G-1

10-nm fiber A relaxed 30-nm chromatin fiber, the state of the chromatin fiber in regions of the nucleus where transcription is currently taking place.

3′ end The end of a nucleic acid strand that carries a free 3′ hydroxyl.

30-nm fiber A chromosomal conformation created by the folding of the nucleosome fiber of DNA and histones.

3-phosphoglycerate (3-PGA) A 3-carbon molecule; two molecules of 3-PGA are the first stable products of the Calvin cycle.

5′ cap The modification of the 5′ end of the primary transcript by the addition of a special nucleotide attached in an unusual chemical linkage.

5′ end The end of a nucleic acid strand containing a free 5′ phosphate group.

ABC model A model of floral development that invokes three activities, A, B, and C, each of which represents the function of a protein or proteins that specify organ identity in each whorl.

abiotic factors Aspects of the physical environment, such as temperature, water availability, or wind, that affect an organism.

abomasum The fourth chamber in the stomach of ruminants, where protein digestion begins.

abscisic acid A plant hormone that triggers stomatal closure, stimulates root elongation, and maintains seed dormancy.

absolute temperature (T) Temperature measured on the Kelvin scale.

absorption The direct uptake of molecules by organisms, commonly to obtain food. In vertebrate digestion, it is the process by which breakdown products are taken up into the bloodstream.

accessory pigment A light-absorbing pigment other than chlorophyll in the photosynthetic membrane; carotenoids are important accessory pigments.

acclimatization An adaptive change in body function to a new environment.

acidic Describes a solution in which the concentration of protons is higher than that of hydroxide ions (the pH is lower than 7).

acoelomate A bilaterian without a body cavity.

acrosome An organelle that surrounds the head of the sperm containing enzymes that enable sperm to transverse the outer coating of the egg.

actin A protein subunit that makes up microfilaments; used by both striated and smooth muscles to contract and generate force.

action potential A brief electrical signal transmitted from the nerve cell body along one or more axon branches.

activation energy (E_A) The energy input necessary to reach the transition state.

activator A synthesized compound that increases the activity of an enzyme.

active site The portion of the enzyme that binds substrate and converts it to product.

active transport The “uphill” movement of substances against a concentration gradient requiring an input of energy.

adaptation In an evolutionary context, the fit between an organism and its environment that results from evolution by natural selection. In sensory reception, the process in which sensory receptors reduce their firing rate when a stimulus continues over a period of time.

adaptive (acquired) immunity The part of the immune system that is specific to given pathogens.

adaptive radiation A period of unusually rapid evolutionary diversification in which natural selection accelerates the rates of both speciation and adaptation in a single lineage.

addition rule The principle that the probability of either of two mutually exclusive outcomes occurring is given by the sum of their individual probabilities.

adenine (A) A purine base.

adenosine triphosphate (ATP) The molecule that provides energy in a form that all cells can readily use to perform the work of the cell. ATP is the universal energy currency for all cells.

adherens junction A beltlike junctional complex composed of cadherins that attaches a band of actin to the plasma membrane.

adrenal glands Paired glands located adjacent to the kidneys that secrete cortisol in times of stress.

adrenal medulla The inner part of the adrenal gland, which is stimulated by the sympathetic nervous system.

adrenaline (epinephrine) A hormone released by the adrenal gland that causes alertness and arousal.

advantageous mutations Genetic changes that improve their carriers’ chances of survival or reproduction.

advertisement display Behavior by which individuals draw attention to themselves or their status.

aerobic Utilizing oxygen.

aerobic metabolism Energy metabolism that uses oxygen gas to oxidize organic compounds, generating ATP; found in Bacteria, Archaea, and the mitochondria of eukaryotic organisms.

age structure The number of individuals within each age group of a population.

agonist muscles Muscle pairs that combine to produce similar motions.

aldosterone A hormone produced by the adrenal glands that stimulates the distal convoluted tubule and collecting ducts to take up more salt and water.

alga (plural, algae) A photosynthetic protist.

alkaloid Any one of a group of nitrogen-bearing compounds that damages the nervous system of animals, produced by some plants as a defensive mechanism.

allantois In the amniotic egg, a membrane that encloses a space where metabolic wastes collect.

allele frequency Among all the alleles of a gene in a population, the proportion that are of a specified allele.

G-2

alleles The different forms of a gene, corresponding to different DNA sequences in each different form.

allopatric Describes populations that are geographically separated from each other.

allosteric effect A change in the activity or affinity of a protein as the result of binding of a molecule to a site other than the active site.

allosteric enzyme An enzyme whose activity is affected by binding a molecule at a site other than the active site. Typically, allosteric enzymes change their shape on binding an activator or inhibitor.

alpha (α) carbon The central carbon atom of each amino acid.

alpha (α) helix One of the two principal types of secondary structure found in proteins.

alternation of generations The life cycle in which a haploid phase, the gametophyte, and a diploid phase, the sporophyte, follow one after the other.

alternative splicing A process in which primary transcripts from the same gene can be spliced in different ways to yield different mRNAs and therefore different protein products.

altruism Self-sacrificial behavior in which an individual’s actions decrease its own fitness while increasing that of another individual.

Alveolata A eukaryotic superkingdom, defined by the presence of cortical alveoli, small vesicles that, in some species, store calcium ions.

alveoli (singular, alveolus) Clusters of tiny thin-walled sacs where gas exchange by diffusion takes place; found at the ends of very fine bronchioles.

amacrine cell A type of interneuron in the retina that communicates between neighboring bipolar cells and ganglion cells, enhancing motion detection and adjusting for changes in illumination of the visual scene.

amine hormone A hormone that is derived from a single aromatic amino acid, such as tyrosine.

amino acid An organic molecule containing a central carbon atom, a carboxyl group, an amino group, a hydrogen atom, and a side chain. Amino acids are the building blocks of proteins.

amino acid replacement A change in the identity of an amino acid at a particular site in a protein resulting from a mutation in the gene.

amino end The end of a polypeptide chain that has a free amino group.

amino group NH₂; a nitrogen atom bonded to two hydrogen atoms, covalently linked to the central carbon atom of an amino acid.

aminoacyl (A) site One of three binding sites for tRNA on the large subunit of a ribosome.

aminoacyl tRNA synthetase An enzyme that attaches a specific amino acid to a specific tRNA molecule.

amnion In the amniotic egg, a membrane surrounding a fluid-filled cavity that allows the embryo to develop in a watery environment.

amniotes The group of vertebrate animals that produces amniotic eggs; this group includes lizards, snakes, turtles, and crocodilians.

amniotic egg An egg that can exchange gases while retaining water, permitting amniotes to reproduce in dry terrestrial habitats that amphibian eggs cannot tolerate.

Amoebozoa A superkingdom of eukaryotes with amoeba-like cells that move and gather food by means of pseudopodia.

Amphibia A monophyletic group of vertebrates, including frogs and salamanders, with an aquatic larval form with gills and an adult terrestrial form that usually has lungs.

amphipathic Having both hydrophilic and hydrophobic regions.

amplified In PCR technology, an alternative term for “replicated.”

amylase An enzyme that breaks down starch into smaller subunits.

anabolism The set of chemical reactions that build molecules from smaller units utilizing an input of energy, usually in the form of ATP. Anabolic reactions result in net energy storage within cells and the organism.

anaerobic metabolism Energy metabolism in the absence of oxygen.

analogous Describes similar characters that evolved independently in different organisms as a result of adaptation to similar environments.

anammox Anaerobic ammonia oxidation; energy metabolism in which ammonium ion is oxidized by nitrite, yielding nitrogen gas as a by-product.

anaphase The stage of mitosis in which the sister chromatids separate.

anaphase I The stage of meiosis I in which the two homologous chromosomes of each bivalent separate as they are pulled in opposite directions, but the sister chromatids remained joined at the centromere.

anaphase II The stage of meiosis II in which the centromere of each chromosome splits and the separated chromatids are pulled toward opposite poles of the spindle.

anchor A membrane protein that attaches to other proteins and helps to maintain cell structure and shape.

angiosperms The flowering plants; angiosperms are a monophyletic group of seed plants characterized by flowers, double fertilization, and fruits.

angiotensin II A hormone that causes the smooth muscles of arterioles throughout the body to constrict, which increases blood pressure as well as stimulates the release of the hormone aldosterone, increasing reabsorption of electrolytes and water by the kidneys.

annealing The coming together of complementary strands of single-stranded nucleic acids by base pairing.

annelid worms A phylum of worms (Annelida) that have a cylindrical body with distinct segments and a bilaterian body plan.

annual clock A biological clock that corresponds closely to a solar year.

anoxygenic Not producing oxygen; anoxygenic photosynthetic bacteria do not gain electrons from water and so do not generate oxygen gas.

antagonist muscles Muscle pairs that pull in opposite directions to produce opposing motions.

G-3

anterior pituitary gland The region of the pituitary gland that forms from epithelial cells that develop and push up from the roof of the mouth; it receives hormones from the hypothalamus that stimulate it to release hormones in turn.

anther In flowering plants, a structure at the top of a stamen consisting of several sporangia in which pollen is produced.

