G-1

GLOSSARY

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′ cap (5 prime) Methylated guanine nucleotide bonded to the 5′ end of eukaryotic mRNA.

5′ end The end of a DNA or RNA strand that has a free phosphate group at the 5′ carbon of the sugar (deoxyribose or ribose).

A

A horizon See topsoil.

abiotic (a′ bye ah tick) [Gk. a: not + bios: life] Nonliving components of the environment including its physical and chemical characteristics. (Contrast with biotic.)

abomasum (a bo mey′ sum) The fourth of the four chambers of the stomach in ruminants; the true stomach.

abortion Any termination of pregnancy, whether induced or natural (in which case it is called a spontaneous abortion), that occurs after a fertilized egg is successfully implanted in the uterus.

abscisic acid (ABA) (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.

aboral The opposite side of the body from the mouth.

absorption spectrum A graph of light absorption versus wavelength of light; shows how much light is absorbed at each wavelength.

absorptive heterotrophs Organisms (primarily fungi) that feed by absorptive heterotrophy, i.e., by secreting digestive enzymes into the environment to break down large food molecules, then absorbing the breakdown products.

absorptive state State in which food is in the gut and nutrients are being absorbed. (Contrast with postabsorptive state.)

accessory fruits A fruit in which some of the flesh develops from tissue exterior to the carpel.

accessory sex organs Anatomical structures that allow transfer of sperm from male to female for internal fertilization. (Contrast with primary sex organs.)

acclimation Refers to increased tolerance for environmental extremes (e.g., extreme cold) after prior exposure to them.

acclimatization Refers to intrinsic seasonal adjustments in the “set points” of an animal’s physiological functioning (e.g., metabolic rate).

acetyl coenzyme A (acetyl CoA) A compound that reacts with oxaloacetate to produce citrate at the beginning of the citric acid cycle; a key metabolic intermediate in the formation of many compounds.

acetylcholine (ACh) A neurotransmitter that carries information across vertebrate neuromuscular junctions and some other synapses. It is then broken down by the enzyme acetylcholinesterase (AChE).

acid [L. acidus: sharp, sour] A substance that can release a proton in solution. (Contrast with base.)

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.

acid rain Rain that has a lower pH than normal as a result of acid-forming precursor molecules introduced into the atmosphere by human activities.

acoelomate An animal that does not have a coelom.

acquired immune deficiency syndrome (AIDS) Decline in immune system function due to depletion of T_H cells and macrophages infected with HIV.

acrosome (a′ krow soam) [Gk. akros: highest + soma: body] The structure at the forward tip of an animal sperm, which is the first to fuse with the egg membrane and enter the egg cell.

ACTH See corticotropin.

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.)

action potentials (APs) Generated by neurons, these are electrical signals that transmit information via waves of depolarization or hyperpolarization of the cell membrane.

action spectrum A graph of a biological process versus light wavelength; shows which wavelengths are involved in the process.

activation energy (E_a) The energy barrier that blocks the tendency for a chemical reaction to occur.

activation gate The property of a voltage-gated Na⁺ ion channel that increases its permeability to Na⁺ ions when the membrane is depolarized.

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 One of the 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.)

adaptive radiation A series of evolutionary events that results in an array (radiation) of related species that live in a variety of environments, differing in the characteristics each uses to exploit those environments.

addition rule The probability of one of two or more events that are mutually exclusive is the sum of their independent probabilities.

adenine (A) (a′ den een) A nitrogen-containing base found in nucleic acids, ATP, NAD, and other compounds.

adeno-associated virus A small, non-pathogenic virus with a single-strand DNA genome, that can be used for gene therapy in humans and does not integrate unto the host chromosomes.

ADH See vasopressin.

adhesion Binding of one cell or substance to another.

adrenal cortex The outer region of the adrenal gland that produces and secretes steroid hormones.

adrenal gland (a dree′ nal) [L. ad: toward + renes: kidneys] An endocrine gland located near the kidneys of vertebrates, consisting of two parts, the adrenal cortex and adrenal medulla.

G-2

adrenal medulla Tissue that makes up the core of the adrenal gland and secretes epinephrine and norepinephrine.

adrenergic receptors G protein-coupled receptor proteins that bind to the hormones epinephrine and norepinephrine, triggering specific responses in the target cells.

adrenocorticotropic hormone (ACTH) See corticotropin.

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] In the presence of oxygen; requiring or using oxygen (as in aerobic metabolism). (Contrast with anaerobic.)

aerotolerant anaerobe A prokaryote that does not require an oxygen atmosphere for survival, but which can live while exposed to O₂.

afferent (af′ ur unt) [L. ad: toward + ferre: to carry] Carrying to, as in neurons that carry impulses to the central nervous system (afferent neurons), or a blood vessel that carries blood to a structure (afferent arterioles). (Contrast with efferent.)

after-hyperpolarization A fall in membrane potential below normal resting level following an action potential.

aggregate fruit A fruit that develops from several carpels from the same flower.

agonist A chemical substance (e.g., a neurotransmitter) that elicits a specific response in a cell or tissue. (Contrast with antagonist.)

AIDS See acquired immune deficiency syndrome.

air sacs Structures in the respiratory system of birds that receive inhaled air; they keep fresh air flowing unidirectionally through the lungs, but are not themselves gas exchange surfaces.

albedo effect (al bee′ doh) [L. albus: white] The solar radiation reflected by the surface of Earth.

alcoholic fermentation See fermentation.

aldosterone (al dohs′ ter own) A steroid hormone produced in the adrenal cortex of mammals. Promotes secretion of potassium and reabsorption of sodium in the kidney.

aleurone layer (al yu′ rown) 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.

allantoic membrane In animal development, an outgrowth of extraembryonic endoderm plus adjacent mesoderm that forms the allantois, a saclike structure that stores metabolic wastes produced by the embryo.

allantois (al′ lun toh is) [Gk. allant: sausage] An extraembryonic membrane enclosing a sausage-shaped sac that stores the embryo’s nitrogenous wastes.

Allee effect An increase in the population growth rate (r) as the population density decreases.

allele (a leel′ ) [Gk. allos: other] The alternate form of a genetic character found at a given locus on a chromosome.

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.

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.

alpha diversity Species diversity at the local or community scale. (Compare with beta diversity, gamma diversity.)

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.

alternative states Different community assemblages that develop at the same location under similar environmental conditions.

altricial Born or hatched in a relatively undeveloped state that requires care and feeding by the parents.

altruistic Pertaining to behavior that benefits other individuals at a cost to the individual who performs it.

alveolus (al ve′ o lus) (plural: alveoli) [L. alveus: cavity] A small, baglike cavity, especially the blind sacs of the lung.

amacrine cells Cells in the retina that communicate laterally between ganglion and bipolar cells.

amensalism A type of interaction in which one species is harmed and the other is unaffected. (Contrast with competition, commensalism, mutualism.)

amine hormones Small hormone molecules synthesized from single amino acids (e.g., thyroxine and epinephrine).

amino acid An organic compound containing both NH₂ and COOH groups. Proteins are polymers of amino acids.

ammonia NH₃, the most common nitrogenous waste. (Compare with urea, uric acid.)

ammonotelic (am moan′ o teel′ ic) [Gk. telos: end] Pertaining to an organism in which the final product of breakdown of nitrogen-containing compounds (primarily proteins) is ammonia. (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.

amniotic fluid The fluid in the amniotic sac that contains mammalian and reptilian embryos.

amphipathic (am′ fi path′ ic) [Gk. amphi: both + pathos: emotion] Of a molecule, having both hydrophilic and hydrophobic regions.

amplitude The magnitude of change over the course of a regular cycle.

amygdala A component of the limbic system that is involved in fear and the memory of fearful experiences.

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 respiration A metabolic pathway in which glucose is catabolized in the absence of oxygen.

analogous In biology, refers to similar structures found in two or more groups of organisms, but with separate, independent evolutionary origins. An example is the wings of bats and the wings of birds, which evolved independently as wings from wingless ancestors.

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) Any of the several male sex steroids (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.)

angiotensin (an′ jee oh ten′ sin) A peptide hormone that raises blood pressure by causing peripheral vessels to constrict. Also maintains glomerular filtration by constricting efferent vessels and stimulates thirst and the release of aldosterone.

angular gyrus A part of the human brain believed to be essential for integrating spoken and written language.

G-3

animal hemisphere The metabolically active upper portion of some animal eggs, zygotes, and embryos; does not contain the dense nutrient yolk. (Contrast with vegetal hemisphere.)

animal physiology The study of animal form and function.

animal pole The point at the end of the developing egg where yolk is least concentrated.

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.)

antagonist A biochemical substance (e.g., a drug) that blocks the normal action of another biochemical substance. (Contrast with agonist.)

anterior pituitary The portion of the vertebrate pituitary gland that derives from gut epithelium. Produces trophic hormones.

anterior Toward or pertaining to the tip or headward region of the body axis. (Contrast with posterior.)

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) See vasopressin.

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, a process in which a cell ingests and digests an antigen, and then exposes fragments of that antigen to the outside of the cell, bound to proteins in the cell’s cell membrane.

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.

antiporter A membrane transport protein that moves one substance in one direction and another in the opposite direction. (Contrast with symporter, uniporter.)

antisense RNA A single-stranded RNA molecule complementary to, and thus targeted against, an mRNA of interest to block its translation.

anus (a′ nus) An opening through which solid digestive wastes are expelled, located at the posterior end of a tubular gut.

aorta (a or′ tah) [Gk. aorte: aorta] The main trunk of the arteries leading to the systemic (as opposed to the pulmonary) circulation.

aortic body A chemosensor in the aorta that senses a decrease in blood supply or a dramatic decrease in partial pressure of oxygen in the blood.

aortic valve A one-way valve between the left ventricle of the heart and the aorta that prevents backflow of blood into the ventricle when it relaxes.

aphasia A deficit in the ability to use or understand words.

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.

appendix A small blind-end sac that is attached to the initial segment of the colon.

aquaporin A transport protein in plant and animal cell membranes through which water passes in osmosis.

aquifer A large pool of groundwater.

archegonium (ar′ ke go′ nee um) The multicellular structure that produces eggs in nonvascular land plants, ferns, and gymnosperms.

archenteron (ark en′ ter on) [Gk. archos: first + enteron: bowel] The earliest primordial animal digestive tract.

arcuate nucleus A group of neurons in the hypothalamus that produce and secrete many neuroendocrines and are involved in regulation of appetite.

arteriole A small blood vessel arising from an artery that feeds blood into a capillary bed.

artery A muscular blood vessel carrying oxygenated blood away from the heart to other parts of the body. (Contrast with vein.)

artificial insemination An infertility treatment that involves the artificial introduction of sperm into the woman’s reproductive tract.

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.

assimilation efficiency The proportion of ingested biomass that consumers assimilate by digestion.

assisted reproductive technologies (ARTs) Any of several procedures that remove unfertilized eggs from the ovary, combine them with sperm outside the body, and then place fertilized eggs or egg–sperm mixtures in the appropriate location in a female’s reproductive tract for development.

association cortex In the vertebrate brain, the portion of the cortex involved in higher-order information processing, so named because it integrates, or associates, information from different sensory modalities and from memory.

asthenosphere (ass thenn′ o sphere) [Gk. asthenes: weak] The viscous, malleable (changeable) layer of Earth’s mantle. It is overlain by the solid lithospheric plates.

astrocyte [Gk. astron: star] A type of glial cell that contributes to the blood–brain barrier by surrounding the smallest, most permeable blood vessels in the brain.

atherosclerosis (ath′ er oh sklair oh′ sis) [Gk. athero: gruel, porridge + skleros: hard] A disease of the lining of the arteries characterized by fatty, cholesterol-rich deposits in the walls of the arteries. When fibroblasts infiltrate these deposits and calcium precipitates in them, the disease becomes arteriosclerosis, or “hardening of the arteries.”

atmosphere A thin layer of gases surrounding Earth composed of nitrogen gas, oxygen, argon, carbon dioxide, and traces of hydrogen, helium, ozone, methane, and many other gases.

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.

atomic weight The average of the mass numbers of a representative sample of atoms of an element, with all the isotopes in their normally occurring proportions. Also called atomic mass.

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.

atrial natriuretic peptide (ANP) A hormone released by the atrial muscle fibers of the heart when they are overly stretched, which decreases reabsorption of sodium by the kidney and thus blood volume.

atrioventricular node A modified node of cardiac muscle that organizes the action potentials that control contraction of the ventricles.

G-4

atrioventricular (AV) valve A heart valve between an atrium and a ventricle.

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). Also, the outer ear.

autocatalysis [Gk. autos: self + kata: to break down] A positive feedback process in which an activated enzyme acts on other inactive molecules of the same enzyme to activate them.

autocrine A chemical signal that binds to and affects the cell that makes it. (Contrast with paracrine.)

autoimmunity An immune response by an organism to its own molecules or cells.

autonomic nervous system (ANS) The portion of the peripheral nervous system that controls such involuntary functions as those of guts and glands. Also called the involuntary nervous system.

autonomous specification Determination of cell fates due to informational molecules originally present in the egg.

autophagy The programmed destruction of a cell’s components.

autopolyploidy The possession of more than two entire chromosome sets that are derived from a single species.

autoregulatory mechanisms In mammalian circulatory systems, local control of blood flow through capillary beds by constriction or dilation of incoming arterioles in response to local metabolite concentrations.

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 some energy source such as sunlight (photoautotrophs) or chemically reduced matter (see chemoautotrophs). (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 concept.)

Avogadro’s number The number of atoms or molecules in a mole (weighed out in grams) of a substance, calculated to be 6.023 ×10²³.

axillary bud A bud that forms in the angle (axil) where a leaf meets a stem.

axon [Gk. axle] The process (branching structure) of a neuron that conducts action potentials away from the cell body. (See also dendrites.)

axon hillock The junction between an axon and the neuron’s cell body; where action potentials are generated.

axon terminal The end portion of an axon, which passes action potentials to another cell. Axon terminals can form synapses and release neurotransmitter.

B

B cell A type of lymphocyte involved in the humoral immune response of vertebrates. Upon recognizing an antigenic determinant, a B cell develops into a plasma cell, which secretes an antibody. (Contrast with T cell.)

B horizon See subsoil.

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.

baroreceptor [Gk. baros: weight] A pressure-sensing cell or organ. Sometimes called a stress receptor.

basal metabolic rate (BMR) The minimum rate of energy turnover in an awake (but resting) bird or mammal that is not expending energy for thermoregulation.

base (1) A substance that can accept a hydrogen ion in solution. (Contrast with acid.) (2) 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. (See complementary base pairing.)

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.

Batesian mimicry The convergence in appearance of an edible species (mimic) with an unpalatable species (model).

behavioral ecology An evolutionary approach to the study of animal behavior that studies how behaviors are adaptive in different environmental conditions.

behaviorism One of two classical approaches to the study of proximate causes of animal behavior, derived from the discoveries of Ivan Pavlov and focused on laboratory studies. (Compare with ethology.)

benign A tumor that grows to a limited extent and does not spread to other parts of the body. (Contrast with malignant.)

beta diversity The change in species number and composition, or turnover of species, across a landscape. (Compare with alpha diversity, gamma diversity.)

β (beta) pleated sheet A type of protein secondary structure; results from hydrogen bonding between polypeptide regions running antiparallel to each other.

biased gene conversion A mechanism of concerted evolution in which a DNA repair system appears biased in favor of using particular nucleotide sequences as templates for repair, resulting in the rapid spread of the favored sequence across all copies of the gene. (See concerted evolution.)

bicuspid valve The left AV valve. So named because it has two leaves.

