which the sensory receptor protein is not an ion channel but typically instead is a G protein–linked receptor.
metagenomics
The practice of analyzing DNA from environmental samples without isolating intact organisms.
metamorphosis
(met′ a mor′ fo sis) [Gk. meta: between + morphe: form, shape] A change occurring between one developmental stage and another, as for example from a tadpole to a frog. (See complete metamorphosis, incomplete metamorphosis.)
metaphase
(met′ a phase) The stage in nuclear division at which the centromeres of the highly supercoiled chromosomes are all lying on a plane (the metaphase plane or plate) perpendicular to a line connecting the division poles.
metapopulation
A “population of populations”—the group of spatially-separated subpopulations that occurs within a defined geographic area.
MHC
See major histocompatibility complex.
microbiomes
The diverse communities of bacteria that live on or within the body and are essential to bodily function.
microcirculation
The part of a closed circulatory system that consists of the smallest diameter blood vessels (i.e., arterioles, capillaries, and venules.)
microfilament
In eukaryotic cells, a fibrous structure made up of actin monomers. Microfilaments play roles in the cytoskeleton, in cell movement, and in muscle contraction.
microgametophyte
In heterosporous plants, the male gametophyte; produces sperm. (Contrast with megagametophyte.)
micronutrient
In plants, a mineral element required in concentrations of less than 100 micrograms per gram of plant dry matter; in animals, a mineral element required in concentrations of less than 100 micrograms per day. (Contrast with macronutrient.)
microphyll
A small leaf with a single vein, found in club mosses and their relatives. (Contrast with megaphyll.)
micropyle
(mike′ roh pile) [Gk. mikros: small + pylon: gate] Opening in the integument(s) of a seed plant ovule through which pollen grows to reach the female gametophyte within.
microRNA
A small, noncoding RNA molecule, typically about 21 bases long, that binds to mRNA to inhibit its translation.
microsporangia
The structures on a heterosporous plant that produce many small microspores (which develop into male gametophytes).
microspore
[Gk. mikros: small + spora: to sow] In plants, a haploid spore that produces a male gametophyte.
microstrobilus
In conifers, male pollen-bearing cone. (Contrast with megastrobilus.)
microtubules
Tubular structures found in centrioles, spindle apparatus, cilia, flagella, and cytoskeleton of eukaryotic cells. These tubules play roles in the motion and maintenance of shape of eukaryotic cells.
microvilli
(sing.: microvillus) Minute, rod-shaped projections of epithelial cells, such as the cells lining the small intestine, that increase their surface area.
midbrain
One of the three regions of the vertebrate brain. Part of the brainstem, it serves as a relay station for sensory signals sent to the cerebral hemispheres, in addition to other functions.
midgut
The first part of the intestine. In mammals, the midgut is also called the small intestine.
midgut fermenter
An animal that has a specialized midgut chamber housing mixed communities of fermenting microbes that assist with the breakdown of food materials
migrate
To periodically move from one location to another, then, after a period of time, return to the original location.
missense mutation
A change in a gene’s sequence that changes the amino acid at that site in the encoded protein. (Contrast with frame-shift mutation, nonsense mutation, silent mutation.)
mitochondrion
(my′ toe kon′ dree un) (plural: mitochondria) [Gk. mitos: thread + chondros: grain] An organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation.
mitosis
(my toe′ sis) [Gk. mitos: thread] Nuclear division in eukaryotes leading to the formation of two daughter nuclei, each with a chromosome complement identical to that of the original nucleus.
mitosomes
Reduced structures derived from mitochondria found in some organisms.
mode
The most frequent value in a sample of observations.
model organism
A species that is widely studied experimentally to understand general principles about biology.
moderately repetitive sequences
DNA sequences repeated 10–1,000 times in the eukaryotic genome. They include the genes that code for rRNAs and tRNAs, as well as the DNA in telomeres.
molds
Sac fungi composed of filamentous hyphae that do not form large ascomata.
molecular biology
Study of the formation, structure and functions of nucleic acids and proteins.
molecular clock
The approximately constant rate of divergence of macromolecules from one another over evolutionary time; used to date past events in evolutionary history.
molecular toolkit
See genetic toolkit.
molecule
A chemical substance made up of two or more atoms joined by covalent bonds or ionic attractions.
molting
The process of shedding part or all of an outer covering, as the shedding of feathers by birds or of the entire exoskeleton by arthropods.
monocots
Angiosperms with a single embryonic cotyledon; one of the two largest clades of angiosperms. (See also eudicots.)
monoecious
(mo nee′ shus) [Gk. mono: one + oikos: house] Pertaining to organisms in which both sexes are “housed” in a single individual that produces both eggs and sperm. (In some plants, these are found in different flowers within the same plant.) Examples include corn, peas, earthworms, hydras. (Contrast with dioecious.)
monohybrid cross
A mating in which the parents differ with respect to the alleles of only one locus of interest.
monomer
[Gk. mono: one + meros: unit] A small molecule, two or more of which can be combined to form oligomers (consisting of a few monomers) or polymers (consisting of many monomers).
monophyletic
(mon′ oh fih leht′ ik) [Gk. mono: one + phylon: tribe] Pertaining to a group that consists of an ancestor and all of its descendants. (Contrast with paraphyletic, polyphyletic.)
monosaccharide
A simple sugar. Oligosaccharides and polysaccharides are made up of monosaccharides.
morphogen
A diffusible substance whose concentration gradient determines a developmental pattern in animals and plants.
morphogenesis
(more′ fo jen′ e sis) [Gk. morphe: form + genesis: origin] The development of form; the overall consequence of determination, differentiation, and growth.
morphological species concept
The definition of a species as a group of individuals that look alike. (Contrast with biological species concept, lineage species concept.)
morphology
(more fol′ o jee) [Gk. morphe: form + logos: study, discourse] The scientific study of organic form, including both its development and function.
mortality
The average individual’s chance of dying during a specified set of life stages, ages, or period of time. Mortality = 1 − survivorship (Contrast with survivorship.)
mosaic development
Pattern of animal embryonic development in which each blastomere contributes a specific part of the adult body. (Contrast with regulative development.)
motor neuron
A neuron that carries signals from the central nervous system to a skeletal muscle, stimulating the muscle to contract. A type of efferent neuron.
motor proteins
Specialized proteins that use energy to change shape and move cells or structures within cells.
mucus
A slippery substance secreted by mucous membranes (e.g., mucosal epithelium). A barrier defense against pathogens in innate immunity in animals.
Muller’s ratchet
The accumulation—“ratcheting up”—of deleterious mutations in the nonrecombining genomes of asexual species.
multi-organ system
A system in which multiple organs work together.
multiple fruit
A fruit derived from carpels of several flowers. An example is a pineapple.
multiplicative growth
In ecology, population growth in which a multiple of the current population size is added to the population during each successive time interval. The multiple of growth may be a constant, or it may change. (Contrast with additive growth.)
multipotent
Having the ability to differentiate into a limited number of cell types. (Contrast with pluripotent, totipotent.)
muscle fiber
A single muscle cell. In the case of skeletal muscle, a multinucleate cell.
mutagen
(mute′ ah jen) [L. mutare: change + Gk. genesis: source] Any agent (e.g., a chemical, radiation) that increases the mutation rate.
mutation
A change in the genetic material not caused by recombination.
mutualism
A mutually beneficial (+/+) interaction between individuals of two different species that causes both to benefit, in terms of increased fitness. (Contrast with amensalism, commensalism.)
mutualistic
A pair of symbiotic organisms in which both partners benefit from the interaction.
mycelium
(my seel′ ee yum) [Gk. mykes: fungus] In the fungi, a mass of hyphae.
mycorrhiza
(my′ ko rye′ za) (plural: mycorrhizae) [Gk. mykes: fungus + rhiza: root] An association of the root of a plant with the mycelium of a fungus.
myelin
(my′ a lin) Concentric layers of cell membrane wrapped around some axons, forming a sheath around the axons; myelin provides an axon with electrical insulation and increases the rate of transmission of action potentials.
myocardium
The muscle tissue of a heart.
MyoD
The protein encoded by the myoblast determining gene. A transcription factor involved in the differentiation of myoblasts (muscle precursor cells).
myofibril
(my′ oh fy′ bril) [Gk. mys: muscle + L. fibrilla: small fiber] A long, longitudinally oriented component of a striated muscle cell that consists of a series of sarcomeres and extends the length of the cell. A myofibril is composed principally of actin and myosin filaments. In cross section, a muscle cell consists of multiple myofibrils.
myogenic heart
A heart in which the electrical impulse to contract during each beating cycle originates in muscle cells or modified muscle cells. (Contrast with neurogenic heart.)
myoglobin
(my′ oh globe′ in) [Gk. mys: muscle + L. globus: sphere] An oxygen-binding molecule found in red muscle cells. A type of hemoglobin, consisting of a single heme unit and a single globin chain per molecule
myosin
One of the two contractile proteins of muscle. (See also actin.)
myosin filaments
Bundles of linked myosin molecules. Also called thick filaments. (See also actin filaments.)
natural history
An observation about nature, obtained outside of a formal hypothesis-testing context, that provides the basis for further investigation.
natural killer cell
A type of lymphocyte that attacks virus-infected cells and some tumor cells as well as antibody-labeled target cells.
natural selection
The differential contribution of offspring to the next generation by various genetic types belonging to the same population. The mechanism of evolution proposed by Charles Darwin.
navigation
The act of moving on a particular course or toward a specific destination using sensory cues to determine direction and position.
necrosis
(nec roh′ sis) [Gk. nekros: death] Premature cell death caused by external agents such as toxins.
negative feedback
In a regulatory system, a type of control that acts to reduce differences that arise between the level of a controlled variable and its set-point level. It tends to stabilize the controlled variable at a level close to the set-point level. (Contrast with positive feedback.)
negative-sense RNA
RNA that is complementary to mRNA. Before it can be translated, it must be converted to positive-sense RNA by an RNA polymerase.
nematocyst
(ne mat′ o sist) [Gk. nema: thread + kystis: cell] An elaborate, threadlike structure produced by cells of jellyfishes and other cnidarians, used chiefly to paralyze and capture prey.
neoteny
(knee ot′ enny) [Gk. neo: new, recent; tenein: to extend] The retention of juvenile or larval traits by the fully developed adult organism.
nephron
(nef′ ron) [Gk. nephros: kidney] The functional unit of a vertebrate kidney, consisting of a structure for receiving a filtrate of blood plasma and a long tubule that modifies the filtrate by secretion and reabsorption.