Anthropocene Period The modern era, so named to reflect the dominant impact of humans on Earth.

antibody A large protein produced by plasma cells that binds to molecules called antigens.

anticodon The sequence of three nucleotides in a tRNA molecule that base pairs with the corresponding codon in an mRNA molecule.

antidiuretic hormone (ADH) A posterior pituitary gland hormone that acts on the kidneys and controls the water permeability of the collecting ducts, thus regulating the concentration of urine that an animal excretes; also known as vasopressin.

antigen Any molecule that leads to the production of antibodies.

antigenic drift The gradual process in which a high rate of mutation leads to changes in the amino acid sequences of antigens, thus allowing a population of viruses to evolve over time and evade memory T and B cells.

antigenic shift Reassortment of RNA strands in the viral genome, leading to sudden changes in cell-surface proteins, thus making it difficult to predict from year to year which virus strains will be most prevalent and therefore what vaccine will be most effective.

antigenic variation The encoding of a protein at different times by any one of a number of different genes.

antigen-presenting cell A type of cell (including macrophages, dendritic cells, and B cells) that takes up an antigen and returns portions of it to the cell surface bound to MHC class II proteins.

antiparallel Oriented in opposite directions; the strands in a DNA duplex are antiparallel.

aorta A large artery through which oxygenated blood flows from the left ventricle to the head and rest of the body.

aortic body A sensory structure of the vertebrate aorta that monitors the levels of oxygen and carbon dioxide and the pressure of blood moving through the body.

aortic valve A valve beween the left ventricle and the aorta.

apical dominance The suppression of growth of axillary buds by the shoot apical meristem.

apomixis A process in some species of flowering plants in which seeds can develop even in the absence of fertilization.

appendicular Describes the part of the vertebrate skeleton that consists of the bones of the limbs, including the shoulder and pelvis.

appendix A narrow, tubelike structure that extends from the cecum. The appendix is a vestigial structure that has no clear function in nonherbivorous animals.

aquaporin A protein channel that allows water to flow through the plasma membrane more readily by facilitated diffusion.

aqueous Watery.

aqueous humor A clear watery liquid that fills the interior region in front of the lens of the vertebrate eye.

Archaea One of the three domains of life, consisting of single-celled organisms with a single circular chromosome and no true nucleus that divide by binary fission and differ from bacteria in many aspects of their cell and molecular biology.

Archaeplastida A eukaryotic superkingdom of photosynthetic organisms; includes the land plants.

Ardi A specimen of Ardipethecus ramidus, an important early hominin, dating from about 4.4 million years ago.

arteriole A small branch of an artery.

artery A large, high-pressure vessel that moves blood flow away from the heart to the tissues.

Arthropoda A monophyletic group of animals that includes insects and contains more than half of all known animal species; distinguished by their segmented bodies and jointed legs.

artificial selection A form of directional selection analogous to natural selection, but without the competitive element; successful genotypes are selected by the breeder, not by competition.

ascomycetes A monophyletic fungal subgroup of the Dikarya, making up 64% of all fungal species, in which nuclear fusion and meiosis take place in an elongated saclike cell called an ascus; also called sac fungi.

asexual reproduction The reproduction of cells or single-celled organisms by cell division; offspring are clones of the parent.

assimilation The process by which organisms take up nutrients from the environment.

associative learning (conditioning) Learning that two events are correlated.

astrocyte A type of star-shaped glial cell that contributes to the blood–brain barrier by surrounding blood vessels in the brain and thus limiting the size of compounds that can diffuse from the blood into the brain.

atom The basic unit of matter.

atomic mass The mass of the atom determined by the number of protons and neutrons.

ATP synthase An enzyme that couples the movement of protons through the enzyme with the synthesis of ATP.

atrioventricular (AV) node A specialized region of the heart containing pacemaker cells that transmit action potentials from the sinoatrial nodes to the ventricles of the heart.

atrioventricular (AV) valve A valve between the right atrium and the right ventricle and between the left ventricle and left atrium.

atrium (plural, atria) A heart chamber that receives blood from the lungs or the rest of the body.

auditory cortex The area of the brain that processes sound.

autocrine signaling Signaling between different parts of a cell; the signaling cell and the responding cell are one and the same.

G-4

autoimmune disease A disease in which tolerance is lost and the immune system becomes active against antigens of the host.

autonomic nervous system The involuntary component of the peripheral nervous system, which controls internal functions of the body such as heart rate, blood flow, digestion, excretion, and temperature.

autosome Any chromosome other than the sex chromosomes.

autotroph Any organism that is able to convert carbon dioxide into glucose, thus providing its own organic source of carbon.

auxin A plant hormone that causes shoots to elongate and guides vascular differentiation.

avirulent In plants, describes pathogens that damage only a small part of the plant because the host plant is able to contain the infection. In animals, describes nonpathogenic microorganisms.

axial Describes the part of the vertebrate skeleton that consists of the skull and jaws of the head, the vertebrae of the spinal column, and the ribs.

axillary bud A meristem that forms at the base of each leaf.

axon The fiberlike extension from the cell body of a neuron that transmits signals away from the nerve’s cell body; the output end of a nerve cell.

axon hillock The junction of the nerve cell body and its axon.

B lymphocyte (B cell) A cell type that matures in the bone marrow of humans and produces antibodies.

Bacteria One of the three monophyletic domains of life, consisting of single-celled organisms with a single circular chromosome but no nucleus that divide by binary fission and differ from archaeons in many aspects of their cell and molecular biology.

bacteriochlorophyll A light-harvesting pigment closely related to the chlorophyll found in plants, algae, and cyanobacteria.

bacteriophage Virus that infects bacterial cells.

balancing selection Natural selection that acts to maintain two or more alleles of a given gene in a population.

ball-and-socket joint A joint that allows rotation in three axes, like the hip and shoulder.

basal lamina A specialized form of extracellular matrix that underlies and supports all epithelial tissues.

base A nitrogen-containing compound that makes up part of a nucleotide.

base excision repair A specialized repair system in which an incorrect DNA base and its sugar are both removed and the resulting gap is repaired.

base stacking Stabilizing hydrophobic interactions between bases in the same strand of DNA.

basic Describes a solution in which the concentration of protons is lower than that of hydroxide ions (the pH is higher than 7).

basidiomycetes A monophyletic fungal subgroup of the Dikarya, including smuts, rusts, and mushrooms, in which nuclear fusion and meiosis take place in a club-shaped cell called a basidium; also called club fungi.

basilar membrane The membrane that, with the cochlear duct, separates the upper and lower canal of the cochlea.

basophil A type of granulocyte that, along with eosinphils, defends against parasitic infections but also contributes to allergies.

behaviorally isolated Describes individuals that only mate with other individuals on the basis of specific courtship rituals, songs, and other behaviors.

beta (β) sheet One of the two principal types of secondary structure found in proteins.

beta-(β-)oxidation The process of shortening fatty acids by a series of reactions that sequentially remove two carbon units from their ends.

bilateral symmetry Symmetry on both sides of a midline; animals with bilateral symmetry have a distinct head and tail, marking front and back, with a single plane of symmetry running between them at the midline.

Bilateria The monophyletic group of animals with bilateral symmetry.

bilayer A two-layered structure of the cell membrane with hydrophilic “heads” pointing outward toward the aqueous environment and hydrophobic “tails” oriented inward, away from water.

bile A fluid produced by the liver that aids in fat digestion by breaking large clusters of fats into smaller lipid droplets.

binary fission The process by which cells of bacteria or archaeons divide.

binding affinity The tightness of the binding between the receptor and the signaling molecule.

biodiversity Biological diversity; the aggregate number of species, or, more broadly, also the diversity of genetic sequences, cell types, metabolism, life history, phylogenetic groups, communities, and ecosystems.

biodiversity hotspots Relatively small areas that have unusually high numbers of endemic species and that are under threat from human activities.

biogeochemical cycles The cycling of carbon and other biologically important elements through the biosphere.

biological species concept (BSC) As described by Ernst Mayr, the concept that “species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.” The BSC is the most widely used and accepted definition of a species, but cannot be applied to Bacteria or Archaea.

biologist A scientist who studies life.

biology The science of life and how it works.

biome The distinctive and stable assemblage of species found over a broad region of Earth; terrestrial biomes are each recognized by their distinctive vegetation.

biomineralization The precipitation of minerals by organisms, as in the formation of skeletons.

biotrophic pathogen A plant pathogen that obtains resources from living cells.

G-5

biparental inheritance A type of inheritance in which the organelles in the offspring cells derive from those in both parents.

bipedal Moving by two feet and habitually walking upright.

bipolar cell A type of interneuron in the retina that adjusts its release of neurotransmitter in response to input from rod and cone cells.

biotic factors Any aspect of organisms that affects another organism.

bivalent The four-stranded structure consisting of two pairs of sister chromatids aligned along their length and held together by chiasmata.

bivalves A group of mollusks that includes clams, oysters, and mussels; they have an enclosing skeleton in which two hard shells are connected by a flexible hinge.

bladder A hollow organ in mammals and fishes for the storage and elimination of urine.

blastocyst A hollow sphere produced by cells in the morula that move in relation to one another, pushing against and expanding the membrane that encloses them. A blastocyst forms from the blastula, has an inner cell mass, and occurs only in mammals.

blastula A fluid-filled ball of undifferentiated cells formed after the fertilized egg has undergone several rounds of mitotic cell division following the morula stage.

blending inheritance The now-discredited model in which heredity factors transmitted by the parents become intermingled in the offspring instead of retaining their individual genetic identities.

blood The circulatory fluid in vertebrates.

bone marrow A fatty tissue between trabeculae and within the central cavity of a bone that contains many important cell populations.

bottleneck An extreme, usually temporary, reduction in population size that often results in marked genetic drift.

Bowman's capsule A membranous sac that encases the glomerulus.

brain The centralized concentration of neurons in an organ that processes complex sensory stimuli from the environment or from anywhere in the body.

brainstem Part of the vertebrate brain, formed from the midbrain, which activates the forebrain by relaying information from lower spinal levels.

bronchiole Any one of the fine branches of secondary bronchi.

bryophytes A paraphyletic group of nonvascular plants that includes the mosses, liverworts, and hornworts.

bud scale One of many like structures formed from leaf primordia that, together, protect shoot apical meristems from desiccation and damage due to cold.

budding A form of asexual reproduction in fungi, plants, and some animals in which a bud forms on the organism and eventually breaks off to form a new organism that is smaller than its parent.

bulbourethral glands Glands below the prostate gland that produce a clear fluid that lubricates the urethra for passage of the sperm.

bulk flow The movement of molecules through organisms due to pressure differences at rates beyond those possible by diffusion across a concentration gradient.

bundle sheath A cylinder of cells that surrounds each vein in C₄ plants in which carbon dioxide is concentrated in bundle sheath cells, suppressing photorespiration.