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.)

bile A secretion of the liver made up of bile salts synthesized from cholesterol, various phospholipids, and bilirubin (the breakdown product of hemoglobin). Emulsifies fats in the small intestine.

binary fission Reproduction of a prokaryote by division of a cell into two comparable progeny cells.

bindin Species specific recognition molecule on the sperm acrosomal process.

binocular vision Overlapping visual fields of an animal’s two eyes; allows the animal to see in three dimensions.

binomial nomenclature [Gk. two names] A taxonomic naming system in which each species is given a binomial, a genus name followed by a species name.

biodiversity The diversity of important ecological entities that span multiple spatial scales, from genes to populations to species to communities.

biofilm A community of microorganisms embedded in a polysaccharide matrix, forming a highly resistant coating on almost any moist surface.

biogeochemical cycle Movement of inorganic elements such as water, nitrogen, phosphorus, and carbon through living organisms and the physical environment.

biogeographic region Distinct regions of biota characterized by different species that occur at the continental-scale. (Contrast with biome.)

biogeography The scientific study of the distribution and diversity of organisms across Earth.

G-5

bioinformatics The use of computers and/or mathematics to analyze complex biological information, such as DNA sequences.

biological control The use of natural enemies (predators, parasites, or pathogens) to reduce the population density of an economically damaging (pest) species.

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.)

biology [Gk. bios: life + logos: study] The scientific study of living things.

biome (bye′ ome) A major division of the ecological communities of Earth, characterized primarily by distinctive vegetation and the annual patterns of temperature and precipitation. A given biogeographic region contains many different biomes.

biosphere (bye′ oh sphere) All regions of Earth (terrestrial and aquatic) and Earth’s atmosphere in which organisms can live.

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 living cells or organisms to produce materials useful to humans.

biotic (bye ah′ tick) [Gk. bios: life] Living components of the environment such as organisms. (Contrast with abiotic.)

biotic resistance The ability of resident species to exclude or slow the population growth of non-native species.

bipedal locomotion Terrestrial locomotion by means of two rear limbs or legs.

bipolar cells Cells in the retina that communicate between photoreceptors and ganglion cells.

blastocoel (blass′ toe seal) [Gk. blastos: sprout + koilos: hollow] The central, hollow cavity of a blastula.

blastocyst (blass′ toe cist) An early embryo formed by the first divisions of the fertilized egg (zygote). In mammals, a hollow ball of cells.

blastoderm A layer of undifferentiated cells that are a very early stage of development prior to the specification of the embryonic axis.

blastodisc (blass′ toe disk) An embryo that forms as a disc 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.

blastopore The opening created by the invagination of the vegetal pole during gastrulation of animal embryos.

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. (Contrast with blastodisc.)

block to polyspermy Any of several responses to entry of a sperm into an egg that prevent more than one sperm from entering the egg.

blood clotting A cascade of events involving platelets and circulating proteins (clotting factors) that seals damaged blood vessels.

blood plasma (plaz′ muh) The liquid portion of blood, in which blood cells and other particulates are suspended.

blood–brain barrier The selective impermeability of blood vessels in the brain that prevents most chemicals from diffusing from the blood into the brain.

blue-light receptors Pigments in plants that absorb blue light (400–500 nm). These pigments mediate many plant responses including photo-tropism, stomatal movements, and expression of some genes.

BMR See basal metabolic rate.

body cavity An internal, fluid-filled space, enclosed by mesoderm in triploblastic animals.

body plan The general structure of an animal, the arrangement of its organ systems, and the integrated functioning of its parts.

Bohr effect A shift in the O₂-binding curve of hemoglobin in response to excess H⁺ ions such that the hemoglobin releases more O₂ in tissues where pH is low.

bone A rigid component of vertebrate skeletal systems that contains an extracellular matrix of insoluble calcium phosphate crystals as well as collagen fibers.

Bowman’s capsule An elaboration of the renal tubule, composed of podocytes, that surrounds and collects the filtrate from the glomerulus.

brain The centralized integrative center of a nervous system.

brainstem The portion of the vertebrate brain between the spinal cord and the forebrain, made up of the medulla, pons, and midbrain.

brassinosteroids Plant steroid hormones that mediate light effects promoting the elongation of stems and pollen tubes.

Broca’s area A portion of the human brain essential for speech. Located in the frontal lobe just in front of the primary motor cortex.

bronchioles The smallest airways in a vertebrate lung, branching off the bronchi.

bronchus (plural: bronchi) The major airway(s) branching off the trachea into the vertebrate lung.

brown fat In mammals, fat tissue that is specialized to produce heat. It has many mitochondria and capillaries, and a protein that uncouples oxidative phosphorylation.

bud A protuberance on a stem that contains an undeveloped leaf, shoot, or flower.

budding Asexual reproduction in which a more or less complete new organism grows from the body of the parent organism, eventually detaching itself.

buffer A substance that can transiently accept or release hydrogen ions and thereby resist changes in pH.

bulbourethral glands Secretory structures of the human male reproductive system that produce a small volume of an alkaline, mucoid secretion that helps neutralize acidity in the urethra and lubricate it to facilitate the passage of semen.

bulbus arteriosus The last chamber of the fish heart.

bulk flow The movement of a solution from a region of higher pressure potential to a region of lower pressure potential.

bulk transport See solvent drag.

bundle of His Fibers of modified cardiac muscle that conduct action potentials from the atria to the ventricular muscle mass.

bundle sheath cell Part of a tissue that surrounds the veins of plants.

C

C horizon See parent rock.

C₃ plants Plants that produce 3PG as the first stable product of carbon fixation in photosynthesis and use ribulose bisphosphate as a CO₂ receptor.

C₄ plants Plants that produce oxaloacetate as the first stable product of carbon fixation in photosynthesis and use phosphoenolpyruvate as a CO₂ acceptor. C₄ plants also perform the reactions of C₃ photosynthesis.

calcitonin Hormone produced by the thyroid gland; lowers blood calcium and promotes bone formation. (Contrast with parathyroid hormone.)

calcitriol A hormone derived from vitamin D whose actions include stimulating the cells of the digestive tract to absorb calcium from ingested food.

calorie [L. calor: heat] The amount of heat required to raise the temperature of 1 gram of water by 1ºC. Physiologists commonly use the kilocalorie (kcal) as a unit of measure (1 kcal = 1,000 calories). Nutritionists also use the kilocalorie, but refer to it as the Calorie (Cal, 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.

CAM See crassulacean acid metabolism.

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.

cancellous bone A type of bone with numerous internal cavities that make it appear spongy, although it is rigid. (Contrast with compact bone.)

G-6

capillaries [L. capillaris: hair] Very small tubes, especially the smallest blood-carrying vessels of animals between the termination of the arteries and the beginnings of the veins. Capillary beds are networks of capillaries where materials are exchanged between the blood and the interstitial fluid.

capsid The outer shell of a virus that encloses its nucleic acid.

capsule In some prokaryotes, a dense layer of polymers that surrounds the cell wall.

carbohydrases Digestive enzymes that hydrolyze carbohydrates.

carbohydrates Organic compounds containing carbon, hydrogen, and oxygen in the ratio 1:2:1 (i.e., with the general formula C_nH_2nO_n). Common examples are sugars, starch, and cellulose.

carbon skeleton The chains or rings of carbon atoms that form the structural basis of organic molecules. Other atoms or functional groups are attached to the carbon atoms.

carbonic anhydrase (CA) An enzyme that catalyzes the hydration of CO₂ to produce carbonic acid.

carboxylase An enzyme that catalyzes the addition of carboxyl functional groups (O=C—OH) to a substrate.

cardiac cycle 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, and is responsible for the beating of, the heart. Characterized by branching 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, blood, and vessels are of a circulatory system.

carnivore [L. carn: flesh + vorare: to devour] An organism that eats animal tissues. (Contrast with detritivore, herbivore, omnivore.)

carnivorous plant Plant capable of trapping and digesting animals to obtain nutrients.

carnivory An act of predation in which the predator and prey are both animals. (Contrast with herbivory, parasitism, or omnivory).

carotid body A chemosensor in the carotid artery that senses a decrease in blood supply or a dramatic decrease in partial pressure of oxygen in the blood.

carpel (kar′ pel) [Gk. karpos: fruit] The organ of the flower that contains one or more ovules.

carrier proteins Proteins that bind another substance to transport it from one location to another; carriers bind small molecules and transport them across membranes.

carrying capacity (K) The maximum number of individuals in a population (i.e., maximum population size) that can be supported by the resources present in a given environment.

cartilage bone A type of bone that begins its development as a cartilaginous structure resembling the future mature bone, then gradually hardens into mature bone. (Contrast with membranous bone.)

cartilage In vertebrates, a tough connective tissue found in joints, the outer ear, and elsewhere. Forms the entire skeleton in some animal groups.

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.)

catabolite repression In the presence of abundant glucose, the diminished synthesis of catabolic enzymes for other energy sources.

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.)

cation exchange The process in soil by which cations bound to soil particles are released from the particles and made available to plant roots in soil water through the binding of hydrogen ions to the soil particles.

CCK See cholecystokinin.

Cdk See cyclin-dependent kinase.

cDNA See complementary DNA.

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.

cecum (see′ cum) [L. blind] A blind branch off the large intestine. In many nonruminant mammals, the cecum contains a colony of microorganisms that contribute to the digestion of food.

cell The simplest structural unit of a living organism. In multicellular organisms, many individual cells serve as the building blocks of tissues and organs.

cell adhesion The binding of one cell to another, often mediated by noncovalent forces.

cell body The portion of the neuron that contains the nucleus and gives rise to dendrites and an axon.

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 cycle The stages through which a cell passes between one mitotic division and the next. Includes all stages of interphase and mitosis. (See mitosis.)

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 The membrane that surrounds the cell, regulating the entry and exit of molecules and ions. Every cell has a cell membrane, and it is also called the plasma membrane.

cell potency In multicellular organisms, an undifferentiated cell’s potential to become a cell of a specific type. (See multipotent, pluripotent, totipotent.)

cell recognition Binding of cells to one another mediated by membrane proteins or carbohydrates.

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.

cellular specialization In multicellular organisms, the division of labor such that different cell types become responsible for different functions (e.g., reproduction or digestion) within the organism.

cellulose (sell′ you lowss) A straight-chain polymer of glucose molecules, used by plants as a structural supporting material.

central dogma The premise that information flows from DNA to RNA to polypeptide (protein).

central nervous system (CNS) That portion of the nervous system that is the site of most information processing, storage, and retrieval; in vertebrates, the brain and spinal cord. (Contrast with peripheral nervous system.)

central vacuole In plant cells, a large organelle that stores the waste products of metabolism and maintains turgor.

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.

G-7

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 brain and sensory organs at the anterior end of the animal.

cerebellum (sair uh bell′ um) [L. diminutive of cerebrum: brain] The brain region that controls muscular coordination; located at the anterior end of the hindbrain.

cerebral cortex The thin layer of gray matter (neuronal cell bodies) that overlies the cerebrum.

cerebral hemispheres The bilateral divisions of the cerebrum.

cerebrum (su ree′ brum) [L. brain] The dorsal anterior portion of the forebrain, making up the largest part of the brain of mammals; the chief coordination center of the nervous system and the major information-processing areas of the vertebrate brain consists of two cerebral hemispheres.

cervix (sir′ vix) [L. neck] The opening of the uterus into the vagina.

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.

chaperone A protein that guards other proteins by counteracting molecular interactions that threaten their three-dimensional structure.

character displacement An evolutionary phenomenon in which species that compete for the same limiting resources diverge in morphology and/or behavior.

character In genetics, an observable feature, such as eye color. (Contrast with trait.)

chemical bond An attractive force stably linking two atoms.

chemical equilibrium See equilibrium.

chemical evolution The theory that life originated through the chemical transformation of inanimate substances.

chemical reaction The change in the composition or distribution of atoms of a substance with consequent alterations in properties.

chemical synapse Neural junction at which neurotransmitter molecules released from a presynaptic cell induce changes in a postsynaptic cell. (Contrast with electrical synapse.)

chemically gated channel A type of membrane channel that opens or closes depending on the presence or absence of a specific molecule that binds either to the channel protein itself or to a separate receptor that alters the three-dimensional shape of the channel protein.

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 obtains energy by oxidizing inorganic substances, using some of that energy to fix carbon. Also known as a chemolithotroph. (Contrast with chemoheterotroph, photoautotroph, photoheterotroph.)

chemoheterotroph An organism that must obtain both carbon and energy from organic substances. (Contrast with chemoautotroph, photoautotroph, photoheterotroph.)

chemoreceptor A receptor protein that binds to specific molecules (such as odorant molecules or pheromones) in the environment.

chemosynthesis The use of energy from inorganic chemical compounds by archaea and bacteria to fix CO₂ and produce carbohydrates using the Calvin cycle.

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.

chiasma (kie az′ muh) (plural: chiasmata) [Gk. cross] An X-shaped connection between paired homologous chromosomes in prophase I of meiosis. A chiasma is the visible manifestation of crossing over between homologous chromosomes.

chief cells One of three types of secretory cell found in the gastric pits of the stomach wall. Chief cells secrete the protein-digesting enzyme pepsin. (See parietal cells.)

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.

cholecystokinin (CCK) (ko′ luh sis tuh kai′ nin) A hormone produced and released by the lining of the duodenum when it is stimulated by undigested fats and proteins. It stimulates the gallbladder to release bile and slows stomach activity.

chorion (kor′ ee on) [Gk. khorion: afterbirth] The outermost of the membranes protecting mammal, bird, and reptile embryos; in mammals it forms part of the placenta.

chromatin The nucleic acid–protein complex that makes up eukaryotic chromosomes.

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.

chylomicrons (ky low my′ cron) Particles of lipid coated with protein, produced in the gut from dietary fats and secreted into the extracellular fluids.

chyme (kime) [Gk. kymus: juice] Created in the stomach; a mixture of ingested food with the digestive juices secreted by the salivary glands and the stomach lining.

cilia (sil′ ee ah) (singular: cilium) [L. eyelashes] Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than flagella.

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 physiological system consisting of a muscular pump (heart), a fluid (blood or hemolymph), and a series of conduits (blood vessels) that transports materials around the body.

cis-trans isomers In molecules with a double bond (typically between two carbon items), identifies on which side of the double bond similar atoms or functional groups are found. If they are on the same side, the molecule is a cis isomer; in a trans isomer, similar atoms are on opposite sides of the double bond. (See isomers.)

clade [Gk. klados: branch] A monophyletic group made up of an ancestor and all of its descendants.

class A group of things defined by particular attributes.

class I MHC proteins Cell surface proteins that participate in the cellular immune response directed against virus-infected cells.

class II MHC proteins Cell surface proteins that participate in the cell–cell interactions (of T-helper cells, macrophages, and B cells) of the humoral immune response.

class switching Occurs when a B cell changes the immunoglobulin class it synthesizes (e.g., a B cell making IgM switches to making IgG).

cleavage The first few cell divisions of an animal zygote. (See also complete cleavage, incomplete cleavage.)

climate The long-term average atmospheric conditions (temperature, precipitation, humidity, wind direction and velocity) at a particular place over longer time (years to millennia). (Contrast with weather.)

climax community The final stage of succession; a community that is capable of perpetuating itself under local climatic and soil conditions and persists for a relatively long time.

clinal variation [Gk. klinein: to lean] Gradual change in the phenotype of a species over a geographic gradient.

G-8

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.)

clumped dispersion The spatial arrangement of individuals within a population, characterized by clumped spatial patterns. (Compare with random dispersion, regular (uniform) dispersion.)

CO (CONSTANS) Gene coding for a transcription factor that activates the synthesis of florigen (FT); involved in the induction of flowering.

co-repressor In the regulation of bacterial operons, a molecule that binds to the repressor, causing it to change shape and bind to the operator, thereby inhibiting transcription.

coastal zone The marine life zone that extends from the shoreline to the edge of the continental shelf. Characterized by relatively shallow, well-oxygenated water and variable temperatures and salinities.

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.

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.)

coelomate Possessing a coelom.

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.

coevolution Evolutionary process in which an adaptation in one species leads to the evolution of an adaptation in a species with which it interacts.

cohesion The tendency of molecules (or any substances) to stick together.

cohort life table A life table in which the fate of a group of individuals born during the same time frame (a cohort) is followed from birth to death.