Nernst equation
A mathematical equation that calculates the electrical potential difference required for electrochemical equilibrium across a membrane that is permeable to a single type of ion, when the ion differs in concentration on the two sides of the membrane.
nerve
A structure consisting of many neuronal axons bound together like wires in a telephone cable.
nerve impulse
See action potential.
nerve nets
Diffuse, loosely connected aggregations of nervous tissues in certain non-bilatarian animals such as cnidarians.
nervous tissue
Tissue specialized for processing and communicating information; one of the four major tissue types in multicellular animals.
net primary productivity (NPP)
The rate at which primary producers in a given area (or volume) convert energy they have captured from sunlight or other abiotic sources into energy stored in the molecules that make up their tissues, and which therefore is available for use by primary consumers. NPP is often estimated as the gain in biomass of primary producers in a unit of time.
neural crest cells
During vertebrate neurulation, cells that migrate outward from the neural plate; they are pluripotent cells that form the sensory nervous system, autonomic nervous system, many bones of the skull, pigment cells, and other tissues
neuroendocrine cells
See neurosecretory cells.
neurogenic heart
A heart in which the electrical impulse to contract during each beating cycle originate in neurons. (Contrast with myogenic heart.)
neuroglia
See glial cells.
neurohemal organ
A specialized organ in the circulatory system that is made up of axon terminals of neurosecretory cells in association with a well-developed bed of capillaries.
neuromuscular junction
Synapse (point of contact) where a motor neuron axon stimulates a muscle fiber (muscle cell).
neuron
(noor′ on) [Gk. neuron: nerve] A nervous system cell that can generate and conduct action potentials along an axon to a synapse with another cell.
neurosecretory cells
Neurons that synthesize and release hormones into the blood. Also called neuroendocrine cells.
neurotransmitter
At a synapse, a substance produced and released by the presynaptic cell that diffuses across the synapse and excites or inhibits the postsynaptic cell.
neurulation
Stage in vertebrate development during which the nervous system begins to form.
neutron
(new′ tron) One of the three fundamental particles of matter (along with protons and electrons), with mass slightly larger than that of a proton and no electrical charge.
niche
(nitch) [L. nidus: nest] In ecology, the abiotic and biotic conditions under which a given species can persist, and the functional role of the species in its community.
nitrifiers
Chemoautotrophic bacteria that oxidize ammonia to nitrate in soil and in seawater.
nitrogen fixation
Conversion of atmospheric nitrogen gas (N2) to a water-soluble, biologically-usable form (usually ammonium, NH4+), in nature by free-living or symbiotic microbes, but also by industrial processes. (Compare with denitrification.)
nitrogen fixers
Organisms that convert atmospheric nitrogen gas into a chemical form (ammonia) that is usable by the nitrogen fixers themselves as well as by other organisms.
nitrogenase
An enzyme complex found in nitrogen-fixing bacteria that mediates the stepwise reduction of atmospheric N2 to ammonia and which is strongly inhibited by oxygen.
nitrogenous wastes
The potentially toxic nitrogen-containing end products resulting from the break down of proteins and nucleic acids. They are usually eliminated from the body by excretion.
node
[L. nodus: knob, knot] (1) In plants, a (sometimes enlarged) point on a stem where a leaf is or was attached. (2) In phylogenetics, a split in a phylogenetic tree when one lineage diverges into two.
node of Ranvier
In a myelinated axon, a gap in the myelin sheath where the axon cell membrane is not covered with myelin.
noncompetitive inhibitor
A nonsubstrate that inhibits the activity of an enzyme by binding to a site other than its active site. (Contrast with competitive inhibitor.)
nondisjunction
Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.
nonneural endocrine cells
Cells that secrete hormones into the blood and that are not neurons or derived from neurons. (See neurosecretory cells.)
nonsense mutation
Change in a gene’s sequence that prematurely terminates translation by changing one of its codons to a stop codon.
nonshivering thermogenesis (NST)
In mammals and some birds, elevation of heat production for thermoregulation by means other than shivering.
nonsynonymous substitution
A change in a gene from one nucleotide to another that changes the amino acid specified by the corresponding codon (i.e., AGC → AGA, or serine → arginine). (Contrast with synonymous substitution.)
nonvascular land plants
Land plants that lack specialized vascular tissues for the conduction of water or nutrients through the plant body. There are three living species of land plants: the liverworts, hornworts, and mosses.
notochord
(no′ tow kord) [Gk. notos: back + chorde: string] A flexible rod of gelatinous material serving as a support in the embryos of all chordates and in the adults of tunicates and lancelets.
NPP
See net primary productivity.
NST
See nonshivering thermogenesis.
nucleic acid
(new klay′ ik) A polymer made up of nucleotides, specialized for the storage, transmission, and expression of genetic information. DNA and RNA are nucleic acids.
nucleic acid hybridization
A technique in which a single-stranded nucleic acid probe is made that is complementary to, and binds to, a target sequence, either DNA or RNA. The resulting double-stranded molecule is a hybrid.
nucleoid
(new′ klee oid) The region that harbors the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not bounded by a membrane.
nucleolus
(new klee′ oh lus) A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.
nucleotide
The basic chemical unit in nucleic acids, consisting of a pentose sugar, a phosphate group, and a nitrogen-containing base.
nucleus
(new′ klee us) [L. nux: kernel or nut] (1) In cells, the centrally located compartment of eukaryotic cells that is bounded by a double membrane and contains the chromosomes. (2) In the brain, an identifiable group of neurons that share common characteristics or functions.
null hypothesis
In statistics, the premise that any differences observed in an experiment are simply the result of random differences that arise from drawing two finite samples from the same population.
obligate aerobe
Organisms that require oxygen for metabolism.
obligate anaerobe
An anaerobic prokaryote that cannot survive exposure to O2.
obligate parasites
Organisms that can only survive and grow in or on other living organisms, to the detriment of the host.
octet rule
Description of processes that atoms undergo whereby they obtain, give up or share electrons such that their outer (valence) shell contains eight electrons.
Okazaki fragments
Newly formed DNA making up the lagging strand in DNA replication. DNA ligase links Okazaki fragments together to give a continuous strand.
oligodendrocyte
A type of glial cell that forms a myelin sheath on axons in the central nervous system.
oligopeptide
Peptide made up of less than 20 amino acids.
oligosaccharide
A polymer containing a small number of monosaccharides.
ommatidium
(plural: ommatidia) [Gk. omma: eye] A single visual unit in the compound eye of an arthropod.
omnivore
[L. omnis: everything + vorare: to devour] An organism that obtains the energy and materials it needs to survive, grow, and reproduce from a variety of trophic levels. (Contrast with carnivore, detritivore, herbivore.)
oncogene
[Gk. onkos: mass, tumor + genes: born] A gene that codes for a protein product that stimulates cell proliferation. Mutations in oncogenes that result in excessive cell proliferation can give rise to cancer. (Contrast with tumor suppressor.)
one gene–one polypeptide
The idea, since shown to be an oversimplification, that each gene in the genome encodes only a single polypeptide—that there is a one-to-one correspondence between genes and polypeptides.
oocyte
The developing ovum.
open circulatory system
Circulatory system in which the blood leaves blood vessels and travels through spaces (sinuses and lacunae) bounded by ordinary tissue cells as it flows through the body (Contrast with closed circulatory system.)
open system
A system that is not isolated from, and interacts with, its surroundings. (Compare with closed system.)
open reading frames
Sequences of DNA within genes that begin with an initiation codon and end with a stop codon.
operator
The region of an operon that acts as the binding site for the repressor.
operon
A genetic unit of transcription, typically consisting of several structural genes that are transcribed together; the operon contains at least two control regions: the promoter and the operator.
order
A group of related families.
ordinal variables
Categorical variables with a natural ordering, such as the grades A, B, C, D, and F.
organ
[Gk. organon: tool] A body part, such as the heart, liver, brain, root, or leaf, that is composed of two or more tissues integrated to perform a distinct function.
organ identity genes
In angiosperms, genes that specify the different organs of the flower.
organelle
(or gan el′) Any of the membrane-enclosed structures within a eukaryotic cell. Examples include the nucleus, endoplasmic reticulum, and mitochondria.
organic fertilizers
Substances added to soil to improve the soil’s fertility; derived from partially decomposed plant material (compost) or animal waste (manure).
organizer
Region of the early amphibian embryo (the dorsal lip of the blastopore) that develops into the notochord and thus establishes the basic body plan. Also known as the primary embryonic inducer.
organogenesis
The formation of organs and organ systems during development.
orientation
A behavioral process in which an organism positions itself or moves in relation to environmental cues such as the sun.
origin of replication (ori)
DNA sequence at which helicase unwinds the DNA double helix and DNA polymerase binds to initiate DNA replication.
orthologs
Genes that are related by orthology.
osmoconformer
See osmotic conformer.
osmolar (Osm)
A unit of measure of osmotic pressure. A 1-osmolar solution is defined to be a solution that behaves osmotically as if it has one Avogadro’s number (6 × 1023) of independent dissolved entities per liter.
osmolarity
The osmotic pressure of a solution expressed in osmolar units.
osmoregulator
See osmotic regulator.
osmosis
(oz mo′ sis) [Gk. osmos: to push] Movement of water across a differentially permeable membrane, from one region to another region where the water potential is more negative.
osmotic conformer
An aquatic animal in which the osmotic pressure of the blood and other extracellular fluids matches the osmotic pressure of the external environment. Also called osmoconformer. (Contrast with osmotic regulator.)
osmotic regulator
An aquatic animal that maintains a relatively constant osmotic pressure in its blood and other extracellular fluids regardless of changes in the osmotic pressure of its external environment. Also called osmoregulator. (Contrast with osmotic conformer.)
osmotic pressure
Pressure exerted by the flow of water through a semipermeable membrane separating two solutions with different concentrations of solute.
outgroup
In phylogenetics, a group of organisms used as a point of reference for comparison with the groups of primary interest (the ingroup).
ovarian follicle
During gamete development in the ovary of a female animal, an oocyte (developing ovum) together with its support cells (theca and granulosa cells).
ovary
(oh′ var ee) [L. ovum: egg] A female organ, in plants or animals, that produces ova (eggs).
overtopping
Plant growth pattern in which one branch differentiates from and grows beyond the others.
oviduct
In mammals, the tube serving to transport eggs from an ovary to the uterus. Also called a fallopian tube.
ovulation
The process of releasing an ovum (egg) from the ovary.
ovule
(oh′ vule) In plants, a structure comprising the megasporangium and the integument, which develops into a seed after fertilization.
ovum
(plural: ova) See egg.
oxidation
(ox i day′ shun) Relative loss of electrons in a chemical reaction; either outright removal to form an ion, or the sharing of electrons with substances having a greater affinity for them, such as oxygen. Most oxidations, including biological ones, are associated with the liberation of energy. (Contrast with reduction.)
oxidation water
See metabolic water.
oxidative phosphorylation
ATP formation in the mitochondrion, associated with flow of electrons through the respiratory chain.
oxidized
Increase in positive charge of an element by removing electron(s); formation of an oxide by adding oxygen.
oxygenase
An enzyme that catalyzes the addition of oxygen to a substrate from O2.
oxygenation
(1) Referring to water, the dissolution of O2 in that water. (2) The combination of respiratory pigments with O2; oxygenation of this sort is reversible and not equivalent to oxidation.
oxygen equilibrium curve
A graph of the amount of O2 per unit of blood volume as a function of the O2 partial pressure of the blood (a measure of the concentration of O2 dissolved in the blood plasma.)
oxytocin
In mammals, a hormone released by the posterior pituitary gland; its major functions are to stimulate contraction of the uterus and the flow of milk from mammary glands.
P-value
The calculated probability of observing a given result by chance sampling, given the null hypothesis is true.
pair rule genes
In Drosophila (fruit fly) development, segmentation genes that divide the early embryo into units of two segments each. Part of a developmental cascade that includes maternal effect genes, gap genes, segment polarity genes, and Hox genes.
paleomagnetic dating
A method for determining the age of rocks based on properties relating to changes in the patterns of Earth’s magnetism over time.
Pangaea
(pan jee′ uh) [Gk. pan: all, every] The single land mass formed when all the continents came together in the Permian period.
para-
[Gk. para: akin to, beside] Prefix indicating association in being along side or accessory to.
parallel phenotypic evolution
The repeated evolution of similar traits, especially among closely related species; facilitated by conserved developmental genes.
paraphyletic
(par′ a fih leht′ ik) [Gk. para: beside + phylon: tribe] Pertaining to a group that consists of an ancestor and some, but not all, of its descendants. (Contrast with monophyletic, polyphyletic.)
parasite
An organism that consumes parts of an organism much larger than itself (known as its host). Parasites sometimes, but not always, kill their host.
parasitism
A +/− interaction between individuals of two different species that causes one individual (the parasite) to benefit (it gains fitness) from consuming part or all of the other (the host), which suffers reduced fitness as a consequence of being consumed.
parasympathetic division
A division of the vertebrate autonomic nervous system that is connected to the central nervous system via cranial and sacral nerves; the parasympathetic and sympathetic divisions tend to exert opposing controls on autonomic effectors. (Contrast with sympathetic division.)
parenchyma
(pair eng′ kyma) A plant tissue composed of relatively unspecialized cells without secondary walls.
parent rock
The soil horizon consisting of the rock that is breaking down to form the soil. Also called bedrock, or the C horizon.
parental (P) generation
The individuals that mate in a genetic cross. Their offspring are the first filial (F1) generation.
parsimony principle
Principle that states that the preferred explanation of observed data is the simplest explanation.
parthenocarpy
Formation of fruit from a flower without fertilization.
passive transport
Diffusion across a membrane; may or may not require a channel or carrier protein. (Contrast with active transport.)
pathogen
(path′ o jen) [Gk. pathos: suffering + genesis: source] An organism that causes disease.
pathogen associated molecular patterns (PAMPs)
The molecules recognized by pattern recognition receptors.
pathogenesis-related (PR) proteins
A component of the hypersensitive response of plants to pathogens. Some PR proteins function as direct defenses against pathogens, others initiate other defensive responses.
pattern formation
In animal embryonic development, the organization of differentiated tissues into specific structures such as wings.
pattern recognition receptors (PRRs)
Proteins made by cells that recognize molecular patterns on pathogens; part of innate immunity.