Burgess Shale A sedimentary rock formation in British Columbia, Canada, that preserves a remarkable sampling of marine life during the initial diversification of animals.

C₃ plant A plant that does not use 4-carbon organic acids to supply the Calvin cycle with carbon dioxide.

C₄ plant A plant in which carbon dioxide is incorporated into 4-carbon organic acids that are then used to supply the Calvin cycle.

cadherin A calcium-dependent adherence protein, important in the adhesion of cells to other cells.

calmodulin A protein that binds with Ca²⁺ and activates the enzyme myosin kinase.

Calvin cycle The process in which carbon dioxide is reduced to synthesize carbohydrates, with ATP and NADPH as the energy sources.

Cambrian explosion A transition period in geologic time during which the body plans characteristic of most bilaterian phyla developed.

canine One of the teeth in carnivores specialized for piercing the body of prey.

capacitation A series of physiological changes that allow the sperm to fertilize the egg.

capillary A very small blood vessel, arranged in finely branched networks connected to arterioles or venules, where gases are exchanged by diffusion with surrounding tissues.

capsid The protein coat that surrounds the nucleic acid of a virus.

carbohydrate An organic molecule containing C, H, and O atoms that provides a source of energy for metabolism and that forms the starting point for the synthesis of all other organic molecules.

carbon cycle The intricately linked network of biological and physical processes that shuttles carbon among rocks, soil, oceans, air, and organisms.

carboxyl end The end of a polypeptide chain that has a free carboxyl group.

carboxyl group COOH; a carbon atom with a double bond to oxygen and a single bond to a hydroxyl group.

carboxylation The first step of the Calvin cycle, in which carbon dioxide absorbed from the air is added to a 5-carbon molecule.

cardiac cycle The contraction of the two atria of the heart followed by contraction of the two ventricles.

cardiac muscle Muscle cells that make up the walls of the atria and ventricles and contract to pump blood through the heart.

cardiac output (CO) The volume of blood pumped by the heart over a given interval of time, the key measure of heart function.

carnivores A monophyletic group of animals, including cats, dogs, seals and their relatives, that consume other animals.

G-6

carotid body A chemosensory structure of the carotid artery that monitors the levels of oxygen and carbon dioxide and the pH of blood moving to the brain.

carpel An ovule-producing floral organ in the center whorl.

carrier A transporter that facilitates movement of molecules.

carrying capacity (K) The maximum number of individuals a habitat can support.

cartilage A type of connective tissue found, for example, in the walls of intervertebral discs and the joint surfaces between adjacent bones.

Casparian strip A thin band of hydrophobic material that encircles each cell of the endodermis of a root, controlling which materials enter the xylem.

catabolism The set of chemical reactions that break down molecules into smaller units and, in the process, produces ATP to meet the energy needs of the cell.

causation A relationship in which one event leads to another.

cavitation The abrupt replacement of the water in a conduit by water vapor, which blocks water flow in xylem.

cecum A chamber that branches off the large intestine; along with the colon, the site of hindgut fermentation.

cell The simplest self-replicating entity that can exist as an independent unit of life.

cell adhesion molecule A cell-surface protein that attaches cells to one another and to the extracellular matrix.

cell cycle The collective name for the steps that make up eukaryotic cell division.

cell division The process by which cells make more cells.

cell junction A complex of proteins in the plasma membrane where a cell makes contact with another cell or the extracellular matrix.

cell theory The theory that the cell is the fundamental unit of life in all organisms and that cells come only from preexisting cells.

cell wall A defining boundary in many organism, external to the cell membrane, that helps maintain the shape and internal composition of the cell.

cell-mediated immunity The ability of T cells, which do not secrete antibodies, to recognize and act against pathogens directly.

cellular blastoderm In Drosophila development, the structure formed by the nuclei in the single-cell embryo when they migrate to the periphery of the embryo and each nucleus becomes enclosed in its own cell membrane.

cellular respiration A series of chemical reactions that convert the energy stored in nutrients into a chemical form that can be readily used by cells.

cellulase The enzyme that breaks down cellulose.

central dogma The theory that information transfer in a cell usually goes from DNA to RNA to protein.

central nervous system (CNS) In vertebrates, the brain and spinal cord; in invertebrates, centralized information-processing ganglia.

centromere A constriction that physically holds sister chromatids together; the site of the attachment of the spindle fibers that move the chromosome in cell division.

centrosome A compact structure that is the microtubule organizing center for animal cells.

cephalization The concentration of nervous system components at one end of the body.

cephalochordates A subphylum of Chordata that shares key features of body organization with vertebrates but lacks a well-developed brain and eyes, has no lateral appendages, and does not have a mineralized skeleton.

cephalopods A monophyletic group of mollusks, including squid, cuttlefish, octopus, and chambered nautilus, with distinctive adaptations such as well-developed eyes and muscular tentacles that capture prey and sense the environment.

cerebellum Part of the vertebrate brain, formed from the hindbrain, which coordinates complex motor tasks by integrating motor and sensory information.

cerebral cortex Part of the vertebrate brain formed from a portion of the forebrain that is greatly expanded in mammals, particularly primates.

cerebrum The outer left and right hemispheres of the cerebral cortex.

cervix The end, or neck, of the uterus.

chain terminator A term for a dideoxynucleotide, which if incorporated into a growing daughter strand stops strand growth because there is no hydroxyl group to attack the incoming nucleotide.

channel A transporter with a passage that allows the movement of molecules through it.

chaperone A protein that helps shield a slow-folding protein until it can attain its proper three-dimensional structure.

character In the discipline of systematics, an anatomical, physiological, or molecular feature of an organism that varies among closely related species.

character state The observed condition of a character, such as presence or absence of lungs or arrangement of petals.

checkpoint One of multiple regulatory mechanism that coordinate the temporal sequence of events in the cell cycle.

chelicerates One of the four main groups of arthropods, including spiders and scorpions, chelicerates have pincer-like claws and are the only arthropods that lack antennae.

chemical bond Any form of attraction between atoms that holds them together.

chemical energy A form of potential energy held in the chemical bonds between pairs of atoms in a molecule.

chemical reaction The process by which molecules are transformed into different molecules.

G-7

chemiosmotic hypothesis The hypothesis that the gradient of protons across a membrane provides a source of potential energy that is converted into chemical energy stored in ATP.

chemoautotroph A microorganism that obtains energy from chemical compounds, not from sunlight.

chemoreceptor A receptor that responds to molecules that bind to specific protein receptors on the cell membrane of the sensory receptor.

chemotroph An organism that derives its energy directly from organic molecules such as glucose.

chiasma (plural, chiasmata) A crosslike structure within a bivalent constituting a physical manifestation of crossing over.

chitin A modified polysaccharide containing nitrogen that makes up the cell walls of fungi and the hard exoskeletons of arthropods.

chloride cell A type of specialized cell in the gills of marine bony fishes that counters the ingestion and diffusion of excess electrolytes into the animal by pumping chloride ions into the surrounding seawater; chloride cells in freshwater fishes have opposite polarity.

chlorophyll The major photosynthetic pigment contained in the thylakoid membrane; it plays a key role in the chloroplast’s ability to capture energy from sunlight. Chlorophyll appears green because it is poor at absorbing green wavelengths.

chloroplast An organelle that converts energy of sunlight into chemical energy by synthesizing simple sugars.

chloroplast genome In photosynthetic eukaryotes, the genome of the chloroplast.

choanocyte A type of cell that lines the interior surface of a sponge; choanocytes have flagella and function in nutrition and gas exchange.

choanoflagellate One of a group of mostly unicellular protists characterized by a ring of microvilli around the cell’s single flagellum.

cholecystokinin (CCK) A peptide hormone that causes the gallbladder to contract and thus release bile into the duodenum.

cholesterol An amphipathic lipid that is a major component of animal cell membranes.

Chondrichthyes Cartilaginous fish, a monophyletic group that includes about 800 species of sharks, rays, and chimaeras.

Chordata One of the three major phyla of deuterostomes, this group includes vertebrates and closely related invertebrate animals such as sea squirts.

chorion In the amniotic egg, a membrane that surrounds the entire embryo along with its yolk and allantoic sac.

chromatin A complex of DNA, RNA, and proteins that gives chromosomes their structure; chromatin fibers are either 30 nm in diameter or, in a relaxed state, 10 nm.

chromatin remodeling The process in which the nucleosomes are repositioned to expose different stretches of DNA to the nuclear environment.

chromosome In eukaryotes, the physical structure in which DNA in the nucleus is packaged; used more loosely to refer to the DNA in bacterial cells or archaeons.

chromosome condensation The progressive coiling of the chromatin fiber, an active, energy-consuming process requiring the participation of several types of proteins.

chytrid A single-celled aquatic fungus with chitin walls that attaches to decomposing organic matter.

cilium (plural, cilia) A hairlike organelle that propels the movement of cells or of substances within cells or out of the body; shorter than a flagellum.

circadian clock A biological clock, on a near-daily cycle, that can be set by external cues and regulates many daily rhythms. In animals, the circadian clock affects feeding, sleeping, hormone production, and core body temperature.

circular muscle Smooth muscle that encircles the body or an organ; in the digestive tract, a circular muscle layer contracts to reduce the size of the lumen. A circular muscle layer contracts alternately with longitudinal muscle to move contents through the digestive tract and to enable locomotion in animals with hydrostatic skeletons.

circulation The movement of a specialized body fluid that carries oxygen and carbon dioxide, and nutrients and waste products, through the body.

cis-regulatory element A short DNA sequence adjacent to a gene, usually at the 54′ end, that interacts with transcription factors.

cisternae (singular, cisterna) The series of flattened membrane sacs that make up the Golgi apparatus.

citric acid cycle The third stage of cellular respiration, in which acetyl-CoA is broken down and more carbon dioxide is released.

cladistics Phylogenetic reconstruction on the basis of shared evolutionary changes in characters, often called synapomorphies.

class In the Linnaean system of classification, a group of closely related orders.

classical conditioning Associative learning in which two stimuli are paired.

cleavage The successive mitotic divisions of the zygote after fertilization, in which the single large egg is divided into many smaller cells.

climax community A mature assembly, a final stage in succession, in which there is little further change in species composition.

clitoris The female homolog of the glans penis.

clonal selection A hypothesis proposing that the antigen instructs the antibody to fold in a particular way so that the two interact in a specific manner; now a central principle of immunology.

clone An individual that carries an exact copy of the nuclear genome of another individual; clones are genetically identical cells or individuals.