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 vertebrates, a tubule that receives urine produced in the nephrons of the kidney and delivers that fluid to the ureter for excretion.

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.)

colon [Gk. kolon] The portion of the gut between the small intestine and the anus. Also called the large intestine.

commensalism [L. com: together + mensa: table] A type of positive interaction in which one species benefits from the interaction while the other is unaffected. (Contrast with amensalism, competition, mutualism.)

communication A signal from one organism (or cell) that alters the functioning or behavior of another organism (or cell).

community A group of species living together at the same place and time.

community function The way a community works as measured by metrics such as plant productivity or stability.

community structure The set of characteristics or patterns that describe a community, including the number, composition, and abundance of species.

compact bone A type of bone with a solid, hard structure. (Contrast with cancellous bone.)

companion cell In angiosperms, a specialized cell found adjacent to a sieve tube element.

comparative experiment Experimental design in which data from various unmanipulated samples or populations are compared, but in which variables are not controlled or even necessarily identified. (Contrast with controlled experiment.)

comparative genomics Computer-aided comparison of DNA sequences between different organisms to reveal genes with related functions.

comparator In a regulatory system, the mechanism for comparing a feedback variable to a set point to generate command signals to effector organs.

competition A nontrophic interaction in which two or more species overlap in the use of at least some of the same required limiting resources, negatively affecting their growth, reproduction, and/or survival. (Contrast with amensalism, commensalism, mutualism.)

competitive coexistence The ability of species to coexist with one another despite sharing limiting resources. (Contrast with competitive exclusion.)

competitive exclusion A result of competition between species for resources, in which one species completely eliminates the other from a given habitat. (Contrast with competitive coexistence.)

competitive inhibitor A nonsubstrate that binds to the active site of an enzyme and thereby inhibits binding of its substrate. (Contrast with noncompetitive inhibitor.)

complement system A group of eleven proteins that play a role in some reactions of the immune system. The complement proteins are not immunoglobulins.

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 diet A mixture of foods that supplies all essential nutrients.

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, and the blastomeres are of similar size. (Contrast with incomplete cleavage.)

complete gut A gut with an entrance and a separate exit.

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.

complex ions Groups of covalently bonded atoms that carry an electric charge (e.g., NH₄⁺, the ammonium ion).

complex life cycle In reference to parasitic species, a life cycle that requires more than one host to complete.

composite transposon Two transposable elements located near one another that transpose together and carry the intervening DNA sequence with them. (See transposable element.)

compound (1) A substance made up of atoms of more than one element. (2) Made up of many units, as in the compound eyes of arthropods.

concentration gradient A difference in concentration of an ion or other chemical substance from one location to another, often across a membrane. (See active transport, facilitated diffusion.)

G-9

concerted evolution The common evolution of a family of repeated genes, such that changes in one copy of the gene family are replicated in other copies of the gene family, and thus evolve “in concert.” (See biased gene conversion, unequal crossing over.)

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.

conditioned reflex A form of associative learning first described by Ivan Pavlov, in which a natural response (such as salivation in response to food) becomes associated with a normally unrelated stimulus (such as the sound of a bell or a metronome).

conduction The transfer of heat from one object to another through direct contact.

cone cells In the vertebrate retina, photoreceptor cells responsible for color vision.

cone In conifers, a reproductive structure consisting of spore-bearing scales extending from a central axis. (Contrast with strobilus.)

confidence interval A numerical interval that is calculated to contain the true value of some parameter of interest at a stated probability level. For example, a 95% confidence interval contains the true value of some parameter of interest in 95% of cases.

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.

connective tissue A type of tissue that connects or surrounds other tissues; its cells are embedded in a collagen-containing matrix. One of the four major tissue types in multicellular animals, including cartilage, bone, blood, and fat.

connexon In a gap junction, a protein channel linking adjacent animal cells.

conservation biology An integrative scientific discipline that relies on principles of ecology, economics, social science, and policy to protect and manage Earth’s biodiversity.

conspecifics Individuals of the same species.

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 proteins Proteins that an organism produces all the time, and at a relatively constant rate.

consumption efficiency The proportion of available biomass that is ingested by consumers.

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.

contraception Birth control methods that prevent fertilization or implantation (conception).

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.

controlled experiment An experiment in which a sample is divided into groups whereby experimental groups are exposed to manipulations of an independent variable while one group serves as an untreated control. The data from the various groups are then compared to see if there are changes in a dependent variable as a result of the experimental manipulation. (Contrast with comparative experiment.)

controlled system A set of components in a physiological system that is controlled by commands from a regulatory system. (Contrast with regulatory system.)

convection The transfer of heat to or from a surface via a moving stream of air or fluid.

convergent evolution Independent evolution of similar features from different ancestral traits.

convergent extension Movements of cells in the sea urchin blastula that form the archenteron. These cells elongate, flatten, and interdigitate to form a tubular structure.

convolutions Foldings of the vertebrate brain’s cerebral cortex into ridges called gyri and valleys called sulci. The level of cortical convolution increases taxonomically and is especially extensive in humans.

coprophagy The consumption of feces.

copulation Reproductive behavior that results in a male depositing sperm in the reproductive tract of a female.

coral bleaching The loss of color in coral colonies, usually because of the loss or decline of symbiotic zooxanthellae.

Coriolis effect The deflection of air or water as a result of differences in Earth’s rotational speed at different latitudes.

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 The clear, transparent tissue that covers the eye and allows light to pass through to the retina.

coronary artery An artery that serves the cardiac muscles.

coronary thrombosis A blood clot in a blood vessel of the heart.

corpora allata The insect endocrine gland that secretes juvenile hormone.

corpora cardiaca An insect endocrine gland that secretes PTTH.

corpus luteum (kor′ pus loo′ tee um) (plural: corpora lutea) [L. yellow body] A structure formed from a follicle after ovulation; produces hormones important to the maintenance of pregnancy.

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).

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).

corticosteroids Steroid hormones produced and released by the cortex of the adrenal gland.

corticotropin A tropic hormone produced by the anterior pituitary hormone that stimulates cortisol release from the adrenal cortex. Also called adrenocorticotropic hormone (ACTH).

corticotropin-releasing hormone (CRH) A hormone produced by the hypothalamus that controls the release of cortisol from the anterior pituitary.

cortisol A corticosteroid that mediates stress responses.

cost–benefit approach An approach to evolutionary studies that assumes an animal has a limited amount of time and energy to devote to each of its activities, and that each activity has fitness costs as well as benefits. (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 flow An arrangement that promotes the maximum exchange of heat, or of a diffusible substance, between two fluids by having the fluids flow in opposite directions through parallel vessels close together.

countercurrent heat exchange In “hot” fishes, an adaptation of the circulatory system such that arterial blood flowing into the muscles is warmed by venous blood flowing out of the muscles, thereby conserving body heat by countercurrent exchange.

countercurrent multiplier The mechanism that increases the concentration of the interstitial fluid in the mammalian kidney through countercurrent flow in the loops of Henle and selective permeability and active transport of ions by segments of the loops of Henle.

coupled human–natural system ecology The practice of encouraging biodiversity and sustainability in systems where humans and nature are intricately linked.

G-10

coupled transporters Membrane proteins that transport two substances across a membrane, often with the transport of one driving the transport of another.

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.

cranial nerves The 12 pairs of nerves that go directly from the brain to different parts of the body without going down the spinal cord.

crassulacean acid metabolism (CAM) A metabolic pathway enabling the plants that possess it to store carbon dioxide at night and then perform photosynthesis during the day with stomata closed.

creatine phosphate (CP) A phosphorylated creatine molecule that is abundant in skeletal muscle and can shuttle high energy phosphate from mitochondria to myofibrils.

CRISPR In archaea, clustered regularly interspaced short palindromic sequences in DNA that are adjacent to sequences from invading viruses. When the two sequences are transcribed, the RNA binds a complementary RNA that binds a nuclease, Cas9, that can cleave and inactivate the genome of the invading virus. This mechanism can be adapted for inactivating and mutating any gene.

critical period See sensitive period.

crop A simple food storage sac, the first of two stomachlike organs in many animals (including reptiles, earthworms, and various insects). (See also gizzard.)

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.

crosstalk Interactions between different signal transduction pathways.

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.

crypsis [Gk. kryptos: hidden] The resemblance of an organism to some part of its environment, which helps it to escape detection by enemies.

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 cilia-bearing plates. Ctenophores move by beating the cilia on their eight ctenes.

cumulus A thick gelatinous layer that protects a mammalian ovum.

cupula Gelatinous swelling in the semicircular canals of the vestibular system. A cupula encloses hair cell stereocilia that react to shifting fluid in the canal ducts.

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 AMP See cAMP.

cyclic electron transport In photosynthetic light reactions, the flow of electrons that produces ATP but not 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, gene products whose spatial distribution may determine such things as embryonic axes.

cytoplasmic segregation The asymmetrical distribution of cytoplasmic determinants in a developing animal embryo.

cytoplasmic streaming The flow of cytoplasm around fungal and plant cells.

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.

cytotoxic T (T_C) cells Cells of the cellular immune system that recognize and directly eliminate virus-infected cells. (Contrast with T-helper cells.)

D

DAG See diacylglycerol.

daily torpor A daily reduction in metabolic rate and body temperature to conserve energy.

dalton (DA) A measure of mass for atoms equal to the atomic mass unit.

data Quantified observations about a system under study.

daughter chromosomes During mitosis, the separated chromatids from the beginning of anaphase onward.

dead space The lung volume that fails to be ventilated with fresh air (because the lungs are never completely emptied during exhalation).

declarative memory Memory of people, places, events, and things that can be consciously recalled and described.

decomposer See detritivore.

decomposition The physical and chemical breakdown of detritus by bacteria and detritivores, leading to the release of nutrients and energy.

deductive logic Logical thought process that starts with a premise believed to be true then predicts what facts would also have to be true to be compatible with that premise. (Contrast with inductive logic.)

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 and beriberi) caused by chronic lack of any essential nutrient.

delayed hypersensitivity Allergic response in which a T cell clone resulting from binding to an antigen-presenting cell releases of cytokines over a period of hours to days, resulting in such effects as inflammation and rash. (Contrast with immediate hypersensitivity.)

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.

demographic stochasticity Fluctuations in population size as the result of random differences among individuals in reproduction and survival.

demography The study of how births, deaths, immigration, and emigration affect population changes over time.

denaturation Loss of activity of an enzyme or nucleic acid molecule as a result of structural changes induced by heat or other means.

dendrites [Gk. dendron: tree] Branching fibers (processes) of a neuron. Dendrites are usually relatively short compared with the axon, and commonly carry information to the neuronal cell body.

denitrifiers Bacteria that release nitrogen to the atmosphere as nitrogen gas (N₂).

density-dependent Factors such as limiting resources, predators, or pathogens that have a negative effect on population size as a consequence of the density of individuals. (Contrast with density-independent.)

density-independent Factors such as the physical environment that have a negative effect on population size independent of the density of the population. (Contrast with density-dependent.)

dentine The layer of dense bony material beneath the enamel of the tooth.

deoxyribose A five-carbon sugar found in nucleotides and DNA.

depolarization A change in the resting potential across a membrane so that the inside of the cell becomes less negative, or even positive, compared with the outside of the cell. (Contrast with hyperpolarization.)

G-11

deprivation experiments Raising an animal in an environment devoid of behavioral models from which it could learn a species specific behavior to see if the behavior is still expressed.

derived trait A trait that differs from the ancestral trait. (Contrast with synapomorphy.)

dermal tissue system The system of cells that forms 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.

desmosome (dez′ mo sowm) [Gk. desmos: bond + soma: body] An adhering junction between animal cells.

desmotubule A membrane extension connecting the endoplasmic reticulum of two plant cells that traverses the plasmodesma.

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 (becomes determined).

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 (detritus) of other organisms, releasing nutrients and energy. (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.

diacylglycerol (DAG) In hormone action, the second messenger produced by hydrolytic removal of the head group of certain phospholipids.

diaphragm (dye′ uh fram) [Gk. diaphrassein: barricade] (1) A sheet of muscle that separates the thoracic and abdominal cavities in mammals; responsible for breathing. (2) A method of birth control 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.

diencephalon The portion of the vertebrate forebrain that develops into the thalamus and hypothalamus.

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. (Contrast with active transport, diffusion.)

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 a body derived from only two embryonic cell layers (ectoderm and endoderm, but no mesoderm) (Contrast with monoblastic, 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 (notably among insects) in which hatchlings look like miniature versions of adults. (Contrast with metamorphosis.)

direct fitness That component of fitness resulting from an organism producing its own offspring. (Contrast with inclusive fitness.)

direct interaction An interaction that occurs between two species, such as predation, competition, or a positive interaction. (Compare with indirect interaction.)

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).

discoidal cleavage In animal development, a type of incomplete cleavage that is common in fishes, reptiles, and birds, the eggs of which contain a dense yolk mass.

discrete variables Quantitative variables that take on only whole number values.

dispersal Movement of individuals into (immigration) or out of (emigration) an existing population.

dispersion patterns The spatial arrangement of individuals in space within a population. (See regular, random, and clumped patterns.)

disruptive selection Selection in which phenotypes at both extremes of the population distribution are favored. (Contrast with directional selection, stabilizing selection.)

dissociation constant (K_D) A type of equilibrium constant measuring the tendency of a two substances that are bound together to separate into the two smaller components. In cell signaling, the two substances are a ligand and receptor.

distal convoluted tubule The portion of a renal tubule from where it reaches the renal cortex, just past the loop of Henle to where it joins a collecting duct. (Compare with proximal convoluted tubule.)

disturbance An abiotic event that physically or chemically injures or kills some individuals, creating opportunities for other individuals to grow and/or reproduce. (Contrast with stress.)

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 helicase An enzyme that unwinds the double helix.

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 testing In human genetics, the determination of genotype by analysis of DNA sequence.

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.)

dormancy A condition in which normal activity is suspended, as in some spores, seeds, and buds.

G-12

dorsal [L. dorsum: back] Toward or pertaining to the back or upper surface. (Contrast with ventral.)

dorsal lip In amphibian embryos, the dorsal segment of the blastopore. Also called the organizer, this region directs the development of nearby embryonic regions.

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).

double helix Refers to DNA and the (usually right-handed) coil configuration of two complementary, antiparallel strands.

duodenum (do′ uh dee′ num) The beginning portion of the vertebrate small intestine. (See also ileum, jejunum.)

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.)

E

early successional species See pioneer species.

easterlies Prevailing winds that blow from east to west at high latitudes. (Compare with westerlies.)

ecdysone (eck die′ sone) [Gk. ek: out of + dyo: to clothe] In insects, a hormone that induces molting.

ecology [Gk. oikos: house] The scientific study of the interrelationships of organisms with their living (biotic) and nonliving (abiotic) environments.

ecosystem (eek′ oh sis tum) The organisms of a particular community together with the physical and chemical environment in which they live.

ecosystem engineering species Species that create, modify, or maintain physical habitat for themselves and other species, and can be keystone or foundation species as well.

ecosystem science A branch of ecology that considers how energy flows and nutrients cycle through communities.

ecosystem values Measures of the economic and biophysical values of ecosystem services to humans.

ecosystems services Benefits that humans obtain from ecosystems such as natural products, protective and regulating services, and recreational, aesthetic, and spiritual enrichment.

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.

ectoparasite A parasite that lives on the surface of its host organism. (Compare with endoparasite.)

ectotherm [Gk. ektos: outside + thermos: heat] An animal that is dependent on external heat sources for regulating its body temperature (Contrast with endotherm.)

edge effects Abiotic and biotic changes to the edge of a habitat brought about by habitat fragmentation.

effective population size The number of individuals within a population that can contribute offspring to the next generation of the population.

effector A component of a physiological system that responds to information by effecting changes (making change happen) in the internal environment; examples include muscles and the secretory cells of the digestive tract.