PCR
See polymerase chain reaction.
pedigree
The pattern of transmission of a genetic trait within a family.
pelagic zone
[Gk. pelagos: open sea] The “open water” zone of oceans that lies beyond the continental shelves.
penetrance
The proportion of individuals with a particular genotype that show the expected phenotype.
pentaradial symmetry
Symmetry in five or multiples of five; a feature of adult echinoderms.
peptidyl transferase
A catalytic function of the large ribosomal subunit that consists of two reactions: breaking the bond between an amino acid and its tRNA in the P site, and forming a peptide bond between that amino acid and the amino acid attached to the tRNA in the A site.
peptides
Molecule containing two or more amino acids.
peptide hormones
Hormone molecules made up of strings of amino acids. They are water soluble. (Compare to steroid hormones, amine hormones.)
peptide bond
The bond between amino acids in a protein; formed between a carboxyl group and amino group (—CO—NH—) with the loss of water molecules.
peptidoglycan
The cell wall material of many bacteria, consisting of a single enormous molecule that surrounds the entire cell.
per capita birth rate (b)
The number of offspring that the average individual produces in some specified interval of time.
per capita death rate (d)
The average individual’s chance of dying in some specified interval of time.
per capita growth rate (r)
The average individual’s contribution to total population growth rate in some specified interval of time, which equals per capita birth rate (b) minus per-capita death rate (d).
perennial
(per ren′ ee al) [L. per: throughout + annus: year] A plant that survives from year to year. (Contrast with annual, biennial.)
perfect flower
A flower with both stamens and carpels; a hermaphroditic flower. (Contrast with imperfect flower.)
perfusion
The flow of blood through a tissue or organ.
pericycle
[Gk. peri: around + kyklos: ring or circle] In plant roots, tissue just within the endodermis, but outside of the root vascular tissue. Meristematic activity of pericycle cells produces lateral root primordia.
periderm
The outer tissue of the secondary plant body, consisting primarily of cork.
peripheral membrane proteins
Proteins associated with but not embedded within the plasma membrane. (Contrast with integral membrane proteins.)
peripheral nervous system (PNS)
The portion of the nervous system other than the central nervous system (CNS). It consists of neurons and parts of neurons located outside the CNS. (Contrast with central nervous system.)
peristalsis
(pair′ i stall′ sis) Wavelike muscular contractions proceeding along a tubular organ, propelling the contents along the tube.
peroxisome
An organelle that houses reactions in which toxic peroxides are formed and then converted to water.
petal
[Gk. petalon: spread out] In an angiosperm flower, a sterile modified leaf, nonphotosynthetic, frequently brightly colored, and often serving to attract pollinating insects.
petiole
(pet′ ee ole) [L. petiolus: small foot] The stalk of a leaf.
phage
(fayj) See bacteriophage.
phage therapy
A strategy for treating bacterial infections using bacteriophage.
phagocyte
[Gk. phagein: to eat + kystos: sac] One of two major classes of white blood cells; one of the nonspecific defenses of animals; ingests invading microorganisms by phagocytosis.
phagocytosis
Endocytosis by a cell of another cell or large particle.
phagosome
Membrane-enclosed vesicle inside a cell that results from infolding of the cell membrane and enclosing a particle to be taken into the cell.
pharmacogenomics
The study of how an individual’s genetic makeup affects his or her response to drugs or other agents, with the goal of predicting the effectiveness of different treatment options.
pharming
The use of genetically modified animals to produce medically useful products in their milk.
phenotype
(fee′ no type) [Gk. phanein: to show] The observable properties of an individual resulting from both genetic and environmental factors. (Contrast with genotype.)
phenotypic plasticity
The ability of an individual animal to express two or more phenotypes during its life. (See also acclimation.)
pheromone
(feer′ o mone) [Gk. pheros: carry + hormon: excite, arouse] A chemical substance that is released into the external environment by an individual and that brings about specific behavioral responses in other individuals of the same species.
phloem
(flo′ um) [Gk. phloos: bark] In vascular plants, the vascular tissue that transports sugars and other solutes from sources to sinks.
phospholipid
A lipid containing a phosphate group; an important constituent of cellular membranes. (See lipid.)
phospholipid bilayer
The basic structural unit of biological membranes; a sheet of phospholipids two molecules thick in which the phospholipids are lined up with their hydrophobic “tails” packed tightly together and their hydrophilic, phosphate-containing “heads” facing outward. Also called lipid bilayer.
photic zone
The surface zone of lakes or oceans that is penetrated by sunlight.
photoautotroph
An organism that obtains energy from light and carbon from carbon dioxide. (Contrast with chemoautotroph, chemoheterotroph, photoheterotroph.)
photoheterotroph
An organism that obtains energy from light but must obtain its carbon from organic compounds. (Contrast with chemoautotroph, chemoheterotroph, photoautotroph.)
photomorphogenesis
In plants, a process by which physiological and developmental events are controlled by light.
photon
(foe′ ton) [Gk. photos: light] A quantum of visible radiation; a “packet” of light energy.
photoperiod
Day length; the number of hours of daylight in a 24-hour day.
photoreceptor
(1) In plants, a pigment that triggers a physiological response when it absorbs a photon. (2) In animals, a sensory receptor cell that senses and responds to light energy.
photosynthesis
(foe tow sin′ the sis) [literally, “synthesis from light”] Metabolic processes carried out by green plants and cyanobacteria, by which visible light is trapped and the energy used to convert CO2 into organic compounds.
photosynthetic lamellae
Elaborate internal membrane systems found in cyanobacteria that are used for photosynthesis.
photosystem
[Gk. phos: light + systema: assembly] A light-harvesting complex in the chloroplast thylakoid composed of pigments and proteins.
photosystem I
In photosynthesis, the complex that absorbs light at 700 nm, passing electrons to ferrodoxin and thence to NADPH.
photosystem II
In photosynthesis, the complex that absorbs light at 680 nm, passing electrons to the electron transport chain in the chloroplast.
phototropins
A class of blue light receptors that mediate phototropism and other plant responses.
phototropism
[Gk. photos: light + trope: turning] A directed plant growth response to light.
phycoerythrin
A red accessory photosynthetic pigment found in red algae.
phylogenetic tree
A graphic representation of lines of descent among organisms or their genes.
phylogeny
(fy loj′ e nee) [Gk. phylon: tribe, race + genesis: source] The evolutionary history of a particular group of organisms or their genes.
phylum
(plural: phyla) A group of related classes in biological classification.
physical geography
The spatial distribution of Earth’s climates and physical features.
phytoalexins
Substances toxic to pathogens, produced by plants in response to fungal or bacterial infection.
phytochrome
(fy′ tow krome) [Gk. phyton: plant + chroma: color] A plant pigment regulating a large number of developmental and other phenomena in plants. It has two isomers: Pr, which absorbs red light, and Pfr, which absorbs far red light. Pfr is the active form.
phytomers
In plants, the repeating modules that compose a shoot, each consisting of one or more leaves, attached to the stem at a node; an internode; and one or more axillary buds.
phytoplankton
Photosynthetic plankton.
phytoremediation
A form of bioremediation that uses plants to clean up environmental pollution.
pie chart
A circular figure that displays proportions of different classes of data in an observed sample.
pigment
A substance that absorbs visible light.
pinocytosis
Endocytosis by a cell of liquid containing dissolved substances.
pistil
[L. pistillum: pestle] The structure of an angiosperm flower within which the ovules are borne. May consist of a single carpel, or of several carpels fused into a single structure. Usually differentiated into ovary, style, and stigma.
pith
In plants, relatively unspecialized tissue found within a cylinder of vascular tissue.
pituitary gland
A small endocrine gland attached to the base of the brain in vertebrates. Many of its hormones control the activities of other endocrine glands. Also known as the hypophysis. (See also posterior pituitary gland, anterior pituitary gland.)
placenta
(pla sen′ ta) The organ in female mammals that provides for the nourishment of the fetus and elimination of the fetal waste products.
plankton
Aquatic organisms that drift with the current. Photosynthetic members of the plankton are referred to as phytoplankton.
planula
(plan′ yew la) [L. planum: flat] A free-swimming, ciliated larval form typical of the cnidarians.
plasma
(plaz′ muh) The liquid portion of blood, in which blood cells are suspended. The part of the blood that is left after the blood cells are removed.
plasma cell
An antibody-secreting cell that develops from a B cell; the effector cell of the humoral immune system.
plasmid
A DNA molecule distinct from the chromosome(s); that is, an extrachromosomal element; found in many bacteria. May replicate independently of the chromosome.
plasmodesma
(plural: plasmodesmata) [Gk. plassein: to mold + desmos: band] A cytoplasmic strand connecting two adjacent plant cells.
plasmogamy
The fusion of the cytoplasm of two cells. (Contrast with karyogamy.)
plate tectonics
[Gk. tekton: builder] The scientific study of the structure and movements of Earth’s lithospheric plates, which are the cause of continental drift.
platelet
In blood, a membrane-bounded body without a nucleus, arising as a fragment of a cell in the bone marrow of mammals. Important to blood-clotting action.
pluripotent
[L. pluri: many + potens: powerful] Having the ability to form all of the cells in the body. (Contrast with multipotent, totipotent.)
pneumatophores
Root, sometimes submerged, that function in gas exchange with the environment.
poikilotherm
An animal in which the body temperature matches the temperature of the external environment and varies as the external temperature varies. Also called an ectotherm.
point mutation
A mutation that results from the gain, loss, or substitution of a single nucleotide.
polar covalent bond
A covalent bond in which the electrons are drawn to one nucleus more than the other, resulting in an unequal distribution of charge.
polar nuclei
In angiosperms, the two nuclei in the central cell of the megagametophyte; following fertilization they give rise to the endosperm.
polarity
(1) In chemistry, the property of unequal electron sharing in a covalent bond that defines a polar molecule. (2) In development, the difference between one end of an organism or structure and the other.
pollen
[L. pollin: fine flour] In seed plants, microscopic grains that contain the male gametophyte (microgametophyte) and gamete (microspore).
pollen tube
A structure that develops from a pollen grain through which sperm are released into the megagametophyte.
pollination
The process of transferring pollen from an anther to the stigma of a pistil in an angiosperm or from a strobilus to an ovule in a gymnosperm.
poly A tail
A long sequence of adenine nucleotides (50–250) added after transcription to the 3′ end of most eukaryotic mRNAs.
poly-
[Gk. poly: many] A prefix denoting multiple entities.
polymer
[Gk. poly: many + meros: unit] A large molecule made up of similar or identical subunits called monomers. (Contrast with monomer.)
polymerase chain reaction (PCR)
An enzymatic technique for the rapid production of millions of copies of a particular stretch of DNA where only a small amount of the parent molecule is available.
polymorphic
(pol′ lee mor′ fik) [Gk. poly: many + morphe: form, shape] Coexistence in a population of two or more distinct traits.
polyp
(pah′ lip) [Gk. poly: many + pous: foot] In cnidarians, a sessile, asexual life cycle stage.
polypeptide
A large molecule made up of many amino acids joined by peptide linkages. Large polypeptides are called proteins.