G-8

closed circulatory system A circulatory system made up of a set of internal vessels and a heart that functions as a pump to move blood to different regions of the body.

Cnidaria A phylum characterized by radial symmetry, two germ tissues in the embryo, a closed internal gastric cavity, and well-developed tissues but not organs; includes jellyfish, sea anemones, and corals.

cochlea A coiled chamber within the skull containing hair cells that convert pressure waves into an electrical impulse that is sent to the brain.

cochlear duct A fluid-filled cavity in the cochlea, next to the upper canal, that houses the organ of Corti.

codon A group of three adjacent nucleotides in RNA that specifies an amino acid in a protein or that terminates polypeptide synthesis.

coelacanth A genus of two species of lobe-finned fish found off the coast of Africa and thought to have been extinct for 80 million years but which is still living today; along with lungfish, the nearest relative of tetrapods.

coelom A body cavity surrounding the gut.

coenocytic Containing many nuclei within one giant cell; the nucleus divides multiple times, but the nuclei are not partitioned into individual cells.

coenzyme Q (CoQ) In respiration, a mobile electron acceptor that transports electrons from complexes I and II to complex III in the electron transport chain and moves protons from the mitochondrial matrix to the intermembrane space.

coevolution The process in which species evolve together, each responding to selective pressures from the other.

cofactor A substance that associates with an enzyme and plays a key role in its function.

cognition The ability of the brain to process and integrate complex sources of information, interpret and remember past events, solve problems, reason, and form ideas.

cohesion Attraction between molecules; one consequence of cohesion is high surface tension.

cohort A group of the individuals born at a given time.

collagen A strong protein fiber found in bone, artery walls, and other connective tissues.

collecting duct A type of duct in the vertebrate kidney where urine collects.

colon Part of the hindgut and the site of reabsorption of water and minerals; also known as the large intestine.

combinatorial control Regulation of gene transcription by means of multiple transcription factors acting together.

commensalism An interaction between species in which one partner benefits with no apparent effect on the other.

communication The transfer of information between two individuals, the sender and the receiver.

community The set of all populations found in a given place.

compact bone Dense, mineralized bone tissue that forms the walls of a bone’s shaft.

companion cell In angiosperms, a cell that carries out cellular functions such as protein synthesis for sieve tubes.

comparative genomics The analysis of the similarities and differences in protein-coding genes and other types of sequence in the genomes of different species.

competition An interaction in which the use of a mutually needed resource by one individual or group of individuals lowers the availability of the resource for another individual or group.

competitive exclusion The result of an antagonistic interaction in which one species is prevented from occupying a particular habitat or niche.

complement system The collective name for certain proteins circulating in the blood that participate in innate immune function and thus complement other parts of the immune system.

complementary Describes the relationship of purine and pyrimidine bases, in which the base A pairs only with T and G pairs only with C.

complex carbohydrate A long, branched chain of monosaccharides.

complex trait A trait that is influenced by multiple genes as well as by the environment.

compound eye An eye structure found in insects and crustaceans that consists of a number of ommatidia, individual light-focusing elements.

concordance The percentage of cases in which both members of a pair of twins show the trait when it is known that at least one member shows it.

cone cell A type of photoreceptor cell on the retina that detects color.

conjugation The direct cell-to-cell transfer of DNA, usually in the form of a plasmid.

conservation biology The efforts by biologists and policymakers to address the challenge of sustaining biodiversity in a changing world crowded with people.

conserved Describes sequences that are similar in different organisms.

constant (C) Describes an unchanging region of the H and L chains.

constitutive Describes expression of a gene that occurs continuously.

constitutive defense A defense that is always active.

consumer An organism that obtains the carbon it needs for growth and reproduction from the foods it eats and gains energy by respiring food molecules; heterotrophic organisms of all kinds that directly consume primary producers or consume those that do.

contractile ring In animal cells, a ring of actin filaments that forms at the equator of the cell perpendicular to the axis of what was the spindle at the beginning of cytokinesis.

contractile vacuole A type of cellular compartment that takes up excess water and waste products from inside the cell and expels them into the external environment.

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control group The group that is not exposed to the variable in an experiment.

cooperative binding The increase in binding affinity with additional binding of O₂.

copy-number variation (CNV) Differences among individuals in the number of copies of a region of the genome.

Coriolis effect The phenomenon in which, because of Earth’s counterclockwise rotation about its axis, winds and oceanic currents in the Northern Hemisphere deflect to the right, and those in the Southern Hemisphere deflect to the left.

cork cambium Lateral meristem that renews and maintains an outer layer that protects the stem against herbivores, mechanical damage, desiccation, and fire.

cornea The transparent portion of the sclera in the front of the vertebrate eye.

corpus luteum A temporary endocrine structure that secretes progesterone.

correlation The co-occurrence of two events or processes; correlation does not imply causation.

corridors Areas that provide species with routes for migration from one reserve to another.

cortex In a stem, the region between the epidermis and the vascular bundles, composed of parenchyma cells. In the mammalian brain, the highly folded outer layer of gray matter, about 4 mm thick, made up of densely packed neuron cell bodies and their dendrites. In the mammalian renal system, the outer layer of the kidney.

co-speciation A process in which two groups of organisms speciate in response to each other and at the same time, producing matching phylogenies.

countercurrent exchange A mechanism in which two fluids flow in opposite directions, exchanging properties.

countercurrent multiplier A system that generates a concentration gradient as two fluids move in parallel but in opposite directions.

covalent bond A chemical bond formed by a shared pair of electrons holding two different atoms together.

CpG island A cluster of CpG sites on a DNA strand where cytosine (C) is adjacent to guanosine (G); the “p” represents the phosphate in the backbone.

cranial nerve In vertebrates, a nerve that links specialized sensory organs to the brain; most contain axons of both sensory and motor neurons.

craniates A subphylum of Chordata, distinguished by a bony cranium that protects the brain; also known as vertebrates.

crassulacean acid metabolism (CAM) A mechanism in plants that helps balance carbon dioxide gain and water loss by capturing carbon dioxide into 4-carbon organic acids at night, when transpiration rates are low, and then using it to supply the Calvin cycle during the day while stomata remain closed.

Crenarchaeota One of the three major divisions of Archaea; includes acid-loving microorganisms.

CRISPR (clustered regularly interspaced short palindromic repeats) A method of DNA editing in which any sequence in the genome can be replaced with any other sequence; the phrase describes the organization of the viral DNA segments in certain bacterial genomes from which the DNA editing method derives.

crisscross inheritance A pattern in which an X chromosome present in a male in one generation is transmitted to a female in the next generation, and in the generation after that can be transmitted back to a male.

Cro-Magnon The first known population of Homo sapiens in Europe, named for the site in France where specimens were first described.

crop The last part of the foregut, which serves as an initial storage and digestive chamber; also known as the stomach.

cross-bridge The binding of the head of a myosin molecule to actin at a specific site between the myosin and actin filaments.

cross-bridge cycle Repeated sequential interactions between myosin and actin filaments at cross-bridges that cause a muscle fiber to contract.

crosscurrent Running across another current at 90 degrees.

crossover The physical breakage, exchange of parts, and reunion between non-sister chromatids.

crustaceans One of the four main groups of arthropods, including lobsters, shrimp, and crabs; distinguished by two pairs of antennae and their branched legs or other appendages.

ctenophores Comb-jellies; species in this phylum have a radial body plan but a flow-through gut; they propel themselves by cilia arranged like a comb along the long axis of the body.

cultural transmission The transfer of information among individuals through learning or imitation.

cuticle In leaves, a protective layer of a waxy substance secreted by epidermal cells that limits water loss; also, an exoskeleton that covers the bodies of invertebrates such as nematodes and arthropods.

C-value paradox The disconnect between genome size and organismal complexity (the C-value is the amount of DNA in a reproductive cell).

cyanobacteria A monophyletic group of bacteria capable of oxygenic photosynthesis.

cyclic electron transport An alternative pathway for electrons during the Calvin cycle that increases the production of ATP.

cyclin A regulatory protein whose levels rise and fall with each round of the cell cycle.

cyclin-dependent kinase (CDK) A kinase that is always present within the cell but active only when bound to the appropriate cyclin.

cytochrome-b₆ f complex (cyt) Part of the photosynthetic electron transport chain, through which electrons pass between photosystem II and photosystem I.