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.

effector mechanisms Adaptations that enable an organism to alter its physiological or behavioral state.

effector protein In cell signaling, a protein responsible for the cellular response to a signal transduction pathway.

efferent (ef′ ur unt) [L. ex: out + ferre: to bear] Carrying outward or away from, as in neurons that carry impulses outward from the central to the peripheral nervous system (efferent neurons), or a blood vessel that carries blood away from a structure (efferent arterioles). (Contrast with afferent.)

egg In all sexually reproducing organisms, the female gamete; in birds, reptiles, and some other vertebrates, a structure within which early embryonic development occurs. (See also amniote egg, ovum.)

ejaculatory duct The duct that carries semen from the seminal vesicle to the urethra.

electrical synapse A type of synapse at which action potentials spread directly from presynaptic cell to postsynaptic cell. (Contrast with chemical synapse.)

electrocardiogram (ECG or EKG) A graphic recording of electrical potentials from the heart.

electrochemical gradient The concentration gradient of an ion across a membrane plus the voltage difference across that membrane.

electroencephalogram (EEG) A graphic recording of electrical potentials from the brain.

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.

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 a simpler substance by ordinary chemical means.

elicitors Molecules associated with plant pathogens that attach to specialized receptors in plants to initiate a defensive response.

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.

embolus (em′ buh lus) [Gk. embolos: stopper] A circulating blood clot. Blockage of a blood vessel by an embolus or a bubble of gas is called an embolism. (Contrast with thrombus.)

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 an egg, uterus, or seed.

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 stem cell (ESC) A pluripotent cell in the blastocyst.

emigration Dispersal of individuals out of an existing population. (Contrast with immigration.)

enamel The hard material, composed primarily of calcium phosphate, that covers the tooth.

endemic (en dem′ ik) [Gk. endemos: native] A species that occurs in a particular location and nowhere else on Earth.

endergonic A chemical reaction in which the products have higher free energy than the reactants, thereby requiring free energy input to occur. (Contrast with exergonic.)

endocrine gland (en′ doh krin) [Gk. endo: within + krinein: to separate] An aggregation of secretory cells that secretes hormones into the blood. The endocrine system consists of all endocrine cells and endocrine glands in the body that produce and release hormones. (Contrast with exocrine gland.)

endocrines Chemical signals released by cells that enter the blood and affect cells anywhere in the body that have appropriate receptors. Also known as hormones.

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.

G-13

endogenous retroviruses Retroviruses that have been incorporated into the host’s genome.

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 epithelial lining of the uterus.

endoparasite A parasite that lives inside the body of its host organism. (Compare with ectoparasite.)

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.

endorphins Molecules in the mammalian brain that act as neurotransmitters in pathways that control pain.

endoskeleton [Gk. endo: within + skleros: hard] An internal skeleton covered by other, soft body tissues. (Contrast with exoskeleton.)

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.

endosymbiosis theory [Gk. endo: within + sym: together + bios: life] The theory that the eukaryotic cell evolved via the engulfing of one prokaryotic cell by another.

endotherm [Gk. endo: within + thermos: heat] An animal that can control its body temperature by the expenditure of its own metabolic energy. (Contrast with ectotherm.)

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.)

energetic cost The difference between the energy an animal expends in performing a behavior and the energy it would have expended had it rested. (Compare with opportunity cost, risk cost.)

energy budget A quantitative description of all paths of energy exchange between an animal and its environment.

energy The capacity to do work or move matter against an opposing force. The capacity to accomplish change in physical and chemical systems.

enhancers Regulatory DNA sequences that bind transcription factors that either activate or increase the rate of transcription.

enkephalins Molecules in the mammalian brain that act as neurotransmitters in pathways that control pain.

enteric nervous system The nerve nets in the submucosa and between the smooth muscle layers of the vertebrate gut.

enthalpy (H) The total energy of a system.

entrain To advance or delay an organism’s circadian clock each day so that it is in phase with the light-dark cycle of the organism’s environment.

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.

enveloped virus A virus enclosed within a phospholipid membrane derived from its host cell.

environmental genomics Sequencing technique used when biologists are unable to work with the whole genome of a prokaryote species but instead examine individual genes collected from a random sample of the organism’s environment.

environmentalism The use of ecological knowledge, along with economics, ethics, and many other considerations, to inform both personal decisions and public policy relating to stewardship of natural resources and ecosystems.

enzyme–substrate complex (ES) An intermediate in an enzyme-catalyzed reaction; consists of the enzyme bound to its substrate(s).

epiblast The upper or overlying portion of the avian blastula which is joined to the hypoblast at the margins of the blastodisc.

epiboly The movement of cells over the surface of the blastula toward the forming blastopore.

epidermis [Gk. epi: over + derma: skin] In plants and animals, the outermost cell layers. (Only one cell layer thick in plants.)

epididymis (epuh did′ uh mus) [Gk. epi: over + didymos: testicle] Coiled tubules in the testes that store sperm and conduct sperm from the seminiferous tubules to the vas deferens.

epigenetics The scientific study of changes in the expression of a gene or set of genes that occur without change in the DNA sequence.

epiglottis The flap of tissue that covers the entrance of the larynx to prevent food and liquid from entering the trachea.

epinephrine (ep i nef′ rin) [Gk. epi: over + nephros: kidney] The “fight or flight” hormone produced by the medulla of the adrenal gland; it also functions as a neurotransmitter. Also known as adrenaline.

epiphyseal plates Plates of cartilage near the ends of long bones where bone growth occurs up to adolescence when growth ceases.

epistasis Interaction between genes in which the presence of a particular allele of one gene determines whether another gene will be expressed.

epithelial tissue A type of animal tissue made up of sheets of cells that lines or covers organs, makes up tubules, and covers the surface of the body; one of the four major tissue types in multicellular animals.

epitope See antigenic determinant.

epoch A subdivision of a period in a geological time scale.

equilibrium Any state of balanced opposing forces and no net change.

ER See endoplasmic reticulum.

erection The state of the penis in which it is stiffened by engorgement with blood.

error signal In regulatory systems, any difference between the set point of the system and its current condition.

ERV See expiratory reserve volume.

erythrocyte (ur rith′ row site) [Gk. erythros: red + kytos: container] A red blood cell.

erythropoietin (EPO) (ur rith′ row poi′ i tn) A hormone produced by the kidney in response to lack of oxygen that stimulates the production of red blood cells.

ESC See embryonic stem cell.

esophagus (i soff′ i gus) [Gk. oisophagos: gullet] That part of the gut between the pharynx and the stomach.

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.

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.)

estivation (ess tuh vay′ shun) [L. aestivalis: summer] A state of dormancy and hypometabolism that occurs during the summer; usually a means of surviving drought and/or intense heat. (Contrast with hibernation.)

estrogen Any of several steroid sex hormones; produced chiefly by the ovaries in mammals.

estrus (es′ trus) [L. oestrus: frenzy] The period of heat, or maximum sexual receptivity, in some female mammals. Ordinarily, estrus is also the time of release of eggs in the female.

estuary An aquatic habitat in which salt water and fresh water mix, as when a river meets the ocean. Includes such ecosystems as salt marshes, mangrove forests, mudflats, and seagrass beds.

ether linkage The linkage of two hydro-carbons by an oxygen atom (HC—O—CH). Ether linkages are characteristic of the membrane lipids of the Archaea. (Contrast with ester linkage.)

G-14

ethology [Gk. ethos: character + logos: study] An approach to the study of animal behavior that focuses on studying many species in natural environments and addresses questions about the evolution of behavior. (Compare with behaviorism.)

ethylene One of the plant growth hormones, the gas H₂C═CH₂. Involved in fruit ripening and other growth and developmental responses.

etiolated Process in flowering plants grown in the absence of light; characterized by long weak stems, smaller leaves due to longer internodes, and a pale yellow color.

euchromatin Diffuse, uncondensed chromatin. Contains active genes that will be transcribed into mRNA. (Contrast with heterochromatin.)

eudicots Angiosperms with two embryonic cotyledons. (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, prokaryotic archaea, and bacteria. (Contrast with prokaryotes.)

eusocial Pertaining to a social group that includes nonreproductive individuals, as in honey bees.

eustachian tube A connection between the middle ear and the throat that allows air pressure to equilibrate between the middle ear and the outside world. Also called the pharyngotympanic tube.

eutrophication (yoo trofe′ ik ay′ shun) [Gk. eu: truly + trephein: to flourish] A process in aquatic ecosystems initiated by a pulse of nutrients that results in an increase of algal growth that may lead to decomposition and depleted oxygen conditions (hypoxia).

evaporation The transition of water from the liquid to the gaseous phase.

evapotranspiration The evaporative transfer of heat and water from the surfaces of plants into the atmosphere, which reduces air temperature and increases moisture.

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. (See macroevolution, microevolution. 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 radiation The proliferation of many species within a single evolutionary lineage.

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.

ex vivo gene therapy Alteration of the human genome sequence in the cells removed from the body and then reimplanted into the body for medical benefit.

excision repair DNA repair mechanism that removes damaged DNA and replaces it with the appropriate nucleotide.

excretory systems In animals, organs that maintain the volume, solute concentration, and composition of the extracellular fluid by excreting water, solutes, and nitrogenous wastes in the form of urine.

executive functions Cognitive processes (e.g., attentional control, inhibitory control, reasoning, problem solving, planning) that allow goal-directed control of thought and behavior.

exergonic A chemical reaction in which the products of the reaction have lower free energy than the reactants, resulting in a release of free energy. (Contrast with endergonic.)

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.)

expanding triplet repeat A three-base-pair sequence in a human gene that is unstable and can be repeated a few to hundreds of times. Often, the more the repeats, the less the activity of the gene involved. Expanding triplet repeats occur in some human diseases such as Huntington’s disease and fragile-X syndrome.

expiratory reserve volume (ERV) The amount of air that can be forcefully exhaled beyond the normal tidal expiration. (Contrast with inspiratory reserve volume, tidal volume, vital capacity.)

exploitation competition Competition in which both species reduce the quantities of their shared but limited resources. (Contrast with interference competition.)

exponential growth A pattern of population growth characterized by a multiplicative pattern in which, as the number of individuals increases, the number of new individuals added per unit time accelerates. (Contrast with logistic growth.)

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.

extensor A muscle that extends an appendage. (Contrast with flexor.)

external fertilization The release of gametes into the environment; typical of aquatic animals. Also called spawning. (Contrast with internal fertilization.)

external gills Highly branched and folded extensions of the body surface that provide a large surface area for gas exchange with water; typical of larval amphibians and many larval insects.

extracellular fluid In closed circulatory systems refers to the fluid in the circulatory system and the fluid outside it.

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, yolk sac.)

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.

eye cups Photosensory organs in flatworms; components of one of the simplest visual systems in animals.

F

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.)

facilitation A positive interaction. See positive interaction.

facultative A type of species interaction that is optional to at least one species involved in the interaction. (Compare with obligate.)

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. (Compare with obligate anaerobe.)

facultative parasite An organism that can parasitize other living organisms but is also capable of growing independently.

fast-twitch fibers Skeletal muscle fibers that can generate high tension rapidly, but fatigue rapidly (“sprinter” fibers). Characterized by an abundance of enzymes of glycolysis. (Compare to slow-twitch fibers.)

G-15

fat (1) A triglyceride that is solid at room temperature. (Contrast with oil.) (2) Adipose tissue, one type of connective tissue. (See brown fat.)

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.)

FD (FLOWERING LOCUS D) Gene coding for a transcription factor in the shoot apical meristem that binds to florigen; involved in the induction of flowering.

feather A structure growing from the skin of theropods (living birds and their extinct relatives) that consists of a hollow shaft fringed with vanes of barbs. Functions to support flight, plumage display, and thermoregulation.

feces [L. faeces: dregs] Waste excreted from the digestive system.

fecundity (m_x) The average number of offspring produced by each individual (or female) within the population.

feedback In regulatory systems, information about the relationship between the set point of the system and its current state. (Contrast with feedforward information.)

feedforward information In regulatory systems, information that changes the set point of the system. (Contrast with feedback.)

fenestrations Small holes in the walls of some capillaries.

fermentation (fur men tay′ shun) [L. fermentum: yeast] The anaerobic degradation of a substance such as glucose to smaller molecules such as lactic acid or alcohol with the extraction of energy.

fertilization Union of gametes. Also known as syngamy.

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.

fiber In angiosperms, an elongated, tapering sclerenchyma cell, usually with a thick cell wall, that serves a support function in xylem. (See also muscle fiber.)

fibrin A protein that polymerizes to form long threads that provide structure to a blood clot.

fibrinogen A circulating protein that can be stimulated to fall out of solution and provide the structure for a blood clot.

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 An equation that describes the factors that determine the rate of diffusion of a molecule from an area of higher concentration to an area of lower concentration.

fight-or-flight response A rapid physiological response to a sudden threat mediated by the hormone epinephrine.

filament In flowers, the part of a stamen that supports the anther.

filter feeder An organism that feeds on organisms much smaller than itself that are suspended in water or air by means of a straining device. Also called suspension feeder.

first filial generation See F₁.

first law of thermodynamics The principle that energy can be neither created nor destroyed.

first polar body The small daughter cell from the first meiotic division of a primary oocyte.

fitness The contribution of a genotype or phenotype to the genetic composition of subsequent generations, relative to the contribution of other genotypes or phenotypes. (See also inclusive fitness.)

fixed action pattern In ethology, a genetically determined behavior that is performed without learning, stereotypic (performed the same way each time), and not modifiable by learning.

fixed In population genetics, the change in a gene pool where there are at least two alleles of a gene to a situation where only one allele remains.

flagellum (fla jell′ um) (plural: flagella) [L. flagellum: whip] Long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes.

flexor A muscle that flexes an appendage. (Contrast with extensor.)

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 A plant hormone involved in the conversion of a vegetative shoot apex to a flower.

flower The sexual structure of an angiosperm.

fluid feeder An animal that feeds on fluids it extracts from the bodies of other organisms; examples include nectar-feeding birds and blood-sucking insects.

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.

foliose Having a leafy growth form.

follicle [L. folliculus: little bag] In female mammals, an immature egg surrounded by nutritive cells.

follicle-stimulating hormone (FSH) A gonadotropin produced by the anterior pituitary.

food web A representation of the trophic or energetic connections among species within a community.

forebrain The region of the vertebrate brain that comprises the cerebrum, thalamus, and hypothalamus. (Compare with hindbrain, midbrain.)

fossil fuels Fuels, including oil, natural gas, coal, and peat, formed over geologic time from organic material buried in anaerobic sediments.

foundation species A species that has large, community-wide effects as a consequence of its large size and abundance, and its provision of habitat or food for other species.

founder effect Random changes in allele frequencies resulting from establishment of a population by a very small number of individuals.

fovea [L. fovea: a small pit] In the vertebrate retina, the area of most distinct vision.

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.)

Frank–Starling law The principle that the stroke volume of the heart increases with increased return of blood to the heart.

free energy (G) Energy that is available for doing useful work, after allowance has been made for the increase or decrease of disorder.

free-running A descriptor of an endogenous rhythm that is not entrained by an environmental cue.

frequency distribution A figure that displays the frequency of different classes of data.

frequency-dependent selection Selection that changes in intensity with the proportion of individuals in a population having the trait.

frontal lobe The largest of the brain lobes in humans; involved with feeling and planning functions; includes the primary motor cortex.

frugivore [L. frugis: fruit + vorare: to devour] An animal that eats fruit.

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.

FT (FLOWERING LOCUS T) Gene that codes for florigen, a small, diffusible protein involved in the induction of flowering.

fugitive species A species that leave an otherwise suitable habitat in order to avoid competition with another species.

full census A count of every individual in a population.