polyphyletic
(pol′ lee fih leht′ ik) [Gk. poly: many + phylon: tribe] Pertaining to a group that consists of multiple distantly related organisms, and does not include the common ancestor of the group. (Contrast with monophyletic, paraphyletic.)
polyploidy
(pol′ lee ploid ee) The possession of more than two entire sets of chromosomes.
polyribosome (polysome)
A complex consisting of a threadlike molecule of messenger RNA and several (or many) ribosomes. The ribosomes move along the mRNA, synthesizing polypeptide chains as they proceed.
polysaccharide
A macromolecule composed of many monosaccharides (simple sugars). Common examples are cellulose and starch.
pool
In ecology, the total amount of any form of matter—of an element or a molecular compound—in a given compartment of an ecosystem.
population
A group of individuals of the same species that live, interact, and reproduce together in a particular geographic area.
population bottleneck
A period during which only a few individuals of a normally large population survive.
population density
The number of individuals in a population per unit of area (for organisms that live in two-dimensional habitats such as the land surface) or volume (for organisms that live in three-dimensional habitats such as air or water).
population size
The total number of individuals in an ecological population.
positional information
In development, the basis of the spatial sense that induces cells to differentiate as appropriate for their location within the developing organism; often comes in the form of a morphogen gradient.
positive feedback
In a regulatory system, a type of control that acts to increase differences that arise between the level of a controlled variable and its set-point level. The period of amplifying deviation is followed by a period in which stabilization is restored in most biological systems. (Contrast with negative feedback.)
positive relationship
A relationship in which two variables tend to vary among observations in the same direction.
positive selection
Natural selection that acts to establish a trait that enhances survival in a population. (Contrast with purifying selection.)
post-
[L. postere: behind, following after] Prefix denoting something that comes after.
posterior
Toward or pertaining to the rear. (Contrast with anterior.)
posterior pituitary gland
A portion of the pituitary gland that is derived from neural tissue. In mammals, it is involved in the release of antidiuretic hormone and oxytocin, both of which are synthesized by hypothalamic neurosecretory cells. (See also pituitary gland, anterior pituitary gland.)
postsynaptic cell
A neuron or effector cell that receives a signal (chemical or electrical) from a presynaptic cell at a synapse.
postzygotic isolating mechanisms
Barriers to the reproductive process that occur after the union of the nuclei of two gametes. (Contrast with prezygotic isolating mechanisms.)
potential difference
See voltage.
potential energy
Energy not doing work, such as the energy stored in chemical bonds. (Contrast with kinetic energy.)
power
In reference to statistical tests: refers to the probability of correctly rejecting a null hypothesis when it is false.
PR proteins
See pathogenesis-related proteins.
PRRs
See pattern recognition receptors.
Precambrian
The first and longest period of geological time, during which life originated.
precursor RNA (pre-mRNA)
Initial gene transcript before it is modified to produce functional mRNA. Also known as the primary transcript.
predator
An organism that kills and eats other organisms.
predation
A +/− interaction between individuals of two different species in which one individual (the predator) gains fitness by consuming the other individual (the prey), which of course loses fitness by being consumed.
pressure flow model
An effective model for phloem transport in angiosperms. It holds that sieve element transport is driven by an osmotically generated pressure gradient between source and sink.
pressure potential (Ψp)
The hydrostatic pressure of an enclosed solution in excess of the surrounding atmospheric pressure. (Contrast with solute potential, water potential.)
presynaptic axon terminal
One of the end processes of an axon, where the axon terminates at a synapse.
presynaptic cell
A neuron or other cell that transmits a signal to a postsynaptic cell at a synapse. (Contrast with postsynaptic cell.)
prey
[L. praeda: booty] An organism consumed by a predator as an energy source.
prezygotic isolating mechanisms
Barriers to the reproductive process that occur before the union of the nuclei of two gametes (Contrast with postzygotic isolating mechanisms.)
primary active transport
Active transport in which ATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient. (Contrast with secondary active transport.)
primary consumer
Organisms that consume primary producers.
primary embryonic inducer
See organizer.
primary endosymbiosis
The engulfment of a cyanobacterium by a larger eukaryotic cell that gave rise to the first photosynthetic eukaryotes with chloroplasts.
primary growth
In plants, growth that is characterized by the lengthening of roots and shoots and by the proliferation of new roots and shoots through branching. (Contrast with secondary growth.)
primary immune response
The first response of the immune system to an antigen, involving recognition by lymphocytes and the production of effector cells and memory cells. (Contrast with secondary immune response.)
primary lysosome
See lysosome.
primary meristem
Meristem that produces the tissues of the primary plant body.
primary producer
A photosynthetic or chemosynthetic organism that synthesizes complex organic molecules from simple inorganic ones.
primary reproductive organs
The gonads: the ovaries in females and the testes (or testicles) in males.
primary structure
The specific sequence of amino acids in a protein. (Contrast with secondary, tertiary, quaternary structure.)
primary transcript
See precursor RNA.
primase
An enzyme that catalyzes the synthesis of a primer for DNA replication.
primer
Strand of nucleic acid, usually RNA, that is the necessary starting material for the synthesis of a new DNA strand, which is synthesized from the 3′ end of the primer.
principle of allocation
The principle that a unit of resource (such as food) cannot be used simultaneously for multiple functions (such as growth and reproduction), but instead must be allocated to one or another function.
pro-
[L.: first, before, favoring] A prefix often used in biology to denote a developmental stage that comes first or an evolutionary form that appeared earlier than another. For example, prokaryote, prophase.
probability
A numerical quantity that expresses the likelihood of an event occurring on a scale from 0 (no chance of the event) to 1 (certainty of the event).
probe
A segment of single-stranded nucleic acid used to identify DNA molecules containing the complementary sequence.
proboscis
A hollow, muscular feeding organ.
procambium
Primary meristem that produces the vascular tissue.
processes
The ways in which the components of a biological system interact (e.g., protein synthesis, nutrient metabolism, grazing).
processive
Pertaining to an enzyme that catalyzes many reactions each time it binds to a substrate, as DNA polymerase does during DNA replication.
products
The molecules that result from the completion of a chemical reaction.
progesterone
[L. pro: favoring + gestare: to bear] A female sex hormone that maintains pregnancy.
prokaryotes
Unicellular organisms that do not have nuclei. (Contrast with eukaryotes.)
prometaphase
The phase of nuclear division that begins with the disintegration of the nuclear envelope.
promoter
A DNA sequence to which RNA polymerase binds to initiate transcription.
prop roots
Adventitious roots in some monocots that function as supports for the shoot.
prophase
(pro′ phase) The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies.
prostaglandin
Any one of a group of specialized lipids with hormone-like functions. It is not clear that they act at any considerable distance from the site of their production.
proteasome
In the eukaryotic cytoplasm, a huge protein structure that binds to and digests cellular proteins that have been tagged by ubiquitin.
protein
(pro′ teen) [Gk. protos: first] Long-chain polymer of amino acids with twenty different common side chains. Occurs with its polymer chain extended in fibrous proteins, or coiled into a compact macromolecule in enzymes and other globular proteins.
protein kinase
(kye′ nase) An enzyme that catalyzes the addition of a phosphate group from ATP to a target protein.
proteoglycan
A glycoprotein containing a protein core with attached long, linear carbohydrate chains.
proteome
The set of proteins that can be made by an organism. Because of alternative splicing of pre-mRNA, the number of proteins that can be made is usually much larger than the number of protein-coding genes present in the organism’s genome.
proteomics
The study of the proteome—the complete complement of proteins produced by an organism.
protocells
Ordered structures enclosed within a membrane that can be formed in the laboratory and are hypothesized to resemble cells when life originated.
protoderm
Primary meristem that gives rise to the plant epidermis.
proton
(pro′ ton) [Gk. protos: first, before] (1) A subatomic particle with a single positive charge. The number of protons in the nucleus of an atom determine its element. (2) A hydrogen ion, H+.
proton pump
An active transport system that uses ATP energy to move hydrogen ions across a membrane, generating an electric potential.
provirus
Double-stranded DNA made by a virus that is integrated into the host’s chromosome and contains promoters that are recognized by the host cell’s transcription apparatus.
proximate explanation
The immediate genetic, physiological, neurological, and developmental explanations for the advantages of an adaptation.
pseudocoelomate
(soo′ do see′ low mate) [Gk. pseudes: false + koiloma: cavity] Having a body cavity, called a pseudocoel, consisting of a fluid-filled space in which many of the internal organs are suspended, but which is enclosed by mesoderm only on its outside.
pseudogene
[Gk. pseudes: false] A DNA segment that is homologous to a functional gene but is not expressed because of changes to its sequence or changes to its location in the genome.
pseudoplasmodium
An aggregate of individuals in cellular slime molds that act in a coordinated fashion to form a fruiting structure.
Punnett square
Method of predicting the results of a genetic cross by arranging the gametes of each parent at the edges of a square.
pupil
The opening in the vertebrate eye through which light passes from the outside to the retina.
purifying selection
The elimination by natural selection of detrimental characters from a population. (Contrast with positive selection.)
purine
(pure′ een) One of the two types of nitrogenous bases in nucleic acids. Each of the purines—adenine and guanine—pairs with a specific pyrimidine.
pyrimidine
(per im′ a deen) One of the two types of nitrogenous bases in nucleic acids. Each of the pyrimidines—cytosine, thymine, and uracil—pairs with a specific purine.
pyruvate oxidation
Conversion of pyruvate to acetyl CoA and CO2 that occurs in the mitochondrial matrix in the presence of O2.
qualitative traits
Traits that differ from one another by discrete qualities.
quantitative trait
Phenotype determined by multiple genes.
quantitative variables
Variables that can take on values along a numerical scale.
quaternary structure
The specific three-dimensional arrangement of protein subunits. (Contrast with primary, secondary, tertiary structure.)
quorum sensing
The use of chemical communication signals to trigger density-linked activities such as biofilm formation in prokaryotes.
R group
The distinguishing group of atoms of a particular amino acid. Also known as a side chain.
radial axis
Radial definition of function in an adult organism or future function in an embryo, as in the developing flower with organs in whorls.
radial cleavage
Embryonic development in some deuterostomes in which the planes of cell division are parallel and perpendicular to the animal-vegetal axis of the embryo. Considered the basic deuterostome cleavage pattern. (Compare to spiral cleavage.)
radial symmetry
The condition in which any two halves of a body are mirror images of each other, providing the cut passes through the center; a cylinder cut lengthwise down its center displays this form of symmetry.
radicle
An embryonic root.
radiometric dating
A method for determining the age of objects such as fossils and rocks based on the decay rates of radioactive isotopes.
rain shadow
The relatively dry area on the down-wind side of a mountain range.
range
The largest minus the smallest observed value for a variable in a sample.
rarity advantage
Any situation in which individuals of a species lose less fitness from interspecific competition when they are rare than when they are common.
reactant
A chemical substance that enters into a chemical reaction with another substance.
reaction center
A group of electron transfer proteins that receive energy from light-absorbing pigments and convert it to chemical energy by redox reactions.
receptor
See receptor protein, sensory receptor cell.
receptor endocytosis
Endocytosis initiated by macromolecular binding to a specific membrane receptor. Also called receptor-mediated endocytosis.
receptor protein
A protein that can bind to a specific molecule, or detect a specific stimulus, within the cell or in the cell’s external environment.
recessive
In genetics, an allele that does not determine phenotype in the presence of a dominant allele. (Contrast with dominance.)
recombinant
Pertaining to an individual, meiotic product, or chromosome in which genetic materials originally present in two individuals end up in the same haploid complement of genes.
recombinant DNA
A DNA molecule made in the laboratory that is derived from two or more genetic sources.
recombination frequency
The proportion of offspring of a genetic cross that have phenotypes different from the parental phenotypes due to crossing over between linked genes during gamete formation.
red blood cell (RBC)
A cell in the blood of an animal that contains hemoglobin and transports O2. Also called an erythrocyte.
redox reaction
A chemical reaction in which one reactant becomes oxidized and the other becomes reduced. Short for reduction–oxidation reaction.
reduction
Gain of electrons by a chemical reactant; any reduction is accompanied by an oxidation. (Contrast with oxidation.)
regulation
The maintenance of internal conditions at an approximately constant level while external conditions vary. (Contrast with conformity.)
regulative development
A pattern of animal embryonic development in which the fates of the first blastomeres are not absolutely fixed. (Contrast with mosaic development.)
regulators
Animals that maintain relatively constant internal conditions in the presence of changing external conditions (i.e., the animals exhibit regulation.) (Contrast with conformers.)
regulatory T cells (Treg)
The class of T cells that mediates tolerance to self antigens.
reinforcement
The evolution of enhanced reproductive isolation between populations due to natural selection for greater isolation.
release-inhibiting hormones
See inhibiting hormones.
releasing hormone (RH)
A hormone secreted by neuroendocrine cells in the hypothalamus of a vertebrate that travels to the anterior pituitary gland through the hypothalamo–hypophysial portal system and stimulates the secretion of a hormone by a specific population of anterior pituitary endocrine cells.
replication fork
A point at which a DNA molecule is replicating. The fork forms by the unwinding of the parent molecule.
reporter gene
A genetic marker included in recombinant DNA to indicate the presence of the recombinant DNA in a host cell.
repressor
A protein encoded by a regulatory gene that can bind to a promoter and prevent transcription of the associated gene. (Contrast with activator.)
reproductive isolation
Condition in which two divergent populations are no longer exchanging genes. Can lead to speciation.
reproductive signals
External cues that stimulate cells to divide or organisms to reproduce.