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cytochrome c The enzyme to which electrons are transferred in complex III of the electron transport chain.

cytokine A chemical messenger released by phagocytes that recruits other immune cells to the site of injury or infection.

cytokinesis In eukaryotic cells, the division of the cytoplasm into two separate cells.

cytokinin A plant hormone that stimulates cell division.

cytoplasm The contents of the cell other than the nucleus.

cytosine (C) A pyrimidine base.

cytoskeleton In eukaryotes, an internal protein scaffold that helps cells to maintain their shape and serves as a network of tracks for the movement of substances within cells.

cytosol The region of the cell inside the plasma membrane but outside the organelles; the jelly-like internal environment that surrounds the organelles.

cytotoxic T cell One of a subpopulation of T cells, activated by cytokines released from helper T cells, that can kill other cells.

daughter strand In DNA replication, the strand synthesized from a parental template strand.

day-neutral plant A plant that flowers independently of any change in day length.

decomposer An organism that breaks down dead tissues, feeding on the dead cells or bodies of other organisms.

delayed hypersensitivity reaction Reactions initiated by helper T cells, which release cytokines that attract macrophages to the site of exposure, which is typically the skin.

deleterious mutations Genetic changes that are harmful to an organism.

deletion A missing region of a gene or chromosome.

demography The study of the size, structure, and distribution of populations over time, including changes in response to birth, aging, migration, and death.

denaturation The unfolding of proteins by chemical treatment or high temperature; the separation of paired, complementary strands of nucleid acid.

dendrite A fiberlike extension from the cell body of a neuron that receives signals from other nerve cells or from specialized sensory endings; the input end of a nerve cell.

dendritic cell A type of cell with long cellular projections that is typically part of the natural defenses found in the skin and mucous membranes.

denitrification The process in which some bacteria use nitrate as an electron acceptor in respiration.

density-dependent Describes processes affecting populations that are influenced by the number of individual organisms, such as the use of resources or susceptibility to predation or parasitism.

density-independent Describes factors such as severe drought that influence population size without regard for the density of the population.

deoxyribonucleic acid (DNA) A linear polymer of four subunits; the information archive in all organisms.

deoxyribose The sugar in DNA.

depolarization An increase in membrane potential

dermis The layer of skin beneath the epidermis, consisting of connective tissue, hair follicles, blood and lymphatic vessels, and glands. It supports the epidermis both physically and by supplying it with nutrients and provides a cushion surrounding the body.

desiccation Excessive water loss; drying out.

desiccation tolerance A suite of biochemical traits that allows cells to survive extreme dehydration without damage to membranes or macromolecules.

desmosome A buttonlike point of adhesion that holds the plasma membranes of adjacent cells together.

deuterostome A bilaterian in which the blastopore, the first opening to the internal cavity of the developing embryo, becomes the anus. The taxonomic name is Deuterostomia and includes humans and other chordates.

development The process in which a fertilized egg undergoes multiple rounds of cell division to become an embryo with specialized tissues and organs.

diabetes mellitus A disease that results when the control of blood-glucose levels by insulin fails.

diaphragm A domed sheet of muscle at the base of the lungs that separates the thoracic and abdominal cavities and contracts to drive inhalation.

diaphysis The middle region of a bone; blood vessels invading the diaphysis and epiphysis trigger the transformation of cartilage into bone.

diastole The relaxation of the ventricles, allowing the heart to fill with blood.

dideoxynucleotide A nucleotide lacking both the 2′ and 3′ hydroxyl groups on the sugar ring.

dietary mineral A chemical element other than carbon, hydrogen, oxygen, or nitrogen that is required in the diet and must be obtained in food; see also minerals.

differentiation The process in which cells become progressively more specialized as a result of gene regulation.

diffusion The random motion of individual molecules, with net movement occurring where there are areas of higher and lower concentration of the molecules.

digestive tract Collectively, the passages that connect the mouth, digestive organs, and anus; also known as the gut.

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Dikarya A vast fungal group that includes about 98% of all described fungal species and in which dikaryotic cells are formed.

dikaryotic (n + n) Having two haploid nuclei, one from each parent, in each cell.

diploid Describes a cell with two complete sets of chromosomes.

directional selection A form of selection that selects against one of two extremes and leads over time to a change in a trait.

dispersal The process in which some individuals colonize a distant place far from the main source population.

display A pattern of behavior that is species specific and tends to be highly repeatable and similar from one individual to the next.

disruptive selection A form of selection that operates in favor of extremes and against intermediate forms, selecting against the mean.

distal convoluted tubule The third portion of the renal tubule, in which urea is the principal solute and into which other wastes from the bloodstream are secreted.

disturbance A severe physical impact on a habitat that has density-independent effects on populations of interacting species.

divergence The slow accumulation of differences between duplicate copies of a gene that occurs over time.

DNA editing Techniques that allow researchers to “rewrite” the nucleotide sequence of DNA so that specific mutations can be introduced into genes.

DNA fingerprinting The analysis of a small quantity of DNA to uniquely identify an individual; also known as DNA typing.

DNA ligase An enzyme that uses the energy in ATP to close a nick in a DNA strand, joining the 3′ hydroxyl of one end to the 5′ phosphate of the other end.

DNA polymerase An enzyme that is a critical component of a large protein complex that carries out DNA replication.

DNA replication The process of duplicating a DNA molecule, during which the parental strands separate and new partner strands are made.

DNA transposons Repeated DNA sequences that replicate and can move from one location to another in the genome by DNA replication and repair.

DNA typing The analysis of a small quantity of DNA to uniquely identify an individual; also known as DNA fingerprinting.

domain One of the three largest limbs of the tree of life: Eukarya, Bacteria, or Archaea.

dominant The trait that appears in the heterozygous offspring of a cross between homozygous genotypes.

donor In recombinant DNA technology, the source of the DNA fragment that is inserted into a cell of another organism.

dormancy Describes a time period in the life of an organism when growth, development, and metabolism slow down or stop; in plants, a state in which seeds are prevented from germinating.

dorsal nerve cord A nerve cord that develops in a location dorsal to the notochord; this embryonic feature is unique to chordates.

dosage The number of copies of each gene in a chromosome.

dosage compensation The differential regulation of X-chromosomal genes in females and in males.

double bond A covalent bond in which covalently joined atoms share two pairs of electrons.

double fertilization In angiosperms, the process in which two sperm from a single pollen tube fuse with (1) the egg and (2) the two haploid nuclei of the central cell.

double helix The structure formed by two strands of complementary nucleotides that coil around each other.

Down syndrome A condition resulting from the presence of an extra copy of chromosome 21; also known as trisomy 21.

downstream gene A gene that functions later than another in development.

duodenum The initial section of the small intestine, into which food enters from the stomach.

duplication A region of a chromosome that is present twice instead of once.

duplication and divergence The process of creating new genes by duplication followed by change in sequence over evolutionary time.

dynamic instability Cycles of shrinkage and growth in microtubules.

dynein A motor protein that carries cargo away from the plasma membrane toward the minus ends of microtubules.

eardrum In mammals, another name for the tympanic membrane, which transmits airborne sounds into the ear.

Echinodermata One of the three major phyla of deuterosomes, defined by five-part symmetry; this group includes sea urchins and sea stars.

echolocation Using sound waves to locate an object; bats find insect prey by emitting short bursts of high-frequency sound that bounce off surrounding objects and are reflected to the bat’s ears.

ecological footprint The quantification of individual human claims on global resources by adding up all the energy, food, materials, and services used and estimating how much land is required to provide those resources.

ecological isolation Pre-zygotic isolation between individuals that specialize ecologically in different ways.

ecological niche A complete description of the role a species plays in its environment.

ecological species concept (ESC) The concept that there is a one-to-one correspondence between a species and its niche.

ecology The study of how organisms interact with one another and with their physical environment in nature.

ecosystem A community of organisms and the physical environment it occupies.

ecosystem services Benefits to humans provided by biodiversity, such as cleaner air and water, greater primary productivity, improved resilience to environmental disruption, and untapped sources of food and molecular compounds for use in medicine and agriculture.

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ectoderm The outer germ layer, which differentiates into epithelial cells, pigment cells in the skin, nerve cells in the brain, and the cornea and lens of the eye.

ectomycorrhizae One of the two main types of mycorrhizae; ectomycorrhizae produce a thick sheath of fungal cells (hyphae) that surround, but do not penetrate, root cells.

ectotherm An animal that obtains most of its heat from the environment.

ejaculatory duct The duct through which sperm travel from the vas deferens to the urethra.

elastin A protein fiber found in artery walls that provides elasticity.

electrochemical gradient A gradient that combines the charge gradient and the chemical gradient of protons and other ions.

electromagnetic receptor A receptor that responds to electrical, magnetic, or light stimuli.

electron A negatively charged particle that moves around the atomic nucleus.

electron carrier A molecule that carries electrons (and energy) from one set of reactions to another.

electron donor A molecule that loses electrons.

electron transport chain The system that transfers electrons along a series of membrane-associated proteins to a final electron acceptor, using the energy released as electrons move down the chain to produce ATP.

electronegativity The ability of atoms to attract electrons.

electroreceptor A sensory receptor found in some fish that enables them to detect weak electrical signals emitted by other organisms.

element A pure substance, such as oxygen, copper, gold, or sodium, that cannot be further broken down by the methods of chemistry.

elongation The process in protein translation in which successive amino acids are added one by one to the growing polypeptide chain.

elongation factor A protein that breaks the high-energy bonds of the molecule GTP to provide energy for ribosome movement and elongation of a growing polypeptide chain.

embryo An early stage of multicellular development that results from successive mitotic divisions of the zygote.

endemic species Species found in one place in the world and nowhere else.

endergonic Describes reactions with a positive ΔG that are not spontaneous and so require an input of energy.

endocrine signaling Signaling by molecules that travel through the bloodstream.

endocrine system A system of cells and glands that secretes hormones and works with the nervous system to regulate an animal’s internal physiological functions.

endocytosis The process in which a vesicle buds off from the plasma membrane, bringing material from outside the cell into that vesicle, which can then fuse with other organelles.

endoderm The germ layer that differentiates into cells of the lining of the digestive tract and lung, liver cells, pancreas cells, and gallbladder cells.