G-16

functional genomics The assignment of functional roles to the proteins encoded by genes identified by sequencing entire genomes.

functional group (1) A characteristic combination of atoms that contributes specific properties (such as charge or polarity) when attached to larger molecules (e.g., carboxyl group, amino group). (2) A group of species that function in similar ways, whether or not they use the same resources. (Compare guild.)

functional residual volume (FRV) The residual volume plus the expiratory reserve volume.

fundamental niche A species’ niche as defined by its physiological capabilities. (Contrast with realized niche.)

G

G protein A membrane protein involved in signal transduction; characterized by binding GDP or GTP.

G protein–coupled 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.

G0 A resting phase in which a cell is not preparing for the cell division cycle.

G1 In the cell cycle, the gap between the end of mitosis and the onset of the S phase.

G1-to-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.)

gallbladder In the human digestive system, an organ in which bile is stored.

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 that leads to the production of gametes. Called oogenesis when referring to processes in the ovary and spermatogenesis in the testis.

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.)

gamma diversity Species diversity within a geographic region; the regional species pool. (Compare with alpha diversity, beta diversity.)

ganglion (gang′ glee un) (plural: ganglia) [Gk. lump] A cluster of neurons that have similar characteristics or function.

ganglion cells Cells at the front of the human retina that transmit information from the bipolar cells to the brain.

gap genes In Drosophila 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.

gap junction A 2.7-nanometer gap between cell membranes of two animal cells, spanned by protein channels. Gap junctions allow chemical substances or electrical signals to pass from cell to cell.

gastric pits Deep infoldings in the walls of the stomach lined with secretory cells.

gastrin A hormone secreted by cells in the lower region of the stomach that stimulates the secretion of digestive juices as well as movements of the stomach.

gastrointestinal system See gut.

gastrovascular cavity Serving for both digestion (gastro) and circulation (vascular); in particular, the central cavity of the body of jellyfish and other cnidarians.

gastrovascular system A branching body cavity in Cnidaria and Platyhelminthes that has one opening to the environment and serves to digest food and distribute nutrients to the 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 into a gastrula, an embryo with three germ layers and distinct body axes.

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 duplication The generation of extra copies of a gene in a genome over evolutionary time. A mechanism by which genomes can acquire new functions.

gene expression The transcription and translation into a protein of the information (nucleotide sequence) contained in a gene.

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 pool All of the different alleles of all of the genes existing in all individuals of a population.

gene therapy Treatment of a genetic disease by providing patients with cells containing functioning alleles of the genes that are nonfunctional in their bodies.

gene tree A graphic representation of the evolutionary relationships of a single gene in different species or of the members of a gene family.

gene-for-gene concept 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.

generation time (G) The average age of the parents of all the offspring produced within a cohort.

genetic code The set of instructions, in the form of nucleotide triplets, that translate a linear sequence of nucleotides in mRNA into a linear sequence of amino acids in a protein.

genetic drift Changes in gene frequencies from generation to generation as a result of random (chance) processes.

genetic marker (1) In gene cloning, a gene of identifiable phenotype that indicates the presence of another gene, DNA segment, or chromosome fragment. (2) In general, a DNA sequence such as a single nucleotide polymorphism whose presence is correlated with the presence of other linked genes on that chromosome.

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.

genetics The scientific study of the structure, functioning, and inheritance of genes, the units of hereditary information.

genitalia The reproductive organs, especially the external accessory sex organs.

genome (jee′ nome) The complete DNA sequence for a particular organism or individual.

G-17

genome sequencing Determination of the nucleotide base sequence of the entire genome of an organism.

genomic equivalence All somatic cells in a complex organism have the entire genome of that organism.

genomic library All of the cloned DNA fragments generated by the breakdown of genomic DNA into smaller segments.

genomics The scientific study of entire sets of genes and their interactions.

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 within which a species occurs.

geological time scale The division of Earth’s history into ordered, named periods of time, based on major biotic and abiotic changes.

germ cell [L. germen: to beget] A reproductive cell or gamete of a multicellular organism. (Contrast with somatic cell.)

germ layers The three embryonic layers formed during gastrulation (ectoderm, mesoderm, and endoderm). Also called cell layers or tissue layers.

germ line gene therapy Alteration of the human genome sequence in the gametes or cells giving rise to gametes for medical benefit. (Contrast with somatic cell gene therapy.)

germ line mutation Mutation in a cell that produces gametes (i.e., a germ line cell). (Contrast with somatic mutation.)

germination Sprouting of a seed or spore.

gestation (jes tay′ shun) [L. gestare: to bear] The period during which the embryo of a mammal develops within the uterus. Also known as pregnancy.

GH See growth hormone.

GHIH See growth hormone inhibiting hormone.

ghrelin A hormone produced and secreted by cells in the stomach that stimulates appetite.

gibberellin (jib er el′ lin) A class of plant growth hormones playing roles in stem elongation, seed germination, flowering of certain plants, etc.

gizzard (giz′ erd) [L. gigeria: cooked chicken parts] The second of two stomachlike organs in birds, other reptiles, earthworms, and various insects, that grinds up food, sometimes with the aid of fragments of stone. (See also crop.)

glans penis The highly sensitive tip of the mammalian penis.

glia (glee′ uh) [Gk. glia: glue] One of the two classes of neural cells (along with neurons, with which glia interact); glia do not typically conduct action potentials. Types of glia include astrocytes, oligodendrocytes, and Schwann cells.

glomerular filtration rate (GFR) The rate at which the blood is filtered in the glomeruli of the kidney.

glomerulus (glo mare′ yew lus) (plural: glomeruli) [L. glomus: ball] Sites in the kidney where blood filtration takes place. Each glomerulus consists of a knot of capillaries served by afferent and efferent arterioles.

glucagon Hormone produced by alpha cells of the pancreatic islets of Langerhans. Glucagon stimulates the liver to break down glycogen and release glucose into the circulation.

gluconeogenesis The biochemical synthesis of glucose from other substances, such as amino acids, lactate, and glycerol.

glucose [Gk. gleukos: sugar, sweet wine] The most common monosaccharide; the monomer of the polysaccharides starch, glycogen, and cellulose.

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.

glycogen (gly′ ko jen) [Gk. glyk: sweet] An energy storage polysaccharide found in animals and fungi; a branched-chain polymer of glucose, similar to starch.

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.

glycosidic linkage Bond between carbohydrate (sugar) molecules through an intervening oxygen atom (–O–).

glycosylation The addition of carbohydrates to another type of molecule, such as a protein.

glyoxysome (gly ox′ ee soam) An organelle found in plants, in which stored lipids are converted to carbohydrates.

goiter Swelling in the neck resulting from an enlarged thyroid gland.

Goldman equation An equation that calculates membrane potential by taking into consideration the concentration differences of all ions on both sides of the membrane and the relative permeability of the membrane to those ions.

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.

Golgi tendon organ A mechanoreceptor found in tendons and ligaments; provides information about the force generated by a contracting muscle.

gonad (go′ nad) [Gk. gone: seed] An organ that produces gametes in animals: either an ovary (female gonad) or testis (male gonad).

gonadotropin A trophic hormone that stimulates the gonads.

gonadotropin-releasing hormone (GnRH) Hormone produced by the hypothalamus that stimulates the anterior pituitary to secrete 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.

graded membrane potential Small local change in membrane potential caused by opening or closing of ion channels.

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.

granulosa cells Cells of the ovarian follicle of mammals that surround and nurture the oocyte.

gravitropism [L. gravitas: weight, force + 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.

gray matter In the nervous system, tissue that is rich in neuronal cell bodies. (Contrast with white matter.)

greenhouse gases Gases in the atmosphere, such as carbon dioxide and methane, that are transparent to sunlight, but trap heat radiating from Earth’s surface, causing heat to build up at Earth’s surface.

gross primary production (GPP) The total amount of carbon fixed by primary producers in an ecosystem. (Compare with net primary production.)

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.

growth An increase in the size of the body and its organs by cell division and cell expansion.

growth factor A chemical signal that stimulates cells to divide.

growth hormone (GH) A peptide hormone released by the anterior pituitary that stimulates many anabolic processes.

G-18

growth hormone inhibiting hormone (GHIH) Also called somatostatin, a peptide hormone produced in the hypothalamus that inhibits the activity of growth hormone secreting cells in the anterior pituitary.

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).

guild A group of species, often taxonomically distinct, that use the same resources. (Compare with functional group.)

gustation The sense of taste.

gut An animal’s digestive tract.

gymnosperms Seed plants that do not produce flowers or fruits; one of the two major groups of living seed plants. (See also angiosperms.)

gyres Large-scale circular ocean currents caused by prevailing winds and Earth’s rotation.

gyri (singular: gyrus) The ridges (folds) in the cerebral cortex.

H

habitat The particular environment in which an organism lives.

habitat degradation The conversion of an ecosystem by human activities that reduces habitat quality. (Compare with habitat loss.)

habitat loss The conversion of an ecosystem by human activities that reduces habitat quantity. (Compare with habitat degradation.)

hair cell A type of mechanoreceptor in animals. Detects sound waves and other forms of motion in air or water.

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.

halophytes Organisms that can grow in high salinity.

Hamilton’s rule The principle that, for an apparent altruistic behavior to be adaptive, the fitness benefit of that act to the recipient times the degree of relatedness of the performer and the recipient must be greater than the cost to the performer.

haplodiploidy A sex determination mechanism in which diploid individuals (which develop from fertilized eggs) are female and haploid individuals (which develop from unfertilized eggs) are male; typical of hymenopterans.

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 In a sexually reproducing population, the allele frequency at a given locus that is not being acted on by agents of evolution; the conditions that would result in no evolution in a population.

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.

Haversian systems Units of organization in compact bone that reflect the action of intercommunicating osteoblasts.

hCG See human chorionic gonadotropin.

HDLs See high-density lipoproteins.

heart In circulatory systems, a muscular pump that moves extracellular fluid around the body.

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.

hematocrit The proportion of the blood volume that consists of red blood cells.

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.)

hemocoel The primary body cavity of many invertebrates, which contains circulatory fluid.

hemoglobin (hee′ mo glow bin) [Gk. heaema: blood + L. globus: globe] Oxygen-transporting protein found in the red blood cells of vertebrates (and found in some invertebrates).

hemolymph The extracellular fluid in closed circulatory systems.

Hensen’s node In avian embryos, a structure at the anterior end of the primitive groove; determines the fates of cells passing over it during gastrulation.

hepatic portal vein The vein that begins in capillary beds in the gut and ends in capillary beds in the liver.

herbivore (ur′ bi vore) [L. herba: plant + vorare: to devour] An animal that eats plant tissues. (Contrast with carnivore, detritivore, omnivore.)

herbivory (ur′ bi vore) [L. herba: plant + vorare: to devour] An act of predation in which the predator is animal and the prey is a plant or algae. (Contrast with carnivory, parasitism, omnivory.)

hermaphroditism (her maf′ row dite ism) The coexistence of both female and male sex organs in the same organism.

heterochromatin Densely packed, dark-staining chromatin; any genes it contains are usually not transcribed. (Contrast with euchromatin.)

heterochrony [Gk. different time] Alteration in the timing of developmental events, contributing to the evolution of different phenotypes in the adult. (Contrast with heterometry, heterotypy.)

heterocyst A large, thick-walled cell type in the filaments of certain cyanobacteria that performs nitrogen fixation.

heterometry [Gk. different measure] Alteration in the level of gene expression, and thus in the amount of protein produced, during development, contributing to the evolution of different phenotypes in the adult. (Contrast with heterochrony, heterotypy.)

heteromorphic (het′ er oh more′ fik) [Gk. different form] Having a different form or appearance, as two heteromorphic life stages of a plant. (Contrast with isomorphic.)

heterosis The superior fitness of heterozygous offspring as compared with that of their dissimilar homozygous parents. Also called 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.)

heterotherm An animal that regulates its body temperature at a constant level at some times but not others, such as a hibernator.

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 food. (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 heterochrony, heterometry.)

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 hemizygous, homozygous.)

hexose [Gk. hex: six] A sugar containing six carbon atoms.

hibernation [L. hibernum: winter] The state of inactivity of some animals during winter; marked by a drop in body temperature and metabolic rate. (Contrast with estivation.)

high-density lipoproteins (HDLs) Lipoproteins that remove cholesterol from tissues and carry it to the liver; HDLs are the “good” lipoproteins associated with good cardiovascular health.

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. (Compare with forebrain, midbrain.)

G-19

hippocampus [Gk. sea horse] A part of the forebrain that takes part in long-term memory formation.

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.

HIV See human immunodeficiency virus.

holometabolous Undergoing complete metamorphosis.

holoparasite A fully parasitic plant (i.e., one that does not perform photosynthesis). (Contrast with hemiparasite.)

homeobox 180-base-pair segment of DNA found in certain homeotic genes. A specific sequence within the homeobox—the homeodomain—regulates the expression of other genes and through this regulation controls large-scale developmental processes. (See homeotic genes.)

homeodomain See homeobox.

homeostasis (home′ ee o sta′ sis) [Gk. homos: same + stasis: position] The maintenance of a steady state, such as a constant temperature, by means of physiological or behavioral feedback responses.

homeotherms Animals that regulate their body temperatures within a narrow range when confronted with a much greater range of environmental temperatures.

homeotic genes Genes that act during development to determine the formation of an organ from a region of the embryo. (Compare with Hox genes.)

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.

homing In animal navigation, the ability to return to a nest site, burrow, or other specific location.

hominin Lineages that includes modern humans (Homo sapiens) and their extinct ancestors (e.g., Australopithecines, Homo erectus).

homolog (1) In cytogenetics, one of a pair (or larger set) of chromosomes having the same overall genetic composition and sequence. In diploid organisms, each chromosome inherited from one parent is matched by an identical (except for mutational changes) chromosome—its homolog—from the other parent. (2) In evolutionary biology, one of two or more features in different species that are similar by reason of descent from a common ancestor.

homologous pair A pair of matching chromosomes made up of a chromosome from each of the two sets of chromosomes in a diploid organism.

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.)

homotypic Pertaining to adhesion of cells of the same type.

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 hemizygous, heterozygous.)

horizons The horizontal layers of a soil profile, including the topsoil (A horizon), subsoil (B horizon) and parent rock or bedrock (C horizon).

horizontal cells Cells in the retina that communicate laterally between photoreceptors and bipolar cells.

hormone (hore′ mone) [Gk. hormon: to excite, stimulate] A chemical signal produced in minute amounts at one site in a multicellular organism and transported to another site where it acts on target cells.

host An organism that harbors a parasite or symbiont and provides it with nourishment.

Hox genes Conserved homeotic genes found in vertebrates, Drosophila, and other animal groups. Hox genes contain the homeobox and specify pattern and axis formation in these animals.

human chorionic gonadotropin (hCG) A hormone secreted by the placenta that sustains the corpus luteum and helps maintain pregnancy.

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) Human immunodeficiency virus, the retrovirus that causes acquired immune deficiency syndrome (AIDS).

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 See heterosis.

hybrid zone A region of overlap in the ranges of two closely related species where the species may hybridize.

hydrocarbon A compound containing only carbon and hydrogen atoms.

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.

hydrologic cycle The movement of water from the oceans to the atmosphere, to the soil, rivers, and lakes, and back to the oceans.

hydrolysis reaction (highdrol′ 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.)

hydrophobic interaction A weak chemical interaction caused when molecules that are not attracted to water interact to exclude water.

hydroponic Pertaining to a method of growing plants with their roots suspended in nutrient solutions instead of soil.

hydrostatic skeleton A fluid-filled body cavity that transfers forces from one part of the body to another when acted on by surrounding muscles.

hyperaccumulators Plant species that store large quantities of heavy metals such as arsenic, cadmium, nickel, aluminum, and zinc.

hyperpolarization A change in the resting potential across a membrane so that the inside of a cell becomes more negative compared with the outside of the cell. (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.

hypertonic Having a greater solute concentration. Said of one solution compared with another. (Contrast with hypotonic, isotonic.)

hypertonic regulation Maintaining body fluids at a greater solute concentration than the environment. (Contrast with hypotonic regulation.)

hypha (high′ fuh) (plural: hyphae) [Gk. hyphe: web] In the fungi and oomycetes, any single filament.

hypoblast The lower tissue portion of the avian blastula which is joined to the epiblast at the margins of the blastodisc.

hypothalamus The part of the brain lying below the thalamus; it coordinates water balance, reproduction, temperature regulation, and metabolism.