RER
See rough endoplasmic reticulum.
residuals
The deviations, along the y-axis from the linear regression line, of individual observations in a bivariate scatter plot.
resistance exercise
Exercise that consists of relatively short periods of high-intensity muscular actions against a large load, often repeated with intervening interruptions. Also called resistance training. (Contrast with endurance exercise.)
resistance (R) genes
Plant genes that confer resistance to specific strains of pathogens.
resource partitioning
A situation in which competing organisms (usually of different species) differ in their use of resources.
resource
In ecology, the materials (e.g., food, mineral nutrients), energy (e.g., photons of light), or favorable physical conditions, that organisms need to survive and reproduce, and that they make unavailable to other organisms when they use these resources.
respiratory airways
Airways in the lungs where O2 and CO2 are exchanged between the air and blood.
respiratory chain
The terminal reactions of cellular respiration, in which electrons are passed from NAD or FAD, through a series of intermediate carriers, to molecular oxygen, with the concomitant production of ATP.
respiratory gases
Oxygen (O2) and carbon dioxide (CO2).
respiratory minute volume
In an animal with lungs, the total volume of air inhaled and exhaled per minute.
respiratory pigments
Any of the pigments that undergo reversible combination with O2 and thus are able to pick up O2 in certain places in an animal’s body (e.g., the breathing organs) and release it in other places (e.g., systemic tissues). Respiratory pigments include hemoglobin and hemocyanin.
resting potential
The potential difference (voltage) across the cell membrane of a living cell at rest. In cells at rest, the interior is negative to the exterior. (Contrast with action potential.)
restoration ecology
The applied subdiscipline of ecology whose goal is to restore the function of damaged ecosystems.
restriction enzyme
Any of a type of enzyme that cleaves double-stranded DNA at specific sites; extensively used in recombinant DNA technology. Also called a restriction endonuclease.
restriction site
A specific DNA base sequence that is recognized and acted on by a restriction endonuclease.
retina
(rett′ in uh) [L. rete: net] The light-sensitive layer of cells in the vertebrate eye and some invertebrate eyes. In the vertebrate eye, it consists of photoreceptor cells and cells that process signals from the photoreceptor cells.
retrotransposons
Mobile genetic elements that are reverse transcribed into RNA as part of their transfer mechanism. (Contrast with DNA transposons.)
retrovirus
An RNA virus that contains reverse transcriptase. Its RNA serves as a template for cDNA production, and the cDNA is integrated into a chromosome of the host cell.
reverse transcriptase
An enzyme that catalyzes the production of DNA (cDNA), using RNA as a template; essential to the reproduction of retroviruses.
rhizoids
(rye′ zoids) [Gk. root] Hairlike extensions of cells in mosses, liverworts, and a few vascular plants that serve the same function as roots and root hairs in vascular plants. The term is also applied to branched, rootlike extensions of some fungi and algae.
rhizome
(rye′ zome) An underground stem (as opposed to a root) that runs horizontally beneath the ground.
rhodopsin
A visual pigment, found in many molecular forms in the animal kingdom, used in the process of transducing energy of photons of light into changes in the membrane potential of photoreceptor cells.
ribonucleic acid
See RNA.
ribose
A five-carbon sugar in nucleotides and RNA.
ribosomal RNA (rRNA)
Several species of RNA that are incorporated into the ribosome. Involved in peptide bond formation.
ribosome
A small particle in the cell that is the site of protein synthesis.
ribozyme
An RNA molecule with catalytic activity.
ribulose bisphosphate carboxylase/oxygenase
See rubisco.
RNA (ribonucleic acid)
An often single-stranded nucleic acid whose nucleotides use ribose rather than deoxyribose and in which the base uracil replaces thymine found in DNA. Serves as genome from some viruses. (See ribosomal RNA, transfer RNA, messenger RNA, ribozyme.)
RNA interference (RNAi)
A mechanism for reducing mRNA translation whereby a double-stranded RNA, made by the cell or synthetically, is processed into a small, single-stranded RNA, whose binding to a target mRNA results in the latter’s breakdown.
RNA polymerase
An enzyme that catalyzes the formation of RNA from a DNA template.
RNA splicing
The last stage of RNA processing in eukaryotes, in which the transcripts of introns are excised through the action of small nuclear ribonucleoprotein particles (snRNP).
rod cells
Light-sensitive cells in the vertebrate retina; these sensory receptor cells are sensitive to extremely dim light and are responsible for dim-light, black and white vision.
root
(1) In reference to phylogenetic trees: the base (oldest) part of the tree. (2) In reference to plants: the organ responsible for anchoring the plant in the soil, absorbing water and minerals, and producing certain hormones. Some roots are storage organs.
root apical meristem
Undifferentiated tissue at theof the root that gives rise to the organs of the root.
root cap
A thimble-shaped mass of cells, produced by the root apical meristem, that protects the meristem; the organ that perceives the gravitational stimulus in root gravitropism.
root system
The organ system that anchors a plant in place, absorbs water and dissolved minerals, and may store products of photosynthesis from the shoot system.
rotational cleavage
Cleavage pattern typical of eutherian mammals in which the first cleavage is meridional whereas in the second cleavage, one blastomere divides meridionally and the other equatorially.
rough endoplasmic reticulum (RER)
The portion of the endoplasmic reticulum whose outer surface has attached ribosomes. (Contrast with smooth endoplasmic reticulum.)
RT-PCR
Determination of the quantity of an RNA by first converting to it to DNA by reverse transcriptase, and then amplifying the DNA by the polymerase chain reaction.
rubisco
Contraction of ribulose bisphosphate carboxylase/oxygenase, the enzyme that combines carbon dioxide or oxygen with ribulose bisphosphate to catalyze the first step of photosynthetic carbon fixation or photorespiration, respectively.
S phase
In the cell cycle, the stage of interphase during which DNA is replicated. (Contrast with G1, G2, M phase.)
salt glands
(1) In plants, glands on the leaves of some halophytic (salt-loving) plants that secrete salts onto the outer plant surfaces, thereby ridding the tissue fluids of the plants of excess salts. (2) In animals, organs other than kidneys that excrete concentrated salt solutions, thereby ridding the body fluids of excess salts.
sample
A set of observations made from a population.
saprobe
[Gk. sapros: rotten] An organism (usually a bacterium or fungus) that obtains its carbon and energy by absorbing nutrients from dead organic matter.
saprobic
Feeding on dead organic matter.
sarcomere
(sark′ o meer) [Gk. sark: flesh + meros: unit] The contractile unit of a skeletal muscle cell. Each myofibril in a muscle cell consists of a series of sarcomeres.
sarcoplasm
The cytoplasm of a muscle cell.
sarcoplasmic reticulum
The endoplasmic reticulum of a muscle cell.
saturated fatty acid
A fatty acid in which all the bonds between carbon atoms in the hydrocarbon chain are single bonds—that is, all the bonds are saturated with hydrogen atoms. (Contrast with unsaturated fatty acid.)
scaling relationships
The study of the relations between physiological (or morphological) characteristics and body size within sets of phylogenetically related species (e.g., the study of metabolism-weight relations among mammals of different sizes.)
scatter plot
A figure that displays the values of observations for two variables along perpendicular axes.
Schwann cell
A type of glial cell that forms a myelin sheath on axons in the peripheral nervous system.
scion
In horticulture, the bud or stem from one plant that is grafted to a root or root-bearing stem of another plant (the stock).
sclereid
One of the principle types of cells in sclerenchyma.
sclerenchyma
(skler eng′ kyma) [Gk. skleros: hard + kymus: juice] A plant tissue composed of cells with heavily thickened cell walls. The cells are dead at functional maturity. The principal types of sclerenchyma cells are fibers and sclereids.
scrotum
In most mammals, a pouch outside the body cavity that contains the testes (testicles).
seasonality
Predictable variation in the climate of a given place during the course of an annual cycle.
second filial generation
See F2.
second law of thermodynamics
The principle that when energy is converted from one form to another, some of that energy becomes unavailable for doing work.
secondary active transport
A form of active transport that does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport. (Contrast with primary active transport.)
secondary consumer
An organism that consumes primary consumers.
secondary endosymbiosis
The engulfment of a photosynthetic eukaryote by another eukaryotic cell that gave rise to certain groups of photosynthetic eukaryotes (e.g., euglenids).
secondary growth
In plants, growth that contributes to an increase in girth. (Contrast with primary growth.)
secondary immune response
A rapid and intense response to a second or subsequent exposure to an antigen, initiated by memory cells. (Contrast with primary immune response.)
secondary lysosome
Membrane-enclosed organelle formed by the fusion of a primary lysosome with a phagosome, in which macromolecules taken up by phagocytosis are hydrolyzed into their monomers. (Contrast with lysosome.)
secondary metabolite
A compound synthesized by a plant that is not needed for basic cellular metabolism. Typically has an antiherbivore or antiparasite function.
secondary sexual characteristics
Sex-specific properties of nonreproductive tissues and organs.
secondary structure
Of a protein, localized regularities of structure, such as the a helix and the b pleated sheet. (Contrast with primary, tertiary, quaternary structure.)
secrete
To discharge a substance from a cell or gland.
sedimentary rock
Rock formed by the accumulation of sediment grains on the bottom of a body of water.
seed
A fertilized, ripened ovule of a gymnosperm or angiosperm. Consists of the embryo, nutritive tissue, and a seed coat.
seedling
A plant that has just completed the process of germination.
segment polarity genes
In Drosophila (fruit fly) development, segmentation genes that determine the boundaries and anterior–posterior organization of individual segments. Part of a developmental cascade that includes maternal effect genes, gap genes, pair rule genes, and Hox genes.
segmentation
Division of an animal body into segments.
segregation
See law of segregation.
selectable marker
A gene, such as one encoding resistance to an antibiotic, that can be used to identify (select) cells that contain recombinant DNA from among a large population of untransformed cells.
selective permeability
Allowing certain substances to pass through while other substances are excluded; a characteristic of membranes.
self-incompatability
In plants, the possession of mechanisms that prevent self-fertilization.
semelparity
A type of reproductive life history in which individuals are physiologically capable of reproducing only one time during their lives. In semelparous species, individuals are often programmed to die after reproducing once. (Contrast with iteroparity.)
semen
(see′ men) [L. semin: seed] The thick, whitish liquid produced by the male reproductive system in mammals, containing the sperm.
semiconservative replication
The way in which DNA is synthesized. Each of the two partner strands in a double helix acts as a template for a new partner strand. Hence, after replication, each double helix consists of one old and one new strand.
seminiferous tubules
The tubules within the testes (testicles) within which sperm production occurs.
sensor
One of four essential elements of a control system; an organ or cell that detects the current level of the controlled variable.
sensory neuron
A neuron that functions as a sensory receptor cell or that carries a signal from sensory cells or organs to the central nervous system. Also called an afferent neuron.
sensory receptor cell
A cell, typically a neuron, that is specialized to transform the energy of a stimulus into an electric signal. Each type is highly specific in the stimuli to which it ordinarily responds.
sensory receptor proteins
Proteins in sensory receptor cells that respond to sensory input, causing a receptor potential (a graded change of membrane potential) in the receptor cell.
sensory transduction
The transformation of environmental stimuli into neural electric signals in a sensory receptor cell.
sepal
(see′ pul) [L. sepalum: covering] One of the outermost structures of the flower, usually protective in function and enclosing the rest of the flower in the bud stage.
sepsis
Generalized inflammation caused by bacterial infection. Can cause a dangerous drop in blood pressure.
septate
[L. wall] Divided, as by walls or partitions.
septum
(plural: septa) (1) A partition or cross-wall appearing in the hyphae of some fungi. (2) The bony structure dividing the nasal passages.
sequential hermaphrodites
Species in which an individual can be male at some times and female at other times during its lifespan
SER
See smooth endoplasmic reticulum.