endodermis In plants, a layer of cells surrounding the xylem and phloem at the center of the root that controls the movement of nutrients into the xylem. Also, the inner lining of the cnidarian body.

endomembrane system A cellular system that includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, the plasma membrane, and the vesicles that move between them.

endomycorrhizae One of the two main types of mycorrhizae; endomycorrhizal hyphae penetrate into root cells, where they produce highly branched structures (arbuscules) that provide a large surface area for nutrient exchange.

endophyte A fungus that lives within leaves and that may help the host plant by producing chemicals that deter pathogens and herbivores.

endoplasmic reticulum (ER) The organelle involved in the synthesis of proteins and lipids.

endoskeleton The bony skeletal system of vertebrate animals, which lies internal to most of the animal’s soft tissues.

endosperm A tissue formed by many mitotic divisions of a triploid cell, it supplies nutrition to the angiosperm embryo.

endosymbiosis A symbiosis in which one partner lives within the other.

endotherm An animal that produces most of its own heat as by-products of metabolic reactions.

energetic coupling The driving of a non-spontaneous reaction by a spontaneous reaction.

energy A property of objects that can be transferred from one object to another, and that cannot be created or destroyed

energy balance A form of homeostasis in which the amount of energy calories from food taken in equals the amount of calories used over time to meet metabolic needs.

energy intake Sources of energy.

energy use The ways in which energy is expended.

enhancer A specific DNA sequence necessary for transcription.

enthalpy (H) The total amount of energy in a system.

entropy (S) The degree of disorder in a system.

envelope A lipid structure that surrounds the capsids of some viruses.

environmental risk factor A characteristic in a person’s surroundings that increases the likelihood of developing a particular disease.

environmental variation Variation among individuals that is due to differences in the environment.

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enzyme A protein that functions as a catalyst to accelerate the rate of a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell.

eosinophil A type of granulocyte that, along with basophils, defends against parasitic infections but also contributes to allergies.

epidermis In mammals, the outer layer of skin, which serves as a water-resistant, protective barrier. In plants, the outermost layer of cells in leaves, young stems (lacking secondary growth), and roots. Also, the outer layer of the cnidarian body.

epididymis An organ that lies above the testes where sperm become motile and are stored prior to ejaculation.

epigenetic Describes effects on gene expression due to differences in DNA packaging, such as modifications in histones or chromatin structure.

epiglottis A flap of tissue at the bottom of the pharynx that prevents food from entering the trachea and lungs.

epiphysis The end region of a bone; blood vessels invading the epiphysis and diaphysis trigger the transformation of cartilage into bone.

epiphyte A plant that grows high in the canopy of other plants, or on branches or trunks of trees, without contact with the soil.

epistasis Interaction between genes that modifies the phenotypic expression of genotypes.

equational division Another name for meiosis II because cells in meiosis II have the same number of chromosomes at the beginning and at the end of the process.

esophagus Part of the foregut; the passage from the mouth to the stomach.

essential amino acid An amino acid that cannot be synthesized by cellular biochemical pathways and instead must be ingested.

estrogen A hormone secreted by the ovaries that stimulates the development of female secondary sexual characteristics.

estrus cycle A cycle in placental mammals other than humans and chimpanzees characterized by phases in which females are sexually receptive.

ethanol fermentation The fermentation pathway in plants and fungi during which pyruvate releases carbon dioxide to form acetaldehyde and electrons from NADH are transferred to acetaldehyde to produce ethanol and NAD+.

Eukarya The eukaryotes; one of the three domains of life, in which cells have a true nucleus and divide by mitosis.

eukaryote An organism whose cells have a true nucleus.

Eumetazoa The monophyletic group of all animals other than sponges.

Euryarchaeota One of the three major divisions of Archaea; includes acid-loving, heat-loving, methane-producing, and salt-loving microorganisms.

eusocial Describes behavior most commonly observed in species of Hymenopteran insects, in which they have overlapping generations in a nest, cooperative care of the young, and clear and consistent division of labor between reproducers (the queen of a honeybee colony) and nonreproducers (the workers).

eutrophication The process in which added nutrients lead to a great increase in the populations of algae and cyanobacteria.

evaporation The amount of water evaporated from the Earth’s surface, including ponds, rivers, and soil.

evo-devo The short name for evolutionary-developmental biology, a field of study that compares the genetic programs for growth and development in species on different branches of phylogenetic reconstructions.

evolution Changes in the genetic make-up of populations over time, sometimes resulting in adaptation to the environment and the origin of new species.

evolutionarily conserved Characteristics that persist relatively unchanged through diversification of a group of organisms and therefore remain similar in related species.

evolutionarily stable strategy A type of behavior that cannot readily be driven to extinction by an alternative strategy.

excitation-contraction coupling The process that produces muscle force and movement, by excitation of the muscle cell coupled to contraction of the muscle.

excitatory postsynaptic potential (EPSP) A positive change in the postsynaptic membrane potential.

excretion The elimination of waste generated by metabolism.

excretory tubule In renal systems, a type of tube that drains waste products and connects to the outside of the body.

exergonic Describes reactions with a negative ΔG that release energy and proceed spontaneously.

exhalation The expelling of oxygen-poor air by the elastic recoil of the lungs and chest wall.

exit (E) site One of three binding sites for tRNA on the large subunit of a ribosome.

exocytosis The process in which a vesicle fuses with the plasma membrane and empties its contents into the extracellular space or delivers proteins to the plasma membrane.

exon A sequence that is left intact in mRNA after RNA splicing.

exoskeleton A rigid skeletal system that lies external to the animal’s soft tissues.

experimentation A disciplined and controlled way of learning about the world and testing hypotheses in an unbiased manner.

exponential growth The pattern of population increase that results when r (the per capita growth rate) is constant through time.

expressed Turned on or activated, as a gene or protein.

extension (PCR) A step in the polymerase chain reaction (PCR) for producing new DNA fragments in which the reaction mixture is heated to the optimal temperature for DNA polymerase, and each primer is elongated by means of deoxynucleoside triphophosphates.

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extension (joint) The joint motion in which bone segments move apart.

external fertilization Fertilization that takes place outside the body of the female; in aquatic organisms, for example, eggs and sperm are released into the water.

extracellular digestion The process in most animals in which food is isolated and broken down outside a cell, in a body compartment.

extracellular matrix A meshwork of proteins and polysaccharides outside the cell; the main constituent of connective tissue.

extraembryonic membrane In the amniotic egg, one of several sheets of cells that extend out from the developing embryo and form the yolk sac, amnion, allantois, and chorion.

extravasation The process in which phagocytes that travel in the blood move from a blood vessel to the site of infection.

eyecup An eye structure found in flatworms that contains photoreceptors that point up and to the left or right.

F₁ generation The first filial, or offspring, generation.

F₂ generation The second filial generation; the offspring of the F₁ generation.

facilitated diffusion Diffusion through a membrane protein, bypassing the lipid bilayer.

facilitation A beneficial indirect interaction between organisms that are independently interacting directly with a third, as when two different organisms attack another organism, and the attack of each is aided as a result.

facultative In mutualisms, describes one in which one or both sides can survive without the other. In metabolism, describes a means of obtaining energy that is sometimes but not always used.

fallopian tube (oviduct) A tube from each ovary, through one of which a released oocyte passes.

family A group of closely related genera.

fast-twitch Describes muscle fibers that generate force quickly, producing rapid movements, but consume much more ATP than do slow-twitch fibers.

fatty acid A long chain of carbons attached to a carboxyl group; three fatty acid chains attached to glycerol form a triacylglycerol, a lipid used for energy storage.

feature detector A specialized sensory receptor or group of sensory receptors that respond to important signals in the environment.

fermentation A process of breaking down pyruvate through a wide variety of metabolic pathways that extract energy from fuel molecules such as glucose; the partial oxidation of complex carbon molecules to molecules that are less oxidized than carbon dioxide.

ferns and horsetails A monophyletic group of vascular plants that have leaves and disperse by spores.

fertilization The union of gametes to produce a diploid zygote.

fetus In humans, the embryo toward the end of the first trimester.

fiber In angiosperms, a narrow cell with thick walls that provides mechanical support in wood. In animals, a term for a muscle cell, which produces forces within an animal’s body and exerts forces on the environment.

filament In animals, a thin thread of proteins that interacts with other filaments to cause muscles to shorten. In plants, the part of the stamen that supports the anther.

filial imprinting Imprinting in which newborn offspring rapidly learn to treat any animal they see shortly after birth as their mother.

filtration The separation of solids from fluids, as when circulatory pressure pushes fluid containing wastes through specialized filters into an extracellular space.

firing rate The number of action potentials fired over a given period of time.

first law of thermodynamics The law of conservation of energy: Energy can neither be created nor destroyed—it can only be transformed from one form into another.

first-division nondisjunction Failure of chromosome separation in meiosis I.

fitness A measure of the extent to which an individual’s genotype is represented in the next generation

fixed Describes a population that exhibits only one allele at a particular gene.

fixed action pattern (FAP) A sequence of behaviors that, once triggered, is followed through to completion.

flagellum (plural, flagella) An organelle that propels the movement of cells or of substances within cells; longer than a cilium.

flexion The joint motion in which bone segments rotate closer together.

fluid Describes lipids that are able to move in the plane of the cell membrane.

fluid mosaic model A model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins.

flux The rate at which a substance, for example carbon, flows from one reservoir to another.

folding domain A region of a protein that folds in a similar way across a protein family relatively independently of the rest of the protein.

follicle A type of cell that makes up the shell of cells surrounding an oocyte that supports the developing oocyte.

follicle-stimulating hormone (FSH) A hormone secreted by the anterior pituitary gland that stimulates the male and female gonads to secrete testosterone in males and estrogen and progesterone in females.

follicular phase The phase of the menstrual cycle during which FSH acts on granulosa cells, resulting in the maturation of several oocytes, of which, usually, only one becomes completely mature.