G-20

hypothermia Below-normal body temperature.

hypothesis A tentative answer to a question, from which testable predictions can be generated. (Contrast with theory.)

hypotonic Having a lesser solute concentration. Said of one solution in comparing it to another. (Contrast with hypertonic, isotonic.)

hypotonic regulation Maintaining body fluids at a lesser solute concentration than the environment. (Contrast with hypertonic regulation.)

hypoxia A deficiency of oxygen.

hysteresis The inability of a community that has undergone change to shift back to the initial community type, even when the original conditions are restored.

I

igneous rocks Rocks solidified from lava or magma.

ileum The final segment of the small intestine. (See also duodenum, jejunum.)

imbibition Water uptake by a seed; first step in germination.

immediate hypersensitivity A rapid, extensive overreaction of the immune system against an allergen, resulting in the release of large amounts of histamine. (Contrast with delayed hypersensitivity.)

immediate memory A form of memory for events happening in the present that is almost perfectly photographic, but lasts only seconds. (Contrast with long-term memory, short-term memory.)

immigration Dispersal of individuals into an existing population. (Contrast with emigration.)

immunity [L. immunis: exempt from] 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 lining of the uterus.

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 then maintained for life) to some object or other organism.

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.

in vivo gene therapy Alteration of the human genome sequence in cells in the body for medical benefit.

inactivation gate The property of a voltage-gated Na⁺ ion channel that decreases its permeability to Na⁺ when the membrane is depolarized.

inbreeding Breeding of two closely genetically related individuals.

inbreeding depression Reduction of fitness in the offspring of inbreeding.

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. (See also indirect fitness, kin selection. Contrast with direct fitness.)

incomplete cleavage A pattern of cleavage that occurs in many eggs that have a lot of yolk, in which the cleavage furrows do not penetrate all of it. (See also discoidal cleavage, superficial cleavage. 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. (Contrast with direct development, complete metamorphosis.)

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.

indeterminate growth An 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 fitness The component of inclusive fitness due to the reproductive success of related individuals times the percentage of shared genes between those individuals (e.g., 50% for a sibling).

indirect interaction An interaction in which the relationship between two species is mediated by a third (or more) species. (Compare with direct interaction.)

induced A process that occurs only under certain conditions or signals.

induced fit A change in the shape of an enzyme caused by binding to its substrate that exposes the active site of the enzyme.

induced mutation A mutation resulting from exposure to a mutagen from outside the cell. (Contrast with spontaneous mutation.)

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.

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 Produced only in the presence of a particular compound or under particular circumstances. (Contrast with constitutive.)

inducible genes Genes that are expressed only when their products—inducible proteins—are needed.

induction In embryonic development, the process by which a factor produced and secreted by certain cells determines the fates of other cells.

inductive logic Involves making observations and then formulating one or more possible scenarios—hypotheses—that might explain those observations. (Contrast with deductive logic.)

inferior vena cava Large vein that returns deoxygenated blood to the right atrium from the lower body.

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).

ingroup In a phylogenetic study, the group of organisms of primary interest. (Contrast with outgroup.)

initials Cells that perpetuate plant meristems, comparable to animal stem cells. When an initial divides, one daughter cell develops into another initial, while the other differentiates into a more specialized cell.

initiation In molecular biology, the beginning of transcription or translation.

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.

initiation site The place within a promoter where transcription begins.

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 Derived from the mammalian blastula (blastocyst), the inner cell mass that will give rise to the yolk sac (via hypoblast) and embryo (via epiblast).

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).

inositol trisphosphate (IP₃) An intracellular second messenger derived from membrane phospholipids.

inspiratory reserve volume (IRV) The amount of air that can be inhaled above the normal tidal inspiration. (Contrast with expiratory reserve volume, tidal volume, vital capacity.)

G-21

instar (in′ star) An immature stage of an insect between molts.

insular cortex The outer layers of the insular lobe.

insular lobe A region of the cerebrum that folds in between the temporal lobes and the overlying frontal and parietal lobes.

insulin (in′ su lin) [L. insula: island] A hormone synthesized in islet cells of the pancreas that promotes the conversion of glucose into the storage material, glycogen.

integral membrane proteins Proteins that are at least partially embedded in the plasma membrane. (Contrast with peripheral membrane proteins.)

integrin In animals, a transmembrane protein that mediates the attachment of epithelial cells to the extracellular matrix.

integument [L. integumentum: covering] A protective surface structure. In gymnosperms and angiosperms, a layer of tissue around the ovule that will become the seed coat.

interaction strength A measure of the effect of one species (the interactor) on the abundance of another species (the target species).

interaction webs A concept that describes both the trophic (vertical) and non-trophic (horizontal; such as competition, mutualism, commensalism) interactions among the species in a food web.

intercalated discs Structural features of cardiac muscle cells that connect cells together and provide electrical continuity through gap junctions.

intercostal muscles Muscles between the ribs that can augment breathing movements by elevating and suppressing the rib cage.

interference competition Competition in which one species actively interferes with another species’ access to a limiting resource. (Contrast with exploitation competition.)

interference RNA (RNAi) See RNA interference.

interferons Glycoproteins produced by virus-infected animal cells; interferons increase the resistance of neighboring cells to the virus.

intermediate disturbance hypothesis A hypothesis describing how varying degrees of disturbance (or stress or predation) affects species diversity within communities. Species diversity is greatest at intermediate levels of disturbance because competitive exclusion at low levels of disturbance and mortality at high levels of disturbance reduces species diversity.

intermediate filaments A part of the cytoskeleton that includes filaments intermediate in diameter between microtubules and microfilaments.

internal environment In multicellular organisms, includes blood plasma and interstitial fluid (i.e., the extracellular fluids that surround the cells).

internal fertilization The release of sperm into the female reproductive tract; typical of most terrestrial animals. (Contrast with external fertilization.)

internal gills Gills enclosed in protective body cavities; typical of mollusks, arthropods, and fishes.

internal skeleton See endoskeleton.

interneuron A neuron that communicates information between two other neurons.

internode The region between two nodes of a plant stem.

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 interactions Interactions between members of two or more species.

interstitial fluid Extracellular fluid that is not contained in the vessels of a circulatory system.

intertidal zone A nearshore region of oceans that is periodically exposed to the air as the tides rise and fall.

intestine The portion of the gut following the stomach, in which most digestion and absorption occurs.

intracellular receptor A receptor that binds its ligand inside the cell. An example is the estrogen receptor in mammals.

intraspecific competition Competition for shared limited resources by individuals of the same species.

intron Portion 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 non-native species that reproduces rapidly, spreads widely, and has mostly negative effects on the native species or ecosystems of the region to which it has been introduced.

inversion A rare 180° reversal of the order of genes within a segment of a chromosome.

involution Cell movements that occur during gastrulation of frog embryos, giving rise to the archenteron.

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.

ionic attraction An electrostatic attraction between positively and negatively charged ions.

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.

islets of Langerhans Clusters of hormone-producing cells in the pancreas.

isomers Molecules consisting of the same numbers and kinds of atoms, but differing in the bonding patterns by which the atoms are held together.

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.)

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.

isozymes Enzymes of an organism that have somewhat different amino acid sequences but catalyze the same reaction.

iteroparity Reproducing multiple times in a lifetime. (Contrast with semelparity.)

J

jasmonic acid (jasmonate) A plant hormone involved in triggering responses to pathogen attack as well as other processes.

jejunum (jih jew′ num) The middle division of the small intestine, where most absorption of nutrients occurs. (See also duodenum, ileum.)

joint In skeletal systems, a junction between two or more bones.

juvenile hormone In insects, a hormone maintaining larval growth and preventing maturation or pupation.

juxtacrine A mode of cell communication where the cell producing a signal is in contact with the cell bearing the receptor for that signal.

K

K-strategist A species whose life history strategy allows it to persist at or near the carrying capacity (K) of its environment. (Contrast with r-strategist.)

karyogamy The fusion of nuclei of two cells. (Contrast with plasmogamy.)

karyotype The number, forms, and types of chromosomes in a cell.

K_D See dissociation constant.

keystone species Species that have strong community-wide effects despite their small size and abundance.

kidneys A pair of excretory organs in vertebrates.

kilocalorie (kcal) See calorie.

kin selection That component of inclusive fitness resulting from helping the survival of relatives containing the same alleles by descent from a common ancestor. (Contrast with direct fitness.)

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.

G-22

Koch’s postulates A set of rules for establishing that a particular microorganism causes a particular disease.

L

lacteals The smallest vessels of the lymphatic system.

lactic acid fermentation A metabolic pathway in which glucose is catabolized in the absence of oxygen with the production of lactic acid.

lagging strand In DNA replication, the daughter strand that is synthesized in discontinuous stretches. (See Okazaki fragments. Contrast with leading strand.)

landscape A geographic area consisting of multiple ecosystems or habitats.

larva (plural: larvae) [L. lares: guiding spirits] An immature stage of any animal that differs dramatically in appearance from the adult.

larynx A structure between the pharynx and trachea that contains the vocal cords.

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 root A root extending outward from the taproot in a taproot system; typical of eudicots.

lateralization A phenomenon in humans in which language functions come to reside in one cerebral hemisphere, usually the left.

laticifers (luh tiss′ uh furs) In some plants, elongated cells containing secondary plant products such as latex.

Laurasia The northernmost of the two large continents produced by the breakup of Pangaea.

law of independent assortment See independent assortment.

law of mass action The rate of a chemical reaction is directly proportional to the concentrations of the reacting substances.

law of segregation See segregation.

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.)

LDLs See low-density lipoproteins.

LDP See long-day plant.

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.

leaf primordium (plural: primordia) An outgrowth on the side of the shoot apical meristem that will eventually develop into a leaf.

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.

lek A display ground within which male animals compete for and defend small display areas as a means of demonstrating their territorial prowess and winning opportunities to mate.

lens In the vertebrate eye, a crystalline protein structure that makes fine adjustments in the focus of images falling on the retina.

lenticels Small porous regions on the bark of woody stems and roots in dicots.

leptin A hormone produced by fat cells that is believed to provide feedback information to the brain about the status of the body’s fat reserves.

Leydig cells The cells between the seminiferous tubules of the testes that secrete testosterone.

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 history The lifetime pattern of growth, reproduction, and survival for an average individual.

life history strategy The way in which individuals within and among species allocate resources to growth, reproduction, and survival based on genetic and environmental factors.

life table A summary of how survival and reproductive rates vary with the age, size, or sex of individuals within a population, and used to estimate population growth rate.

ligament A band of connective tissue linking two bones in a joint.

ligand (lig′ and) Any molecule that binds to a receptor site of another (usually larger) molecule.

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.

light-independent reactions Reactions that follow the light reaction and use the energy captured in the light reactions to drive the reduction of CO₂ to form carbohydrates.

lignin A complex, hydrophobic polyphenolic polymer in plant cell walls that crosslinks other wall polymers, strengthening the walls, especially in wood.

limbic system A group of evolutionarily primitive structures in the vertebrate telencephalon that are involved in emotions, drives, instinctive behaviors, learning, and memory.

limiting resource The required resource whose supply (or lack thereof) most strongly influences the size of a population.

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 analysis Genetic analysis in which a gene and its alleles are linked to an unknown gene and its alleles and the location of the latter is determined by its co-segregating with the former in genetic crosses.

lipases Digestive enzymes that break down fats.

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.

lipoproteins Lipids packaged inside a covering of protein so that they can be circulated in the blood.

liposome (lip′ o zome) Spherical structure contained by a membrane of phospholipids. Can be used to deliver drugs to cells.

lithosphere (lith′ o sphere) [Gk. lithos: strong] The crust of sold rock plates that overlays the viscous mantle of Earth. The movements of the lithosphere are the source of plate tectonics. (Contrast with asthenosphere.)

liver A large digestive gland. In vertebrates, it secretes bile and is involved in the formation of blood.

loading In phloem, the process of transferring the products of photosynthesis, usually sugars, into sieve tubes.

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.

logistic growth A pattern of population growth characterized by a multiplicative pattern that eventually levels off as density dependent processes slow growth at some carrying capacity (Contrast with exponential growth.)

long-day plant (LDP) A plant that requires long days (actually, short nights) in order to flower. (Compare to short-day plant.)

long-term depression (LTD) A long-lasting decrease in the responsiveness of a neuron resulting from a period of prior stimulation. (Contrast with long-term potentiation.)

G-23

long-term memory Information that is maintained in the brain and is accessible for long periods of time—hours to years. (Contrast with immediate memory, short-term memory.)

long-term potentiation (LTP) A long-lasting increase in the responsiveness of a neuron resulting from a period of intense stimulation. (Contrast with long-term depression.)

loop of Henle (hen′ lee) Long, hairpin loop of the mammalian renal tubule that runs from the cortex down into the medulla and back to the cortex; creates a concentration gradient in the interstitial fluids in the medulla.

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.)

lottery model A hypothesis proposing that species diversity in communities is maintained by a “lottery” in which resources, made available by the effects of disturbance, stress, or predation, are captured at random by individuals of different species. Also called neutral model.

low-density lipoproteins (LDLs) Lipoproteins that transport cholesterol around the body for use in biosynthesis and for storage; LDLs are the “bad” lipoproteins associated with a high risk of cardiovascular disease.

lower critical temperature The environmental temperature at which a resting endotherm must increase its metabolic rate to avoid a drop in its body temperature.

lumen (loo′ men) [L. lumen: light] The open cavity inside any tubular organ or structure, such as the gut or a renal tubule.

lung An internal organ specialized for respiratory gas exchange with air.

luteinizing hormone (LH) A gonadotropin produced by the anterior pituitary that stimulates the gonads to produce sex hormones.

lymph [L. lympha: liquid] A fluid derived from blood and other tissues that accumulates in intercellular spaces throughout the body and is returned to the blood by the lymphatic system.

lymph node A specialized structure in the vessels of the lymphatic system. Lymph nodes contain lymphocytes, which encounter and respond to foreign cells and molecules in the lymph as it passes through the vessels.

lymphatic duct One of two large lymphatic vessels that drains lymph into a subclavian vein.

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

macroevolution [Gk. makros: large] Evolutionary changes occurring over long time spans and usually involving changes in many traits. (Contrast with microevolution.)

macroglia Non-neuronal cells of the central and peripheral nervous system that provide homeostatic functions for neurons, modulate synaptic connections, provide myelin coverings for axons, and create the blood-brain or blood-spinal cord barrier.

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.)

macroparasites Relatively large parasite species, such as arthropods and worms. (Compare with microparasites.)

MADS box DNA-binding domain in many plant transcription factors that is active in development.

maintenance methylase An enzyme that catalyzes the methylation of the new DNA strand when DNA is replicated.

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.

malignant Pertaining to a tumor that can grow indefinitely and/or spread from the original site of growth to other locations in the body. (Contrast with benign.)

malnutrition A condition caused by lack of any essential nutrient.

Malpighian tubule (mal pee′ gy un) A type of protonephridium found in insects.

mandible A crushing part of arthropod mouth parts, in mammals, the lower jaw bone, in birds the upper or lower part of the beak.

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, Earth’s crust below the solid lithospheric plates.

mark–recapture method A method of estimating population sizes of mobile organisms by capturing, marking, and releasing a sample of individuals, then capturing another sample at a later time.

mass A measure of the quantity of matter present; the greater the mass, the greater the quantity of matter.

mass extinction event 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 The joining of individuals for the purpose of reproduction.