Sertoli cells
Cells in the seminiferous tubules that nurture the developing sperm.
sex
A process by which the genes of two individuals are combined to produce offspring.
sex chromosome
In organisms with a chromosomal mechanism of sex determination, one of the chromosomes involved in sex determination.
sex determination
The process by which the sex (male or female) of an individual becomes established during the conception and development of the individual.
sex pilus
A thin connection between two bacteria through which genetic material passes during conjugation.
sex-linked inheritance
Inheritance of a gene that is carried on a sex chromosome. Also called sex linkage.
sexual reproduction
Reproduction in which the genes of two individuals are combined to produce offspring, typically involving the union of male and female gametes.
sexual selection
Selection by one sex of characteristics in individuals of the opposite sex. Also, the favoring of characteristics in one sex as a result of competition among individuals of that sex for mates.
shared derived trait
See synapomorphy.
shiver
(1) In a mammal or bird, the subtle contraction and relaxation of skeletal muscles that produces heat rather than motion as the primary product. (2) In an insect, contraction of the flight muscles in a nonflying mode to generate heat rather than flight.
shoot apical meristem
Undifferentiated tissue at the apex of the shoot that gives rise to the organs of the shoot.
shoot system
In plants, the organ system consisting of the leaves, stem(s), and flowers.
short-day plant (SDP)
A plant that flowers when nights are longer than a critical length specific for that plant’s species. (Compare to long-day plant.)
short tandem repeat (STR)
A short (1–5 base pairs), moderately repetitive sequence of DNA. The number of copies of an STR at a particular location varies between individuals and is inherited.
sieve tube element
The characteristic cell of the phloem in angiosperms, which contains cytoplasm but relatively few organelles, and whose end walls (sieve plates) contain pores that form connections with neighboring cells.
sigma factor
In prokaryotes, a protein that binds to RNA polymerase, allowing the complex to bind to and stimulate the transcription of a specific class of genes (e.g., those involved in sporulation).
signal sequence
The sequence within a protein that directs the protein to a particular organelle.
signal transduction pathway
The series of biochemical steps whereby a stimulus to a cell (such as a hormone or neurotransmitter binding to a receptor) is translated into a response of the cell.
significance level
A particular threshold of making a Type I error (incorrectly rejecting a true null hypothesis) in a statistical test, selected a priori by the investigator.
silent mutation
A change in a gene’s sequence that has no effect on the amino acid sequence of a protein because it occurs in noncoding DNA or because it does not change the amino acid specified by the corresponding codon. (Contrast with frame-shift mutation, missense mutation, nonsense mutation.)
silent substitution
See synonymous substitution.
simple diffusion
Diffusion that doesn’t involve a direct input of energy or assistance by carrier proteins.
simple epithelium
An epithelium (sheet of cells that lines a body cavity or covers an organ or body surface) consisting of a single cell layer.
simple fruit
A fruit derived from a single ovary; examples include grapes and tomatoes.
single nucleotide polymorphisms (SNPs)
Inherited variations in a single nucleotide base in DNA that differ between individuals.
sink
In plants, any organ that imports the products of photosynthesis, such as roots, developing fruits, and immature leaves. (Contrast with source.)
sister chromatid
Each of a pair of newly replicated chromatids.
sister clades
Two phylogenetic groups that are each other’s closest relatives.
sister species
Two species that are each other’s closest relatives.
skeletal muscle
A type of muscle tissue characterized by multinucleated cells containing highly ordered arrangements of actin and myosin microfilaments that give the tissue a striated appearance under a light microscope. Also called striated muscle. (Contrast with cardiac muscle, smooth muscle.)
sliding filament theory
Mechanism of muscle contraction (use of ATP to produce contractile forces) based on the formation and breaking of crossbridges between actin and myosin filaments, causing the filaments to slide past each other.
slow oxidative cells
Skeletal muscle cells specialized for sustained, relatively low-intensity, aerobic work; contain myoglobin and abundant mitochondria. Also called slow-twitch cells. (Compare to fast glycolytic cells.)
slug
A worm-like organism or colony of organisms; used to refer to unshelled terrestrial mollusks as well as to the mobile, colonial stage of cellular slime molds.
small intestine
See midgut.
small nuclear ribonucleoprotein particle (snRNP)
A complex of an enzyme and a small nuclear RNA molecule, functioning in RNA splicing.
smooth endoplasmic reticulum (SER)
Portion of the endoplasmic reticulum that lacks ribosomes and has a tubular appearance. (Contrast with rough endoplasmic reticulum.)
smooth muscle
Muscle tissue consisting of small, mononucleated muscle cells innervated by the autonomic nervous system. (Contrast with cardiac muscle, skeletal muscle.)
social behavior
Behavior that helps integrate individuals into social groups (societies), and the group behaviors of social groups. (See also society.)
society
A group of individuals of a single species that exhibits some degree of cooperative action.
sodium–potassium (Na+−K+) pump
A membrane protein (anti-porter) that carries out primary active transport of ions; it uses energy from ATP to pump sodium ions out of a cell and potassium ions into the cell. Also called a sodium–potassium ATPase.
soil horizon
See horizons.
soil solution
The aqueous portion of soil, from which plants take up dissolved mineral nutrients.
solute potential (Ψs)
A property of any solution, resulting from its solute contents; it may be zero or have a negative value. The more negative the solute potential, the greater the tendency of the solution to take up water through a differentially permeable membrane. (Contrast with pressure potential, water potential.)
solvent
Liquid in which a substance (solute) is dissolved to form a solution.
somatic cell
[Gk. soma: body] All the cells of the body that are not specialized for reproduction. (Contrast with germ cell.)
somatic mutation
Permanent genetic change in a somatic cell. These mutations affect the individual only; they are not passed on to offspring. (Contrast with germline mutation.)
somite
(so′ might) A segmental block of mesoderm; differentiates to form dermis, axial skeleton (vertebrae, ribs), and skeletal muscle.
soredia
(sing.: soredium) Propagules of lichens consisting of one or a few photosynthetic cells bound by fungal hyphae.
sori
Clusters of stalked sporangia that occur on the underside of fern fronds.
source
In plants, any organ that exports the products of photosynthesis in excess of its own needs, such as a mature leaf or storage organ. (Contrast with sink.)
spawning
See external fertilization.
speciation
(spee′ see ay′ shun) The process of splitting one biological lineage into two biological lineages that evolve independently from one another.
species
(spee′ sees) [L. kind] The base unit of taxonomic classification, consisting of an ancestor–descendant group of populations of evolutionarily closely related, similar organisms. The more narrowly defined “biological species” consists of individuals capable of interbreeding with each other but not with members of other species.
species composition
The identities of all the species that make up a given ecological community.
species concepts
Ways that biologists think about the existence of species.
species diversity
In ecology, a measure of the chance that two individuals in a community are of the same species. Measures of species diversity include the number of distinct species in a community (species richness), and/or the relative abundances of different species (species evenness).
species evenness
A measure of the similarity in the abundances of species in a community. If species are equally abundant, evenness is at a maximum.
species richness
The total number of species in an ecological community.
sperm
[Gk. sperma: seed] See spermatozoa.
spermatozoa
Haploid gametes that are capable of swimming (usually by use of flagella), produced by spermatogenesis in the testis of a male animal. Also called sperm.
spicule
[L. arrowhead] A hard, calcareous skeletal element typical of sponges.
spinal reflex
In a vertebrate, the conversion of sensory neuronal signals to motor neuronal signals in the spinal cord without participation of the brain.
spindle
Array of microtubules emanating from both poles of a dividing cell during mitosis and playing a role in the movement of chromosomes at nuclear division. Named for its shape.
spiral cleavage
Embryonic development in many protostomes in which the plane of cell division is diagonal to the vertical axis of the embryo. (Compare to radial cleavage.)
spirillum
A bacterium that is shaped like a corkscrew.
spleen
Organ that serves as a reservoir for venous blood and eliminates old, damaged red blood cells from the circulation.
splicing
See RNA splicing.
spliceosome
RNA–protein complex that splices out introns from eukaryotic pre-mRNAs.
spontaneous mutation
A genetic change caused by internal cellular mechanisms, such as an error in DNA replication. (Contrast with induced mutation.)
spontaneous ovulation
Ovulation (release of an egg from the ovaries of a female) that results from endogenous processes (processes that originate within the animal), more or less independent of copulation. (Contrast with induced ovulation.)
sporangiophore
A stalked reproductive structure produced by zygospore fungi that extends from a hypha and bears one or many sporangia.
sporangium
(spor an′ gee um) (plural: sporangia) [Gk. spora: seed + angeion: vessel or reservoir] In plants and fungi, any specialized structure within which one or more spores are formed.
spore
[Gk. spora: seed] (1) Any asexual reproductive cell capable of developing into an adult organism without gametic fusion. In plants, haploid spores develop into gametophytes, diploid spores into sporophytes. (2) In prokaryotes, a resistant cell capable of surviving unfavorable periods.
sporocyte
Specialized cells of the diploid sporophyte that will divide by meiosis to produce four haploid spores. Germination of these spores produces the haploid gametophyte.
sporulation
The formation of specialized cells (spores) that are capable of developing into new individuals.
sporophyte
(spor′ o fyte) [Gk. spora: seed + phyton: plant] In plants and protists with alternation of generations, the diploid phase that produces the spores. (Contrast with gametophyte.)
stabilizing selection
Selection against the extreme phenotypes in a population, so that the intermediate types are favored. (Contrast with disruptive selection.)
stamen
(stay′ men) [L. stamen: thread] A male (pollen-producing) unit of a flower, usually composed of an anther, which bears the pollen, and a filament, which is a stalk supporting the anther.
standard amino acid
One of the 20 amino acids that animals use to synthesize proteins.
standard deviation
A measure of the spread of observations in a sample. See Appendix B for the mathematical formula.
standard error
A measure of how close a sample statistic (such as the mean) is likely to be to the true population value.
standard error of the mean
A measure of how close a sample mean is likely to be to the true population value. Calculated by dividing the standard deviation of a sample by the square root of the sample size.
staphylococci
Bacteria of the genus Staphylococcus.
start codon
The mRNA triplet (AUG) that acts as a signal for the beginning of translation at the ribosome. (Contrast with stop codon.)
statistic
A numerical quantity calculated from data.
stele
(steel) [Gk. stylos: pillar] The central cylinder of vascular tissue in a plant stem.
stem
In plants, the organ that holds leaves and/or flowers and transports and distributes materials among the other organs of the plant.
stem cell
In animals, an undifferentiated cell that is capable of continuous proliferation. A stem cell generates more stem cells and a large clone of differentiated progeny cells. (See also embryonic stem cell.)
steroid hormones
Lipid-soluble hormones synthesized from cholesterol. The chemical structure of a steroid hormone includes a distinctive subpart consisting of four interlinked rings. (Compare to peptide hormones, amine hormones.)
stigma
[L. stigma: mark, brand] The part of the pistil at the apex of the style that is receptive to pollen, and on which pollen germinates.
stock
In horticulture, the root or root-bearing stem to which a bud or piece of stem from another plant (the scion) is grafted.
stoma
(plural: stomata) [Gk. stoma: mouth, opening] Small opening in the plant epidermis that permits gas exchange; bounded by a pair of guard cells whose osmotic status regulates the size of the opening.
stop codon
Any of the three mRNA codons that signal the end of protein translation at the ribosome: UAG, UGA, UAA. (Contrast with start codon.)