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food chain The linear transfer of carbon from one organism to another.

food web A map of the interactions that connect consumer and producer organisms within the carbon cycle; the movement of carbon through an ecosystem.

force An interaction that changes the movement of an object, such as a push or pull by one object interacting with another object.

forebrain The region of the vertebrate brain that governs cognitive functions.

foregut The first part of an animal’s digestive tract, including the mouth, esophagus, and stomach.

fossil The remains, impressions, or evidence of a once-living organism, preserved through time, most often in sedimentary rocks or tree resins.

founder event A type of bottleneck that occurs when only a few individuals establish a new population.

fovea The center of the visual field of most vertebrates, where cone cells are most concentrated; the region of greatest acuity.

fragmentation A form of asexual reproduction in which new individuals arise by the splitting of one organism into pieces, each of which develops into a new individual.

frameshift mutation A mutation in which an insertion or deletion of some number of nucleotides that is not a multiple of three causes a shift in the reading frame of the mRNA, changing all following codons.

fraternal (dizygotic) twins Twins that arise when two separate eggs, produced by double ovulation, are fertilized by two different sperm.

free energy The capacity to do work.

frequency of recombination The proportion of recombinant chromosomes among the total number of chromosomes observed.

frontal lobe The region of the brain located in the anterior region of the cerebral cortex, important in decision making and planning.

fruit In angiosperms, the structure that develops from the ovary, sometimes united with adjacent tissues, and serves to protect immature seeds and enhance dispersal once the seeds are mature.

fruiting body A multicellular structure in some fungi that facilitates the dispersal of sexually produced spores.

functional groups Groups of one or more atoms that have particular chemical properties of their own, regardless of what they are attached to.

fundamental niche The full range of climate conditions and food resources that permit the individuals in a species to live.

fungi An abundant and diverse group of heterotrophic eukaryotic organisms, principally responsible for the decomposition of plant and animal tissues.

G protein A protein that binds to the guanine nucleotides GTP and GDP.

G protein-coupled receptor A receptor that couples to G proteins, which bind to the guanine nucleotides GTP and GDP.

G₀ phase The gap phase in which cells pause in the cell cycle between M phase and S phase; may last for periods ranging from days to more than a year.

G₁ phase The gap phase in which the size and protein content of the cell increase and specific regulatory proteins are made and activated in preparation for S-phase DNA synthesis.

G₂ phase The gap phase in which the size and protein content of the cell increase in preparation for M-phase mitosis and cytokinesis.

gain-of-function mutation Any mutation in which a gene is expressed in the wrong place or at the wrong time.

gallbladder The organ in which bile produced by the liver is stored.

gamete A reproductive haploid cell; gametes fuse in pairs to form a diploid zygote. In many species, there are two types of gametes: eggs in females, sperm in males.

gametic isolation Incompatibility between the gametes of two different species.

gametogenesis The formation of gametes.

gametophyte In alternation of generation, describes the haploid multicellular generation that gives rise to gametes.

ganglion (plural, ganglia) A group of nerve cell bodies that processes sensory information received from a local, nearby region, resulting in a signal to motor neurons that control some physiological function of the animal.

ganglion cell A type of interneuron in the retina that synapses with bipolar cells and, if activated, transmits action potentials along the optic nerve to the visual cortex in the brain.

gap junction A type of connection between the plasma membranes of adjacent animal cells that permits materials to pass directly from the cytoplasm of one cell to the cytoplasm of another.

gas exchange The transport of oxygen and carbon dioxide between an organism and its environment.

gastric cavity In cnidarians, a closed internal site where extracellular digestion and excretion take place.

gastrin A peptide hormone produced in the stomach that stimulates cells lining the stomach to increase their production of HCl.

gastropods A group of mollusks consisting of snails and slugs.

gastrula A layered structure formed when the inner cell mass cells of the blastocyst migrate and reorganize.

gastrulation A highly coordinated set of cell movements in which the cells of the blastoderm migrate inward, creating germ layers of cells within the embryo.

gel electrophoresis A procedure to determine the size of a DNA fragment, in which DNA samples are inserted into slots or wells in a gel and a current passed through. Fragments move toward the positive pole according to size.

G-16

gene The unit of heredity; the stretch of DNA that affects one or more traits in an organism, usually through an encoded protein or noncoding RNA.

gene expression The production of a functional gene product.

gene family A group of genes with related functions, usually resulting from multiple rounds of duplication and divergence.

gene flow The movement of alleles from one population to another through interbreeding of some of their respective members.

gene pool All the alleles present in all individuals in a population or species.

gene regulation The various ways in which cells control gene expression.

general transcription factors A set of proteins that bind to the promoter of a gene whose combined action is necessary for transcription.

genetic code The correspondence between codons and amino acids, in which 20 amino acids are specified by 64 codons.

genetic drift A change in the frequency of an allele due to the random effects of finite population size.

genetic incompatibility Genetic dissimilarity between two organisms, such as different numbers of chromosomes, that is sufficient to act as a post-zygotic isolating factor.

genetic information Information carried in DNA, organized in the form of genes.

genetic map A diagram showing the relative positions of genes along a chromosome.

genetic risk factor Any mutation that increases the risk of a given disease in an individual.

genetic test A method of identifying the genotype of an individual.

genetic variation Differences in genotype among individuals in a population.

genetically modified organism (GMO) An organism that has been genetically engineered, such as modified viruses and bacteria, laboratory organisms, agricultural crops, and domestic animals; also known as a transgenic organism.

genome The genetic material transmitted from a parental cell or organism to its offspring.

genome annotation The process by which researchers identify the various types of sequence present in genomes.

genomic rearrangement The process of joining different gene segments as a B cell differentiates to produce a specific antibody.

genotype The genetic makeup of a cell or organism; the particular combination of alleles present in an individual.

genotype-by-environment interaction Unequal effects of the environment on different genotypes, resulting in different phenotypes.

genus (plural, genera) A group of closely related species.

geographic isolation Spatial segregation of individuals.

geologic timescale The series of time divisions that mark Earth’s long history.

germ cells The reproductive cells that produce sperm or eggs and the cells that give rise to them.

germ layers Three sheets of cells, the ectoderm, mesoderm, and endoderm, formed by migrating cells of the gastrula that differentiate further into specialized cells.

germ-line mutation A mutation that occurs in eggs and sperm or in the cells that give rise to these reproductive cells and therefore is passed on to the next generation.

gibberellic acid A plant hormone that stimulates the elongation of stems.

Gibbs free energy (G) The amount of energy available to do work.

gills Highly folded delicate structures in aquatic animals that facilitate gas exchange with the surrounding water.

gizzard In birds, alligators, crocodiles, and earthworms, a compartment with thick muscular walls in the digestive tract where food mixed with ingested rock or sediment is broken down into smaller pieces.

glans penis The head of the human penis.

glial cell A type of cell that surrounds neurons and provides them with nutrition and physical support.

glomeromycetes A monophyletic fungal group of apparently low diversity but tremendous ecological importance that occurs in association with plant roots.

glomerulus A tufted loop of porous capillaries in the vertebrate kidney that filters blood.

glycerol A 3-carbon molecule with OH groups attached to each carbon.

glycogen The form in which glucose is stored in animals.

glycolysis The breakdown of glucose to pyruvate; the first stage of cellular respiration.

glycosidic bond A covalent bond that attaches one monosaccharide to another.

Golgi apparatus The organelle that modifies proteins and lipids produced by the ER and acts as a sorting station as they move to their final destinations.

gonad In mammals, the part of the reproductive system where haploid gametes are produced. Male gonads are testes, where sperm are produced. Female gonads are ovaries, where eggs are produced.

gonadotropin-releasing hormone (GnRH) A hormone released by the hypothalamus that stimulates the anterior pituitary gland to secrete luteinizing hormone and follicle-stimulating hormone.

gram-positive bacteria Bacteria that retain, in their thick peptidoglycan walls, the diagnostic dye developed by Hans Christian Gram. (Bacteria with thin walls, which do not retain the dye, are said to be gram negative.)

G-17

grana (singular, granum) Interlinked structures that form the thylakoid membrane.

granulocyte A type of phagocytic cell that contains granules in its cytoplasm.

granuloma A structure formed by lymphocytes surrounding infected macrophages that helps to prevent the spread of an infection and aids in killing infected cells.

gravitropic Bending in response to gravity. A negative gravitropic response, as in stems, is growth upward against the force of gravity; a positive gravitropic response, as in roots, is with the force of gravity.

gray matter Densely packed neuron cell bodies and dendrites that make up the cortex, a highly folded outer layer of the mammalian brain about 4 mm thick.

greenhouse gas A gas in the atmosphere that allows incoming solar radiation to reach the Earth’s surface, but absorbs radiation re-emitted as heat, trapping it in the atmosphere and causing the temperature to rise.

group selection Selection caused by the differential success of groups rather than individuals.

growth factor Any one of a group of small, soluble molecules, usually the signal in paracrine signaling, that affect cell growth, cell division, and changes in gene expression.

growth plate A region of cartilage near the end of a bone where growth in bone length occurs.

growth ring One of the many rings apparent in the cross section of the trunk of a tree, produced by decreases in the size of secondary xylem cells at the end of the growing season, that make it possible to determine the tree’s age.

guanine (G) A purine base.

guard cell One of two cells surrounding the central pore of a stoma.

gustation The sense of taste.

gut Collectively, the passages that connect the mouth, digestive organs, and anus; also known as the digestive tract.

gymnosperms Seed plants whose ovules are not enclosed in a carpel; gymnosperms include pine trees and other conifers.

habituation The reduction or elimination of a behavioral response to a repeatedly presented stimulus.

hagfish One of the earliest-branching craniates, with a cranium built of cartilage but no jaws; hagfish feed on marine worms and dead and dying sea animals.

hair cell A specialized mechanoreceptor that senses movement and vibration.

half-life The time it takes for an amount of a substance to reach half its original value. Radioactive half-life is the time it takes for half of the atoms in a given sample of a substance to decay.

haploid Describes a cell with one complete set of chromosomes.

haplotype The particular combination of alleles present in any defined region of a chromosome.