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.

mechanically gated channel A molecular channel that opens or closes in response to mechanical force applied to the plasma membrane in which it is inserted.

mechanoreceptor A cell that is sensitive to physical movement and generates action potentials in response.

median The value at which there are equal numbers of larger and smaller observations in a sample.

medulla (meh dull′ luh) (1) The inner, core region of an organ, as in the adrenal medulla (adrenal gland) or the renal medulla (kidneys). (2) The portion of the brainstem that connects to the spinal cord.

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.)

megakaryocytes A bone marrow cell that produces blood platelets.

G-24

megaphyll The generally large leaf of a fern, horsetail, or seed plant, with several to many veins. (Contrast with microphyll.)

megasporangia The plant structures that bear megaspores.

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.

meiosis I See meiosis.

meiosis II See meiosis.

Meissner’s corpuscles Rapidly adapting skin mechanoreceptors sensitive to light touch and vibration. Found primarily in non-hairy skin; provide continuous information about changes in things touching the skin.

melanocyte-stimulating hormone (MSH) A pituitary peptide hormone that stimulates melanocytes to produce the pigment melanin.

melatonin A hormone released by the pineal gland. Involved in photoperiodicity and circadian rhythms.

membrane A phospholipid bilayer forming a barrier that separates the internal contents of a cell from the nonbiological environment, or enclosing the organelles within a cell. The membrane regulates the molecular substances entering or leaving a cell or organelle.

membrane potential The difference in electrical charge between the inside and the outside of a cell, caused by a difference in the distribution of ions.

membranous bone A type of bone that develops by forming on a scaffold of connective tissue. (Contrast with cartilage bone.)

memory cells 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.

menopause In human females, the end of fertility and menstrual cycling.

menstrual cycle A term for the uterine cycle of humans that refers to the monthly sloughing of the uterine lining, or menses.

menstruation The process by which the endometrium breaks down, 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.

Merkel’s discs Slowly adapting skin mechanoreceptors that provide continuous information about anything touching the skin.

mesenchyme (mez′ en kyme) [Gk. mesos: middle + enchyma: infusion] Embryonic or unspecialized cells derived from the mesoderm.

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.

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.

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.)

metanephridia (singular: metanephridium) [Gk. meta: akin to) The paired excretory organs of annelids.

metapopulation A group of geographically isolated populations linked together by dispersal.

metastasis Spread of cells a tumor to regions of the body away from the primary tumor.

methanogens Methane-producing microbes.

methylation The addition of a methyl group (—CH₃) to a molecule.

MHC See major histocompatibility complex.

MI See myocardial infarction.

micelle A particle of lipid covered with bile salts that is produced in the duodenum and facilitates digestion and absorption of lipids.

microbiome The diverse community of bacteria that lives on or within the body of an organism and is essential to bodily function.

microbiota The diverse communities of bacteria that live on or within the body.

microevolution Evolutionary changes below the species level, affecting allele frequencies. (Contrast with macroevolution.)

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.)

microglia Glial cells that act as macrophages and mediators of inflammatory responses in the central nervous system.

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.)

microparasites Parasite species too small to be seen with the naked eye, such as viruses, bacteria, protists, and fungi. (Compare with macroparasites.)

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 (miRNA) A small, noncoding RNA molecule, typically about 21 bases long, that binds to mRNA to inhibit its translation.

microsporangia The plant structures that bear microspores.

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 (singular: microvillus) 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. (Compare with forebrain, hindbrain.)

middle lamella (la mell′ ah) [L. lamina: thin sheet] A layer of polysaccharides that separates plant cells; a shared middle lamella lies outside the primary walls of the two cells.

migration Roundtrip movement of individuals within a population as a consequence of seasonal variation in resources.

mineral nutrients Inorganic ions required by organisms for normal growth and reproduction.

G-25

mineralization The process by which animals, bacteria, and fungi convert organic compounds into inorganic soluble nutrients.

mismatch repair A mechanism that scans DNA after it has been replicated and corrects any base-pairing mismatches.

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.)

missense substitution A change in a gene from one nucleotide to another that results in a change to the corresponding amino acid in the encoded protein. (Contrast with synonymous substitution.)

mitochondria (my′ toe kon′ dree uh) (singular: mitochondrion) [Gk. mitos: thread + chondros: grain] Energy-generating organelles in eukaryotic cells that contain 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.

mitral valve See bicuspid valve. So named because it has a similar shape to a miter (a religious headdress).

mode The most frequent value in a sample of observations.

model organisms Also known as model systems, these include the small group of species that are the subject of extensive research. They are organisms that adapt well to laboratory situations and findings from experiments on them can apply across a broad range of species. Classic examples include white mice and the fruit fly Drosophila.

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.

mole A quantity of a compound whose weight in grams is numerically equal to its molecular weight expressed in atomic mass units. Avogadro’s number of molecules: 6.023 ×10²³ molecules.

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 evolution The scientific study of the mechanisms and consequences of the evolution of macromolecules.

molecular weight The sum of the atomic weights of the atoms in a molecule.

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.

monoblastic Having a body derived from a single embryonic cell layer. (Contrast with diploblastic, triploblastic.)

monocots Angiosperms with a single embryonic cotyledon; one of the two largest clades of angiosperms. (See also eudicots.)

monoculture In agriculture, a large-scale planting of a single crop species. (Contrast with polyculture).

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.

monosomic Pertaining to an organism with one less than the normal diploid number of chromosomes.

morphogen A diffusible substance whose concentration gradient determines a developmental pattern in embryonic 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.

mosaic development Pattern of animal embryonic development in which each blastomere contributes a specific part of the adult body. (Contrast with regulative development.)

motile (mo′ tul) Able to move from one place to another. (Contrast with sessile.)

motor cortex The region of the cerebral cortex that contains motor neurons that directly stimulate specific muscle fibers to contract.

motor end plate The depression in the postsynaptic membrane of the neuromuscular junction where the terminals of the motor neuron sit.

motor proteins Specialized proteins that use energy to change shape and move cells or structures within cells.

motor unit A motor neuron and the muscle fibers it controls.

MSH See melanocyte-stimulating hormone.

mucosa The first layer of the gut that lines the luminal surface.

mucus A viscous substance secreted by mucous membranes (e.g., mucosal epithelium). A barrier defense against pathogens in innate immunity in animals and a protective coating in many animal organ systems.

Muller’s ratchet The accumulation—“ratcheting up”—of deleterious mutations in the nonrecombining genomes of asexual species.

Müllerian mimicry Convergence in appearance of two or more unpalatable species.

multifactorial The interaction of many genes and proteins with one or more factors in the environment. For example, cancer is a disease with multifactorial causes.

multiple fruits A fruit that develops from several carpels of multiple flowers.

multiplication rule The probability of two or more independent events occurring together is the product of their independent probabilities.

multipotent Having the ability to differentiate into a limited number of cell types. (Contrast with pluripotent, totipotent, unipotent.)

muscle fiber A single muscle cell. In the case of skeletal muscle, a syncitial, multinucleate cell.

muscle spindles Mechanoreceptors embedded in skeletal muscle that sense changes in the length of the muscle.

muscle tissue Excitable tissue that can contract through the interactions of actin and myosin; one of the four major tissue types in multicellular animals. There are three types of muscle tissue: skeletal, smooth, and cardiac.

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 type of positive interaction in which both species benefit from the interaction, sometimes in a highly dependent and symbiotic manner. (Contrast with amensalism, commensalism, competition.)

mycelium (my seel′ ee yum) [Gk. mykes: fungus] In the fungi, a mass of hyphae.

mycologists Scientists who study fungi.

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.

G-26

myelin (my′ a lin) Concentric layers of plasma membrane that form a sheath around some axons; myelin provides the axon with electrical insulation and increases the rate of transmission of action potentials.

myocardial infarction (MI) Blockage of an artery that carries blood to the heart muscle; a “heart attack.”

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 polymeric unit of actin or myosin in a muscle.

myoglobin (my′ oh globe′ in) [Gk. mys: muscle + L. globus: sphere] An oxygen-binding molecule found in muscle. Consists of a heme unit and a single globin chain; carries less oxygen than hemoglobin.

myosin One of the two contractile proteins of muscle. (See also actin.)

myxamoeba An independent, unicellular, haploid life stage of cellular slime molds.

N

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.

nauplius (naw′ plee us) [Gk. nauplios: shellfish] A bilaterally symmetrical larval form typical of crustaceans.

necrosis (nec roh′ sis) [Gk. nekros: death] Premature cell death caused by external agents such as toxins.

negative feedback In regulatory systems, information that decreases a regulatory response, returning the system to the set point. (Contrast with positive feedback.)

negative regulation A type of gene regulation in which a gene is normally transcribed, and the binding of a repressor protein to the promoter prevents transcription. (Contrast with positive regulation.)

negative-sense RNA A strand of RNA that is complementary to mRNA.

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 the kidney, consisting of a structure for receiving a filtrate of blood and a tubule that reabsorbs selected parts of the filtrate.

Nernst equation A mathematical statement that calculates the potential across a membrane permeable to a single type of ion that differs in concentration on the two sides of the membrane.

nerve A structure consisting of many neuronal axons and connective tissue.

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 production (NPP) The amount of energy or biomass incorporated into the tissues of primary producers after respiration. (Compare with gross primary production.)

net reproductive rate (R₀) The mean number of offspring produced by an individual in a population during its lifetime.

net secondary production The amount of energy or biomass obtained from the consumption and assimilation of other organisms, taking into account respiration.

neural crest cells During vertebrate neurulation, cells that migrate outward from the neural plate and give rise to connections between the central nervous system and the rest of the body.

neural network An organized group of neurons that contains three functional categories of neurons—afferent neurons, interneurons, and efferent neurons—and is capable of processing information.

neural tube An early stage in the development of the vertebrate nervous system consisting of a hollow tube created by two opposing folds of the dorsal ectoderm along the anterior–posterior body axis.

neurohormone A chemical signal produced and released by neurons that subsequently acts as a hormone.

neuromuscular junction Synapse (point of contact) where a motor neuron axon stimulates a muscle fiber 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.

neurotransmitter A substance produced in and released by a neuron (the presynaptic cell) that diffuses across a synapse and excites or inhibits another cell (the postsynaptic cell).

neurulation Stage in vertebrate development during which the nervous system begins to form.

neutral allele An allele that does not alter the functioning of the proteins for which it codes.

neutral model See lottery model.

neutral theory A view of molecular evolution that postulates that most mutations do not affect the amino acid being coded for, and that such mutations accumulate in a population at rates driven by genetic drift and mutation rates.

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] The set of physical and biological conditions a species requires to grow, reproduce, and survive.

nitrate reduction The process by which nitrate (NO₃^–) is reduced to ammonia (NH₃).

nitrification The oxidation of ammonia (NH₃) to nitrate (NO₃^–) in soil and seawater, carried out by chemoautotrophic bacteria (nitrifiers).

nitrifiers Prokaryotes that can convert ammonium into nitrate.

nitrogen fixation Conversion of atmospheric nitrogen gas (N₂) into a more reactive and biologically useful form (ammonia), which makes nitrogen available to living things. Carried out by nitrogen fixers—bacteria, some of them free-living and others living within plant roots.

nitrogen fixers Organisms capable of fixing atmospheric N₂ to form NH₃.

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.

node [L. nodus: knob, knot] (1) In plants, a (sometimes enlarged) point on a stem where a leaf is or was attached. (2) A region at the anterior end of the primitive grove—the site of ingression of cells during gastrulation. In mammals and birds it serves a function similar to the dorsal lip of the blastopore in amphibians.

node of Ranvier A gap in the myelin sheath covering an axon; the point where the axonal membrane can fire action potentials.

non-disjunction Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.

non-REM sleep A state of deep, restorative sleep characterized by high-amplitude slow waves in the EEG. (Contrast with REM sleep.)

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.)

noncyclic electron transport In photosynthesis, the flow of electrons that forms ATP, NADPH, and O₂.

nonpolar Having electric charges that are evenly balanced from one end to the other. (Contrast with polar.)

nonpolar covalent bond A covalent bond between atoms that has equal sharing of electrons.

nonsense mutation Change in a gene’s sequence that prematurely terminates translation by changing one of its codons to a stop codon. (Contrast with frame-shift mutation, missense mutation, silent mutation.)

G-27

nonsense substitution A change in a gene from one nucleotide to another that prematurely terminates translation by changing one of its codons to a stop codon. (Contrast with synonymous substitution.)

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 All land plants that lack a vascular system.

norepinephrine A neurotransmitter found in the central nervous system and also at the postganglionic nerve endings of the sympathetic nervous system. Also called noradrenaline.

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.

nuclear envelope A double membrane that encloses the cell nucleus.

nucleases Digestive enzymes that break down nucleic acids.

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.

nucleoside A nucleotide without the phosphate group; a nitrogenous base attached to a sugar.

nucleosome A portion of a eukaryotic chromosome, consisting of part of the DNA molecule wrapped around a group of histone molecules, and held together by another type of histone molecule. The chromosome is made up of many nucleosomes.

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.

nutrient A food substance; or, in the case of mineral nutrients, an inorganic element required for completion of the life cycle of an organism.

O

obligate A type of species interaction that is necessary to at least one species involved in the interaction. (Compare with facultative.)

obligate aerobe An organism that requires an oxygen atmosphere to live.

obligate anaerobe An anaerobic prokaryote that cannot survive exposure to O₂. (Compare with facultative anaerobe.)

obligate parasites Organisms that can only survive and grow in or on other living organisms, to the detriment of the host.

occipital lobe One of the four lobes of the brain’s cerebral hemisphere; processes visual information.

ocean zone The marine life zone that extends from the coastal zone into open ocean. Characterized by deep water that varies in its oxygenation and relatively stable temperatures and salinities.

odorant A molecule that can bind to an olfactory receptor.

oil A triglyceride that is liquid at room temperature. (Contrast with fat.)

Okazaki fragments Newly formed DNA making up the lagging strand in DNA replication. DNA ligase links Okazaki fragments together to give a continuous strand.

olfaction (ole fak′ shun) [L. olfacere: to smell] The sense of smell.

olfactory bulb Structure in the vertebrate forebrain that receives and processes input from olfactory receptor neurons.

olfactory receptor neurons (ORNs) Neurons with receptors for different odorants.

oligodendrocyte A type of glial cell that myelinates axons in the central nervous system.

oligosaccharide A polymer containing a small number of monosaccharides.

omasum The third of the four chambers of the stomach in ruminants; concentrates food by water absorption before it enters the true stomach (abomasum).

ommatidia [Gk. omma: eye] The units that make up the compound eye of some arthropods.

omnivore [L. omnis: everything + vorare: to devour] An organism that feeds on multiple trophic levels; also an organism that feeds on animals and primary producers. (Contrast with carnivore, detritivore, herbivore.)

omnivory An act of predation in which the predator is animal and the prey is an animal or a plant. (Contrast with carnivory, herbivory, parasitism.)

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.

one gene–one polypeptide The idea, now known 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.

one-gene, one-enzyme hypothesis See one gene–one polypeptide.

one-gene, one-polypeptide relationship See one gene–one polypeptide.

oogonium (oh′ eh go′ nee um) (plural: oogonia) (1) In some algae and fungi, a cell in which an egg is produced. (2) In animals, the diploid progeny of a germ cell in females.

ootid In oogenesis, the daughter cell of the second meiotic division that differentiates into the mature ovum.

open circulatory system Circulatory system in which extracellular fluid leaves the vessels of the circulatory system, percolates between cells and through tissues, and then flows back into the circulatory system to be pumped out again. (Contrast with closed circulatory system.)

operant conditioning Using a reward or a punishment to control the expression of a behavior by an animal in response to an unrelated stimulus.