STR
See short tandem repeat.
stratigraphy
The study of geological strata, or layers.
stratum
(plural: strata) [L. stratos: layer] A layer of sedimentary rock laid down at a particular time in the past.
stretch-gated channel
A membrane ion channel that opens and closes in response to local stretching or pulling forces applied to the cell membrane.
striated muscle
See skeletal muscle.
strigolactones
Signaling molecules produced by plant roots that attract the hyphae of mycorrhizal fungi.
strobilus
(plural: strobili) One of several conelike structures in various groups of plants (including club mosses, horsetails, and conifers) associated with the production and dispersal of reproductive products.
structural gene
A gene that encodes the primary structure of a protein not involved in the regulation of gene expression.
style
[Gk. stylos: pillar or column] In the angiosperm flower, a column of tissue extending from the tip of the ovary, and bearing the stigma or receptive surface for pollen at its apex.
sub-
[L. under] A prefix used to designate a structure that lies beneath another or is less than another. For example, subcutaneous (beneath the skin); subspecies.
subduction
In plate tectonics, the movement of one plate under another.
subpopulation
In ecology, the portion of a larger metapopulation that occupies a given piece of suitable habitat. See metapopulation.
subsoil
The soil horizon lying below the topsoil and above the parent rock (bedrock); the zone of infiltration and accumulation of materials leached from the topsoil. Also called the B horizon.
substrate
(sub′ strayte) (1) The molecule or molecules on which an enzyme exerts catalytic action. (2) The base material on which a sessile organism lives.
succession
A relatively predictable series of changes in the species composition of a community following a disturbance, that leads ultimately to a community that resembles the original, predisturbance community. (Contrast with ecological transition.)
succulence
In plants, possession of fleshy, water-storing leaves or stems; an adaptation to dry environments.
sun compass
A mechanism by which an animal uses the position of the sun and an internal clock to determine compass direction in navigation.
surface area-to-volume ratio
For any cell, organism, or geometrical solid, the ratio of surface area to volume; this is an important factor in setting an upper limit on the size a cell or organism can attain.
surface tension
The attractive intermolecular forces at the surface of liquid; an especially important property of water.
survivorship
The average individual’s chance of surviving over a specified set of life stages, ages, or period of time—often from birth to a given stage or age. (Contrast with mortality.)
suspensor
In the embryos of seed plants, the stalk of cells that pushes the embryo into the endosperm and is a source of nutrient transport to the embryo.
suspension feeder
An aquatic animal that feeds on objects much smaller than itself that are suspended in water. (See also filter feeder.)
swim bladders
Organs used to regulate buoyancy in fish.
symbiosis
(sim′ bee oh′ sis) [Gk. sym: together + bios: living] The living together of two or more species in a prolonged, interactive relationship.
symbiotic
A relationship in which two or more organisms live in close association with one another.
symmetry
Pertaining to an attribute of an animal body in which at least one plane can divide the body into similar, mirror-image halves. (See bilateral symmetry, radial symmetry.)
sympathetic division
A division of the vertebrate autonomic nervous system that is connected to the central nervous system via thoracic and lumbar spinal nerves; the sympathetic and parasympathetic divisions tend to exert opposing controls over autonomic effectors. (Contrast with parasympathetic division.)
sympatric speciation
(sim pat′ rik) [Gk. sym: same + patria: homeland] Speciation due to reproductive isolation without any physical separation of the subpopulation. (Contrast with allopatric speciation.)
symplast
The continuous meshwork of the interiors of living cells in the plant body, resulting from the presence of plasmodesmata. (Contrast with apoplast.)
synapomorphy
A trait that arose in the ancestor of a phylogenetic group and is present (sometimes in modified form) in all of its members, thus helping to delimit and identify that group. Also called a shared derived trait.
synapse
(sin′ aps) [Gk. syn: together + haptein: to fasten] A specialized type of junction where a neuron meets its target cell (which can be another neuron or some other type of cell) and neurotransmitter molecules serve to carry signals from the presynaptic neuron to the postsynaptic cell across the synaptic cleft.
synaptic plasticity
The process by which synapses in the nervous system of an individual animal can undergo long-term changes in their functional properties.
synonymous (silent) substitution
A change of one nucleotide in a sequence to another when that change does not affect the amino acid specified (i.e., UUA → UUG, both specifying leucine). (Contrast with nonsynonymous substitution, missense mutation, nonsense mutation.)
system
A set of interacting parts in which neither the parts nor the whole can be understood without taking into account the interactions among the parts.
systems analysis
The understanding of a biological system through evaluation of its components and their interactions.
systematics
The scientific study of the diversity and relationships among organisms.
systemic acquired resistance
A general resistance to many plant pathogens following infection by a single agent.
systemic circuit
In a circulatory system, the blood vessels that take blood to and from the systemic tissues. (Contrast with breathing-organ circuit.)
systemic tissues
All tissues other than those of the breathing organs (lungs or gills).
systems analysis
The understanding of a biological system through evaluation of its components and their interactions.
systems biology
The scientific study of an organism as an integrated and interacting system of genes, proteins, and biochemical reactions.
systole
(sis′ tuh lee) [Gk. systole: contraction] During the cardiac cycle, the period of contraction of the chambers of the heart, which drives blood forward in the circulatory system. (Contrast with diastole.)
T tubules
See transverse tubules.
T-helper (TH) cell
Type of T cell that stimulates events in both the cellular and humoral immune responses by binding to the antigen on an antigen-presenting cell; target of the HIV-I virus, the agent of AIDS. (Contrast with cytotoxic T cell.)
taproot
The main root of a plant that is usually thick and grows down into the soil.
taproot system
A root system typical of eudicots consisting of a primary root (taproot) that extends downward by tip growth and outward by initiating lateral roots. (Contrast with fibrous root system.)
target cell
A cell with the appropriate receptor proteins to bind and respond to a particular hormone or other chemical mediator.
TATA box
An eight-base-pair sequence, found about 25 base pairs before the starting point for transcription in many eukaryotic promoters, that binds a transcription factor and thus helps initiate transcription.
taxon
(plural: taxa) [Gk. taxis: arrange, put in order] A biological group (typically a species or a clade) that is given a name.
telomerase
An enzyme that catalyzes the addition of telomeric sequences lost from chromosomes during DNA replication.
telomeres
(tee′ lo merz) [Gk. telos: end + meros: units, segments] Repeated DNA sequences at the ends of eukaryotic chromosomes.
telophase
(tee′ lo phase) [Gk. telos: end] The final phase of mitosis or meiosis during which chromosomes become diffuse, nuclear envelopes re-form, and nucleoli begin to reappear in the daughter nuclei.
template
A molecule or surface on which another molecule is synthesized in complementary fashion, as in the replication of DNA.
template strand
In double-stranded DNA, the strand that is transcribed to create an RNA transcript that will be processed into a protein. Also refers to a strand of RNA that is used to create a complementary RNA.
tendon
A collagen-containing band of tissue that connects a muscle with a bone.
termination
In molecular biology, the end of transcription or translation.
territory
A region that an individual occupies and defends against occupation by other individuals of the same species. (Compare with home range.)
tertiary consumers
An organism that consumes secondary consumers.
tertiary endosymbiosis
The mechanism by which some eukaryotes acquired the capacity for photosynthesis; for example, a dinoflagellate that apparently lost its chloroplast became photosynthetic by engulfing another protist that had acquired a chloroplast through secondary endosymbiosis.
tertiary structure
In reference to a protein, the relative locations in three-dimensional space of all the atoms in the molecule. The overall shape of a protein. (Contrast with primary, secondary, quaternary structures.)
test cross
Mating of a dominant-phenotype individual (who may be either heterozygous or homozygous) with a homozygous-recessive individual.
testis
(tes′ tis) (plural: testes) [L. testis: witness] The male gonad; the organ that produces the male gametes. Also sometimes called a testicle.
testosterone
An androgen (sex steroid hormone), produced by the Leydig cells of the testes.
tetrad
[Gk. tettares: four] During prophase I of meiosis, the association of a pair of homologous chromosomes or four chromatids.
thallus
The vegetative body of a lichen.
thermoneutral zone (TNZ)
[Gk. thermos: temperature] In a homeotherm, the range of ambient temperatures over which the animal is under neither heat nor cold stress and the metabolic rate is constant.
thermoregulation
The maintenance of a relatively constant body temperature.
thick filaments
See myosin filaments.
thin filaments
See actin filaments.
thylakoid
(thigh la koid) [Gk. thylakos: sack or pouch] A flattened sac within a chloroplast. Thylakoid membranes contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.
thymine (T)
Nitrogen-containing base found in DNA.
thyroid-stimulating hormone (TSH)
A hormone, released by the anterior pituitary gland, that signals the thyroid gland to secrete its hormones.
tidal ventilation
Bidirectional pumping of air in and out of lungs, during which air enters and leaves the lungs by the same route. (Contrast with unidirectional ventilation.)
tidal volume
The volume of air that is breathed in and out of the lungs during each breath.
tissue
A group of similar cells organized into a functional unit; usually integrated with other tissues to form part of an organ.
tissue fluid
See interstitial fluid.
tissue system
In plants, any of three organized groups of tissues—dermal tissue, vascular tissue, and ground tissue—that are established during embryogenesis and have distinct functions.
titin
A giant, elongated, elastic protein molecule that in a striated muscle cell spans an entire half-sarcomere from the M band to the Z line.
TNZ
See thermoneutral zone.
topography
Variation in the elevation of Earth’s surface, in the form of mountains, valleys, ocean basins, and so on.
topsoil
The uppermost soil horizon; contains most of the organic matter of soil, but may be depleted of most mineral nutrients by leaching. Also called the A horizon.
totipotent
[L. toto: whole, entire + potens: powerful] Possessing all the genetic information and other capacities necessary to form an entire individual. (Contrast with multipotent, pluripotent.)
toxigenicity
The ability of some pathogenic bacteria to produce chemical substances that harm the host.
trachea
(tray′ kee ah) [Gk. trakhoia: tube] (1) In a vertebrate lung, the principal tube leading into the lungs from the mouth cavity. (2) One of the airways in the tracheal breathing system of an insect.
tracheal breathing system
The type of breathing system found in insects, in which airways (called tracheae and tracheoles) branch throughout the animal’s body so that airways are nearby all cells.
tracheid
(tray′ kee id) A type of tracheary element found in the xylem of nearly all vascular plants, characterized by tapering ends and walls that are pitted but not perforated. (Contrast with vessel element.)
tract
A bundle of axons within a vertebrate central nervous system.
trade-off
Any situation in which a gain in one function, outcome, aspect, or amount comes only at the cost of a loss in other functions, outcomes, aspects, or amounts.
trait
In genetics, a specific form of a character: eye color is a character; brown eyes and blue eyes are traits. (Contrast with character.)
transcription
The synthesis of RNA using one strand of DNA as a template.
transcription factors
Proteins that assemble on a eukaryotic chromosome, allowing RNA polymerase II to perform transcription.
transcription initiation site
The part of a gene’s promoter where synthesis of the gene’s RNA transcript begins.
transfection
Insertion of recombinant DNA into animal cells.
transfer RNA (tRNA)
A family of folded RNA molecules. Each tRNA carries a specific amino acid and anticodon that will pair with the complementary codon in mRNA during translation.
transformation
(1) A mechanism for transfer of genetic information in bacteria in which pure DNA from a bacterium of one genotype is taken in through the cell surface of a bacterium of a different genotype and incorporated into the chromosome of the recipient cell. (2) Insertion of recombinant DNA into a host cell.
transgenic
Containing recombinant DNA incorporated into the genetic material.
transition state
In an enzyme-catalyzed reaction, the reactive condition of the substrate after there has been sufficient input of energy (activation energy) to initiate the reaction.
translation
The synthesis of a protein (polypeptide). Takes place on ribosomes, using the information encoded in messenger RNA.
translational repressor
A protein that blocks translation by binding to mRNAs and preventing their attachment to the ribosome. In mammals, the production of ferritin protein is regulated by a translational repressor.
translocation
(1) In genetics, a rare mutational event that moves a portion of a chromosome to a new location, generally on a nonhomologous chromosome. (2) In vascular plants, movement of solutes in the phloem.
transmembrane protein
An integral membrane protein that spans the phospholipid bilayer.
transpiration
[L. spirare: to breathe] The evaporation of water from plant leaves and stem, driven by heat from the sun, and providing the motive force to raise water (plus mineral nutrients) from the roots.
transpiration–cohesion–tension mechanism
Theoretical basis for water movement in plants: evaporation of water from cells within leaves (transpiration) causes an increase in surface tension, pulling water up through the xylem. Cohesion of water occurs because of hydrogen bonding.
transposon
Mobile DNA segment that can insert into a chromosome and cause genetic change.
transverse tubules (T tubules)
Indentations of the cell membrane at regular intervals over the entire surface of a muscle cell. Transverse tubules conduct electrical excitation into the interior of the cell and thus play a key role in linking electrical excitation with contraction.
triglyceride
A simple lipid in which three fatty acids are combined with one molecule of glycerol.
triploblastic
Having three cell layers. (Contrast with diploblastic.)
trochophore
(troke′ o fore) [Gk. trochos: wheel + phoreus: bearer] A radially symmetrical larval form typical of annelids and mollusks, distinguished by a wheel-like band of cilia around the middle.
trophic
[Gk trophes: nourishment] Referring to feeding.
trophic cascade
Effects of abundances of species at one trophic level on species at other trophic levels. For example, introduction of a tertiary consumer to a community may reduce the population sizes of secondary consumers which it eats, which in turn may increase the population sizes of primary consumers that the secondary consumers eat, which may reduce the population sizes of primary producers that the primary consumers eat.
trophic interactions
The consumer–resource relationships among species in a community.
trophic level
A group of organisms united by obtaining their energy from the same part of the food web of a biological community.
trophoblast
[Gk trophes: nourishment + blastos: sprout] In mammalian development, the outer group of cells of the blastocyst; will form the chorion and thus become part of the placenta, thereby helping to nourish the growing embryo. (Contrast with inner cell mass.)
tropic hormone
A hormone produced by one endocrine gland that controls the secretion of hormones by another endocrine gland. Also called a tropin.
tropics
The latitudes between the Tropic of Cancer and the Tropic of Capricorn (23.5° north and south, respectively), defined by the region of the planet in which the sun is directly overhead at least once during each annual cycle.
tropin
See tropic hormone.
tropomyosin
[troe poe my′ oh sin] A muscle protein that controls the interactions of actin and myosin necessary for muscle contraction.
troponin
A muscle protein that controls the interactions of actin and myosin necessary for muscle contraction.
tube feet
A unique feature of echinoderms; extensions of the water vascular system, which functions in gas exchange, locomotion, and feeding.
tubular fluid
In a kidney, fluid flowing through the lumen of one of the tubular structures (e.g., a nephron) of which the kidney is composed.
tubulin
A protein that polymerizes to form microtubules.
tumor necrosis factor
A family of cytokines (growth factors) that causes cell death and is involved in inflammation.
tumor suppressor
A gene that codes for a protein product that inhibits cell proliferation; inactive in cancer cells. (Contrast with oncogene.)
turgor pressure
[L. turgidus: swollen] See pressure potential.
turnover
In ecology, the change through time or over space in the composition of an ecological community.
Type I error
The incorrect rejection of a true null hypothesis.
Type II error
The incorrect acceptance of a false null hypothesis.
ubiquitin
A small protein that is covalently linked to other cellular proteins identified for breakdown by the proteosome.
ultimate explanation
The historical explanations of the processes that led to the evolution of an adaptation.
ultrafiltration
A process of primary urine formation during which fluid moves out of the blood plasma into tubules in the kidney, driven by the force of blood pressure.
ultraviolet wavelengths
Wavelengths in the electromagnetic spectrum that are too short to be seen by the human eye (i.e., shorter than 0.4 μm).
unidirectional ventilation
Forced flow (of water or air) over or through a breathing organ in a single direction from the point of intake to the point of outflow. (Contrast with tidal ventilation.)
unipotent
An undifferentiated cell that is capable of becoming only one type of mature cell. (Contrast with totipotent, multipotent, pluripotent.)
unsaturated fatty acid
A fatty acid whose hydrocarbon chain contains one or more double bonds. (Contrast with saturated fatty acid.)
uracil (U)
A pyrimidine base found in nucleotides of RNA.
ureotelic
Pertaining to an organism in which the principal final product of the breakdown of nitrogen-containing compounds (primarily proteins) is urea. Mammals and most amphibians are ureotelic. (Contrast with ammonotelic, uricotelic.)
uricotelic
Pertaining to an organism in which the principal final product of the breakdown of nitrogen-containing compounds (primarily proteins) is uric acid or other compounds closely related to uric acid. Insects, spiders, birds, and some other reptiles are uricotelic. (Contrast with ammonotelic, ureotelic.)
urine
(you′ rin) The fluid that the kidney produces and excretes from the body.
vacuole
(vac′ yew ole) Membrane-enclosed organelle in plant cells that can function for storage, water concentration for turgor, or hydrolysis of stored macromolecules.
van der Waals interactions
Weak attractions between atoms resulting from the interaction of the electrons of one atom with the nucleus of another. This type of attraction is about one-fourth as strong as a hydrogen bond.
variable region
The portion of an immunoglobulin molecule or T cell receptor that includes the antigen-binding site and is responsible for its specificity. (Contrast with constant region.)
vas deferens
(plural: vasa deferentia) In a male mammal, the duct that transfers sperm from the testis and epididymis to the urethra for ejaculation.
vascular bundle
In vascular plants, a strand of vascular tissue, including xylem and phloem as well as thick-walled fibers.
vascular cambium
(kam′ bee um) [L. cambiare: to exchange] In plants, a lateral meristem that gives rise to secondary xylem and phloem.
vascular endothelium
In the walls of all blood vessels, the layer of cells—a simple epithelium—that immediately surrounds the blood.
vascular system
All the blood vessels of the circulatory system. (See also cardiovascular system.)
vascular tissue system
The transport system of a vascular plant, consisting primarily of xylem and phloem. (Contrast with dermal tissue system, ground tissue system.)
vasomotor control
Related to changes in the inside (luminal) diameters of blood vessels mediated by contraction and relaxation of smooth muscles in the blood vessel walls. Changes in inside diameter have large effects on rate of blood flow through a vessel.
vasopressin
See antidiuretic hormone.
vector
(1) An agent, such as an insect, that carries a pathogen affecting another species. (2) A plasmid or virus that carries an inserted piece of DNA into a bacterium for cloning purposes in recombinant DNA technology.
vegetal hemisphere
The yolky portion of some animal eggs. (Contrast with animal hemisphere.)
vegetative cells
(1) Generally: non-reproductive cells. (2) The photosynthesizing cells of a cyanobacterial colony.
vegetative reproduction
Asexual reproduction through the modification of stems, leaves, or roots.
vein
[L. vena: channel] A blood vessel that returns blood to the heart. (Contrast with artery.)
ventilation
Forced flow of air or water over or through structures used for breathing (external respiration) or over body surfaces used for external respiration.
ventral
[L. venter: belly, womb] Toward or pertaining to the belly or lower side. (Contrast with dorsal.)
ventricle
In reference to a heart, the principal pumping chamber. The walls of the ventricle are highly muscular, composed of cardiac muscle.
vernalization
[L. vernalis: spring] Events occurring during a required chilling period, leading eventually to flowering.
vertebral column
[L. vertere: to turn] The jointed, dorsal column that is the primary support structure of vertebrates.
vesicle
Within the cytoplasm, a membrane-enclosed compartment that is associated with other organelles; the Golgi complex is one example.
vessel element
A type of tracheary element with perforated end walls; found only in angiosperms. (Contrast with tracheid.)
villus
(vil′ lus) (plural: villi) [L. villus: shaggy hair or beard] A hairlike projection from a membrane; for example, the gut wall is often covered with villi that increase its surface area.
virus
Any of a group of ultramicroscopic particles constructed of nucleic acid and protein (and, sometimes, lipid) that require living cells in order to reproduce. Viruses evolved multiple times from different cellular species.
visceral mass
A centralized, internal portion of a mollusk’s body that contains the heart as well as digestive, excretory, and reproductive organs.
vitamin
[L. vita: life] A carbon compound that an organism cannot synthesize, but nevertheless requires in small quantities for normal growth and health.
voltage
A measure of the difference in electrical charge between two points. Also called potential difference.
voltage-gated channel
A type of gated channel protein, located in a cell membrane, that opens or closes in response to changes in the local membrane potential.
water channel protein
See aquaporin.
water potential (psi, Ψ)
In osmosis, the tendency for a system (a cell or solution) to take up water from pure water through a differentially permeable membrane. Water flows toward the system with a more negative water potential. (Contrast with solute potential, pressure potential.)
water vascular system
In echinoderms, a network of water-filled canals that functions in gas exchange, locomotion, and feeding.
wavelength
The distance between successive peaks of a wave train, such as electromagnetic radiation.
weather
The atmospheric conditions of temperature, humidity, precipitation, and wind speed and direction experienced at a given place and time. (Contrast with climate.)
white blood cells
Cells in the blood plasma that play defensive roles in the immune system. Also called leukocytes.
white matter
In the nervous system, tissue that is rich in myelinated axons and therefore has a glistening white appearance.
wild type
Geneticists’ term for standard or reference type. Deviants from this standard, even if the deviants are found in the wild, are usually referred to as mutant. (Note that this terminology is not usually applied to human genes.)
wood
Secondary xylem tissue.
xeric
Referring to terrestrial animals, those that are able to live steadily in the open air and thus face the full drying power of the terrestrial environment.
xerophyte
(zee′ row fyte) [Gk. xerox: dry + phyton: plant] A plant adapted to an environment with limited water supply.
xylem
(zy′ lum) [Gk. xylon: wood] In vascular plants, the tissue that conducts water and minerals; xylem consists, in various plants, of tracheids, vessel elements, fibers, and other highly specialized cells.
yolk sac
In reptiles (including birds) and mammals, the extraembryonic membrane that forms from the endoderm and lateral plate mesoderm and surrounds the yolk. First source of blood and blood vessels. During development, the yolk sac digests the yolk, and its blood vessels carry products to the embryo.
zeaxanthin
A blue-light receptor involved in the opening of plant stomata.
zone of cell division
The apical and primary meristems of a plant root; the source of all cells of the root’s primary tissues.
zone of cell elongation
The part of a plant root, generally above the zone of cell division, where cells are expanding (growing), primarily in the longitudinal direction.
zone of cell maturation
The part of a plant root, generally above the zone of cell elongation, where cells are differentiating.
zygospore
Multinucleate, diploid cell that is a resting stage in the life cycle of zygospore fungi.
zygote
(zye′ gote) [Gk. zygotos: yoked] The fertilized egg. The cell created by the union of two gametes, in which the gamete nuclei are fused. The earliest stage of the diploid generation.