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.

opportunity cost The sum of the benefits an animal forfeits by not being able to perform some other behavior during the time when it is performing a given behavior. (Compare with energetic cost, risk cost.)

opsin (op′ sin) [Gk. opsis: sight] The protein portion of vertebrate visual pigments; associated with the pigment molecule 11-cis-retinal. (See also rhodopsin.)

optic chiasm [Gk. chiasma: cross] Structure on the lower surface of the vertebrate brain where the two optic nerves come together.

optic nerve The nerve that carries information from the retina of the eye to the brain.

optical isomers Two molecular isomers that are mirror images of each other.

optimal foraging theory The application of a cost–benefit approach to feeding behavior to identify the fitness value of feeding choices.

oral [L. os: mouth] Pertaining to the mouth, or that part of the body that contains the mouth.

orbital A region in space surrounding the atomic nucleus in which an electron is most likely to be found.

ordinal variables Categorical variables with a natural ordering, such as the grades A, B, C, D, and F.

G-28

organ [Gk. organon: tool] A body part, such as the heart, liver, brain, root, or leaf. Organs are composed of different tissues integrated to perform a distinct function. Organs, in turn, are integrated into organ systems.

organ identity genes In angiosperms, genes that specify the different organs of the flower. (Compare with homeotic genes.)

organ of Corti Structure in the inner ear that transforms mechanical forces produced from pressure waves (“sound waves”) into action potentials that are sensed as sound.

organ system An interrelated and integrated group of tissues and organs that work together in a physiological function.

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 that directs early embryonic development. Also known as the primary embryonic organizer.

organogenesis The formation of organs and organ systems during development.

origin of replication (ori ) DNA sequence at which helicase unwinds the DNA double helix and DNA polymerase binds to initiate DNA replication.

ortholog [Gk. ortho: true, direct] Homologous gene whose divergence can be traced to speciation events. (Contrast with paralog.)

osmoconformer An aquatic animal that equilibrates the osmolarity of its extracellular fluid to be the same as that of the external environment. (Contrast with osmoregulator.)

osmolarity The concentration of osmotically active particles in a solution.

osmoregulator An aquatic animal that actively regulates the osmolarity of its extracellular fluid. (Contrast with osmoconformer.)

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.

ossicles Three bones in the middle ear that transmit vibrations from tympanum to the oval window of the inner ear. Individually named the malleus (“hammer”), incus (“anvil”), and stapes (“stirrup”).

osteoblast (oss′ tee oh blast) [Gk. osteon: bone + blastos: sprout] A cell that lays down the protein matrix of bone.

osteoclast (oss′ tee oh clast) [Gk. osteon: bone + klastos: broken] A cell that dissolves bone.

osteocyte (oss′ tee oh site) [Gk. osteon: bone + kytos: cell] An osteoblast that has become enclosed in lacunae within the bone it has built.

otoliths (oh′ teliths) [Gk. ous: ear + lithos: stones] Crystals of calcium carbonate in the inner ear. Also called ear stones.

outgroup In phylogenetics, a group of organisms used as a point of reference for comparison with the groups of primary interest (the ingroup).

oval window The flexible membrane that, when moved by the bones of the middle ear, produces pressure waves in the inner ear.

ovarian cycle (oh var′ ee un) In human females, the monthly cycle of events by which eggs and hormones are produced. (Contrast with uterine cycle).

ovary (oh′ var ee) [L. ovum: egg] Any female organ, in plants or animals, that produces an egg.

overtopping Plant growth pattern in which one branch differentiates from and grows beyond the others.

oviduct In mammals, the tube serving to transport eggs to the uterus or to the outside of the body.

oviparity Reproduction in which eggs are released by the female and development is external to the mother’s body. (Contrast with viviparity.)

ovoviviparity Pertaining to reproduction in which fertilized eggs develop and hatch within the mother’s body but are not attached to the mother by means of a placenta.

ovulation Release of an egg from an ovary.

ovule (oh′ vule) In plants, a structure comprising the megasporangium and the integument, which develops into a seed after fertilization.

ovum (oh′ vum) (plural: ova) [L. egg] The female gamete.

oxaloacetate Four-carbon salt of an acid with the formula HOOC—CO—CO—COOH that is an intermediate in metabolic pathways such as the citric acid cycle.

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–reduction (redox) reaction A reaction in which one substance transfers one or more electrons to another substance. (See oxidation, reduction.)

oxidative phosphorylation ATP formation in the mitochondrion, associated with flow of electrons through the respiratory chain.

oxygenase An enzyme that catalyzes the addition of oxygen to a substrate from O₂.

oxytocin A hormone released by the posterior pituitary that promotes social bonding.

P

P-value The calculated probability of observing a given result by chance sampling, given the null hypothesis is true.

pacemaker cells Cardiac cells that can initiate action potentials without stimulation from the nervous system, allowing the heart to initiate its own contractions.

Pacinian corpuscles (puh sin′ ee uhn) Encapsulated mechanoreceptors in deep skin sensitive to pressure and vibration.

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.

pancreas (pan′ cree us) A gland located near the stomach of vertebrates that secretes digestive enzymes into the small intestine and releases insulin into the bloodstream.

parabronchi Passages in the lungs of birds through which air flows.

paracrine [Gk. para: near] Pertaining to a chemical signal, such as a hormone, that acts locally, near the site of its secretion. (Contrast with autocrine.)

parallel evolution The repeated evolution of similar traits, especially among closely related species; facilitated by conserved developmental genes.

paralog Homologous gene whose divergence can be traced to gene duplication events. (Contrast with ortholog.)

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 [Gk. parasitos: one who eats at another’s table] An organism that kills and/or consumes parts of an organism much larger than itself (known as its host). Parasites sometimes, but not always, kill their host.

parasitic plants Plants that obtains some of their nutrition by growing on other plants.

parasitism An act of predation in which the predator (a parasite) lives symbiotically on or in the prey (its host) and consumes only certain tissues without necessarily killing the host. (Contrast with carnivory, herbivory, omnivory.)

parasympathetic nervous system The division of the autonomic nervous system that works in opposition to the sympathetic nervous system. (Contrast with sympathetic nervous system.)

parathyroid glands Four glands on the posterior surface of the thyroid gland that produce and release parathyroid hormone.

parathyroid hormone (PTH) A hormone secreted by the parathyroid glands that stimulates osteoclast activity and raises blood calcium levels. Also called parathormone. (Contrast with calcitonin.)

parenchyma (pair eng′ kyma) A plant tissue composed of relatively unspecialized cells without secondary walls. (Contrast with collenchyma, sclerenchyma.)

G-29

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.

parietal cells One of three types of secretory cell found in the gastric pits of the stomach wall. Parietal cells produce hydrochloric acid (HCl), creating an acidic environment that destroys many of the harmful microorganisms ingested with food. (See chief cells.)

parietal lobe One of four lobes of the cerebral hemisphere; processes complex stimuli and includes the primary somatosensory cortex.

parsimony principle Preferring the simplest among a set of plausible explanations of any phenomenon.

parthenocarpy Formation of fruit from a flower without fertilization.

parthenogenesis [Gk. parthenos: virgin] Production of an organism from an unfertilized egg.

partial pressures In a mixture of gasses at a specific pressure, the contribution of each gas to that total pressure.

passive transport Diffusion across a membrane; may or may not require a channel or carrier protein. (Contrast with active transport.)

patch clamping Technique for isolating a tiny patch of membrane to allow the study of ion movement through a particular channel.

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) protein A plant protein involved in systemic acquired resistance whose synthesis is induced in response to an infection by a pathogen such as a fungus.

pattern formation In animal embryonic development, the organization of differentiated tissues into specific structures such as wings.

pattern recognition receptors (PRRs) Proteins expressed by cells of the innate immune system in animals or plants that bind to molecules from pathogens containing pathogen associated molecular patterns (PAMPs).

PCR See polymerase chain reaction.

peat Partly decomposed vegetable matter, especially of sphagnum moss, that is typically formed in acidic bogs.

pedigree The pattern of transmission of a genetic trait within a family.

penetrance The proportion of individuals with a particular genotype that show the expected phenotype.

penis An accessory sex organ of male animals that enables the male to deposit sperm in the female’s reproductive tract.

pentaradial symmetry Symmetry in five or multiples of five; a feature of adult echinoderms.

pentose [Gk. penta: five] A sugar containing five carbon atoms.

PEP carboxylase The enzyme that combines carbon dioxide with PEP to form a 4-carbon dicarboxylic acid at the start of C₄ photosynthesis or of crassulacean acid metabolism (CAM).

peptidases Digestive enzymes that break down peptides.

peptide hormones Hormone molecules made up of chains of 2 to 50 amino acids (upper size arbitrary); encoded by genes and produced by translation.

peptide linkage 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.

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.

per capita birth rate (b) In population growth models, the average number of offspring that an individual produces over some time interval.

per capita death rate (d) In population growth models, the average individual’s chance of dying in some time interval.

per capita growth rate (r) In population models, the average individual’s contribution to total population growth rate. In mathematical terms, the difference between per capita birth rate (b) and per capita death rate (d) or (b-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.)

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.

period (1) A category in the geological time scale. (2) The duration of a single cycle in a cyclical event, such as a circadian rhythm.

periodic table The arrangement of elements in order of increasing atomic number, with vertical groupings of similar valence electrons.

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 that transmits information to and from the central nervous system, consisting of neurons that extend or reside outside the brain or spinal cord and their supporting cells. (Contrast with central nervous system.)

peristalsis (pair′ i stall′ sis) Wavelike muscular contractions proceeding along a tubular organ, propelling the contents along the tube.

peritoneum The mesodermal lining of the body cavity in coelomate animals.

peritubular capillaries Small blood vessels that surround the renal tubules.

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.

P_fr See phytochrome.

pH The negative logarithm of the hydrogen ion concentration; a measure of the acidity of a solution. A solution with pH = 7 is said to be neutral; pH values higher than 7 characterize basic solutions, while acidic solutions have pH values less than 7.

phage therapy The therapeutic use of bacteriophages to treat pathogenic bacterial infections.

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 The process by which a cell uses its cell membrane to enclose and engulf a macromolecule or particle in the extracellular environment.

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.

pharynx [Gk. throat] The part of the gut between the mouth and the esophagus.

phenotype (fee′ no type) [Gk. phanein: to show] The observable properties of an individual resulting from both genetic and environmental factors. (Contrast with genotype.)

pheromone (feer′ o mone) [Gk. pheros: carry + hormon: excite, arouse] A chemical substance used in communication between organisms 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.

phloem sap Liquid contents on the phloem tubes.

phosphatidyl inositol-bisphosphate (PIP₂) A phospholipid in the cell membrane that is involved in cell signaling after a ligand binds to a nearby receptor.

G-30

phosphodiester linkage The connection in a nucleic acid strand, formed by linking two nucleotides.

phosphoenolpyruvate (PEP) Phosphorylated salt of a three-carbon acid, formula OPO₃H₂—CO—CO—COOH that is an intermediate in metabolic pathways such as glycolysis.

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.

phosphorylation Addition of a phosphate group.

photic zone The life zone in lakes and oceans that is penetrated by light and therefore supports photosynthetic organisms.

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.

photoperiodism Control of an organism’s physiological or behavioral responses by the length of the day or night (the photoperiod).

photophosphorylation Mechanism for ATP formation in chloroplasts in which electron transport is coupled to the transport of hydrogen ions (protons, H⁺) across the thylakoid membrane. (Compare with chemiosmosis.)

photoreceptors (1) In plants, pigments that trigger a physiological response when they absorb a photon. (2) In animals, the sensory receptor cells that sense and respond to light energy. (See cone cells, rod cells.)

photorespiration Light-driven uptake of oxygen and release of carbon dioxide, the carbon being derived from the early reactions of photosynthesis.

photosensitivity Sensitivity to light.

photosynthates Carbohydrate products of photosynthesis.

photosynthesis (foe tow sin′ the sis) [Gk. creating from light] Metabolic processes carried out by green plants and some microorganisms by which visible light is trapped and the energy used to synthesize compounds such as ATP and glucose.

photosynthetic lamellae The internal membrane system of the cyanobacteria.

photosystem [Gk. phos: light + systema: assembly] A light-harvesting complex in the chloroplast thylakoid composed of pigments and proteins. Photosystem I absorbs light at 700 nm, passing electrons to ferredoxin and from there to NADPH. Photosystem II absorbs light at 680 nm and passes electrons to the electron transport chain in the chloroplast.

photosystem I See photosystem.

photosystem II See photosystem.

phototropin Photoreceptor proteins that mediate light reception in plants.

phototropism [Gk. photos: light + trope: turning] A directed plant growth response to light.

phycoerythrin A red protein pigment that occurs in red algae.

phylogenetic tree A graphic representation of these lines of evolutionary descent.

phylogeny (fy loj′ e nee) [Gk. phylon: tribe, race + genesis: source] The evolutionary history of a particular group of organisms or their genes.

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: P_r, which absorbs red light, and P_fr, which absorbs far red light. P_fr 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.

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.

piloting A form of navigation in which an animal finds its way by remembering landmarks in its environment.

pilus (plural: pili) A structure that links prokaryote cells at the beginning of conjugation.

pineal gland Gland located between the cerebral hemispheres that secretes melatonin.

pinocytosis Endocytosis by a cell of liquid containing dissolved substances.

pioneer species Species that arrive first to primary successional habitats and must deal with extreme conditions; they tend to have life history strategies that maximize population growth (see r-strategists). Also called early succession species.

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 gland attached to the base of the brain in vertebrates. Its hormones control the activities of other glands. Also known as the hypophysis.

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 float in the water column, dependent on currents and wind for movement. Plankton include many protists, some algae, and larval animals.

planula (plan′ yew la) [L. planum: flat] A free-swimming, ciliated larval form typical of the cnidarians.

plaque (plack) [Fr. a metal plate or coin] (1) A circular clearing in a layer (lawn) of bacteria growing on the surface of a nutrient agar gel. (2) An accumulation of prokaryotic organisms on tooth enamel. Acids produced by these microorganisms cause tooth decay. (3) A region of arterial wall invaded by fibroblasts and fatty deposits.

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.

plasmodesmata (singular: plasmodesma) [Gk. plassein: to mold + desmos: band] Cytoplasmic strands 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 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.

pleiotropy (plee′ a tro pee) [Gk. pleion: more] The determination of more than one character by a single gene.

pleural membrane [Gk. pleuras: rib, side] The membrane lining the outside of the lungs and the walls of the thoracic cavity. Inflammation of these membranes is a condition known as pleurisy.

pluripotent [L. pluri: many + potens: powerful] Having the ability to form all of the cells in the body. (Contrast with multipotent, totipotent, unipotent.)

pneumatophores Roots that grow upward out of the water or mud to reach the air and obtain oxygen for the root systems of trees that live in swampy or tidal habitats.

G-31

podocytes Cells of Bowman’s capsule of the nephron that cover the capillaries of the glomerulus, forming filtration slits.

poikilotherms Animals whose body temperatures are largely dependent on environmental temperature.

point mutation A mutation that results from the gain, loss, or substitution of a single nucleotide. (Contrast with deletion, duplication.)

polar A molecule with separate and opposite electric charges at two ends, or poles; the water molecule (H₂O) is the most prevalent example. (Contrast with nonpolar.)

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.

pole plasm Cytoplasm at one end of an insect egg that determines that the cells that form at that location will become germ cells.

pollen grain [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.

polyandry Mating system in which one female mates with multiple males. (Contrast with polygyny.)

polyculture In agriculture, a large scale planting of multiple crop species. (Contrast with monoculture.)

polygyny Mating system in which one male mates with multiple females. (Contrast with polyandry.)

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.)

polyploid (pol′ lee ploid) Possessing more than two entire sets of chromosomes.

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.

pons [L. pons: bridge] Region of the brainstem anterior to the medulla.

population (1) In biology, any group of organisms coexisting at the same time and in the same place and capable of interbreeding with one another. (2) In statistics, a study group of interest.

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 or volume.

population dynamics The patterns and processes of change in populations over space and time.

population growth rate The rate of change in population size over time.

population growth The change in population size over time.

population size The total number of individuals in a population.

portal blood vessels Blood vessels that begin and end in capillary beds.