Hardy-Weinberg equilibrium A state in which particular allele and genotype frequencies do not change over time, implying the absence of evolutionary forces. It also specifies a mathematical relationship between allele frequencies and genotype frequencies.

heart The pump of the circulatory system, which moves blood to different regions of the body.

heart rate (HR) The number of heartbeats per unit time.

heartwood The center of the stem in long-lived trees, which does not conduct water.

heavy (H) chains Two of the four polypeptide chains that make up the simplest antibody molecule.

helicase A protein that unwinds the parental double helix at the replication fork.

helper T cell One of a subpopulation of T cells that help other cells of the immune system by secreting cytokines, thus activating B cells to secrete antibodies.

hematocrit The fraction of red blood cells within the blood of vertebrates.

Hemichordata One of the three major phyla of deuterosomes, this group includes acorn worms and pterobranchs.

hemidesmosome A type of desmosome in which integrins are the prominent cell adhesion molecules.

hemoglobin An iron-containing molecule specialized for oxygen transport.

hemolymph The circulatory fluid in invertebrates.

hemophilia A trait characterized by excessive bleeding that results from a recessive mutation in a gene encoding a protein necessary for blood clotting.

herbivory The consumption of plant parts.

heritability In a population, the proportion of the total variation in a trait that is due to genetic differences among individuals.

heterokaryotic Describes a stage in the life cycle of some fungi, in which plasmogamy is not followed immediately by karyogamy and the cells have unfused haploid nuclei from both parents.

heterotroph An organism that obtains its carbon from organic molecules synthesized by other organisms.

heterozygote advantage A form of balancing selection in which the heterozygote’s fitness is higher than that of either of the homozygotes, resulting in selection that ensures that both alleles remain in the population at intermediate frequencies.

heterozygous Describes an individual who inherits different types of alleles from the parents, or genotypes in which the two alleles for a given gene are different.

hierarchical Describes gene regulation during development, in which the genes expressed at each stage in the process control the expression of genes that act later.

G-18

hindbrain Along with the midbrain, the region of the vertebrate brain that controls basic body functions and behaviors.

hindgut The last part of an animal’s digestive tract, including the large intestine and rectum.

hinge joint A simple joint that allows one axis of rotation, like the elbow and knee.

hippocampus A posterior region of the limbic system involved in long-term memory formation.

histamine A chemical messenger released by mast cells and basophils; an important contributor to allergic reactions and inflammation.

histone code The pattern of modifications of the histone tails that affects the chromatin structure and gene transcription.

histone tail A string of amino acids that protrudes from a histone protein in the nucleosome.

homeodomain The DNA-binding domain in homeotic proteins, whose sequences are very similar from one homeotic protein to the next.

homeostasis The active regulation and maintenance, in animals, organs, or cells, of a stable internal physiological state in the face of a changing external environment.

homeotic gene A gene that specifies the identity of a body part or segment during embryonic development. One type of homeotic gene is known as a Hox gene.

hominins A member of one of the different species in the group leading to humans.

homologous Describes characters that are similar in different species because of descent from a common ancestor.

homologous chromosomes Pairs of chromosomes, matching in size and appearance, that carry the same set of genes; one of each pair was received from the mother, the other from the father.

homozygous Describes an individual who inherits an allele of the same type from each parent, or a genotype in which both alleles for a given gene are of the same type.

horizontal cell A type of interneuron in the retina that communicates between neighboring pairs of photoreceptors and bipolar cells, enhancing contrast through lateral inhibition to sharpen the image.

horizontal gene transfer The transfer of genetic material between organisms that are not parent and offspring.

hormone A chemical signal that influences physiology and development in both plants and animals; in animals hormones are released into the bloodstream and circulate throughout the body.

host cell A cell in which viral reproduction occurs.

host plant A plant species that can be infected by a given pathogen.

hotspot A site in the genome that is especially mutable.

housekeeping gene A gene that is transcribed continually because its product is needed at all times and in all cells.

human chorionic gonadotropin (hCG) A hormone released by the developing embryo that maintains the corpus luteum.

human genome The DNA in the chromosomes present in a human sperm or egg. The term is often used informally to mean all of the genetic material in a human cell or organism.

hybridization Interbreeding between two different varieties or species.

hydrogen bond A weak bond between a hydrogen atom in one molecule and an electronegative atom in another molecule.

hydrophilic “Water loving”; describes a class of molecules with which water can undergo hydrogen bonding.

hydrophobic “Water fearing”; describes a class of molecules poorly able to undergo hydrogen bonding with water.

hydrophobic effect The exclusion of nonpolar molecules by polar molecules, which drives biological processes such as the formation of cell membranes and the folding of proteins.

hydrostatic skeleton A skeletal system in which fluid contained within a body cavity is the supporting element.

hydroxyapatite The calcium phosphate mineral found in bone.

hypersensitive response A type of plant defense against infection in which uninfected cells surrounding the site of infection rapidly produce large numbers of reactive oxygen species, triggering cell wall reinforcement and causing the cells to die, thus creating a barrier of dead tissue.

hyperthermophile An organism that requires an environment with high temperature.

hyphae In fungi, highly branched filaments that provide a large surface area for absorbing nutrients.

hypothalamus The underlying brain region of the forebrain, which interacts with the autonomic and endocrine systems to regulate the general physiological state of the body.

hypothesis A tentative explanation for one or more observations that makes predictions that can be tested by experiments or additional observations.

identical (monozygotic) twins Twins that arise from a single fertilized egg, which after several rounds of cell division, separates into two distinct, but genetically identical, embryos.

IgA One of the five antibody classes, IgA is typically a dimer and the major antibody on mucosal surfaces.

IgD One of the five antibody classes, IgD is a monomer and is typically found on the surface of B cells; it helps initiate inflammation.

IgE One of the five antibody classes, IgE is a monomer that plays a central role in allergies, asthma, and other immediate hypersensitivity reactions.

IgG The most abundant of the five antibody classes, IgG is a monomer that circulates in the blood and is particularly effective against bacteria and viruses.

IgM One of the five antibody classes, IgM is a pentamer in mammals and a tetramer in fish and is particularly important in the early response to infection, activating the complement system and stimulating an immune response.

G-19

ileum A section of the small intestine that, with the jejunum, carries out most nutrient absorption.

imitation Observing and copying the behavior of another.

immediate hypersensitivity reaction A reaction characterized by a heightened or an inappropriate immune response to common antigens.

immunodeficiency Any disease in which part of the immune system does not function properly.

imprinting A form of learning typically seen in young animals in which a certain behavior is acquired in response to key experiences during a critical period of development.

in vitro fertilization (IVF) A process in which eggs and sperm are brought together in a petri dish, where fertilization and early cell divisions occur.

inbred line A true-breeding, homozygous strain.

inbreeding depression A reduction in fitness resulting from breeding among relatives causing homozygosity of deleterious recessive mutations.

incisor One of the teeth in the front of the mouth, used for biting.

incomplete dominance Describes inheritance in which the phenotype of the heterozygous genotype is intermediate between those of homozygous genotypes.

incomplete penetrance The phenomenon in which some individuals with a genotype corresponding to a trait do not show the phenotype, either because of environmental effects or because of interactions with other genes.

incus A small bone in the vertebrate middle ear that helps amplify the sound waves that strike the tympanic membrane.

induced pluripotent stem cell (iPS cell) A cell that has been reprogrammed to become pluripotent by activation of certain genes, most of them encoding transcription factors or chromatin proteins.

inducer A small molecule that elicits gene expression.

inflammation A physiological response of the body to injury that removes the inciting agent if present and begins the healing process.

inhalation The drawing of oxygen-rich blood into the lungs by the expansion of the thoracic cavity.

inhibitor A synthesized compound that decreases the activity of an enzyme.

inhibitory postsynaptic potential (IPSP) A negative change in the postsynaptic membrane potential.

initiation The stage of translation in which methionine is established as the first amino acid in a new polypeptide chain.

initiation factor A protein that binds to mRNA to initiate translation.

innate Describes behaviors that are instinctive and carried out regardless of earlier experience.

innate (natural) immunity The part of the immune system that provides protection in a nonspecific manner against all kinds of infection; it does not depend on exposure to a pathogen.

inner cell mass A mass of cells in one region of the inner wall of the blastocyst from which the body of the embryo develops.

inner ear The part of the vertebrate ear that includes the cochlea and semicircular canals. 

insects The most diverse of the four main groups of arthropods.

instantaneous speciation Speciation that occurs in a single generation.

integral membrane protein A protein that is permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself.

integrin A transmembrane protein, present on the surface of virtually every animal cell, that enables cells to adhere to the extracellular matrix.

intercostal muscles Muscles of vertebrates attached to adjacent pairs of ribs that assist the diaphragm by elevating the ribs on inhalation and depressing them during exhalation.

intermediate filament A polymer of proteins, which vary according to cell type, that combine to form strong, cable-like filaments that provide animal cells with mechanical strength.

intermembrane space The space between the inner and outer mitochondrial membranes.

internal fertilization Fertilization that takes place inside the body of the female.

interneuron A neuron that processes information received by sensory neurons and transmits it to motor neurons in different body regions.

internode The segment between two nodes on a shoot.

interphase The time between two successive M phases.

intersexual selection A form of sexual selection involving interaction between males and females, as when females choose from among males.

interspecific competition Competition between individuals of different species.

intervertebral disc A fluid-filled support structure found between the bony vertebrae of the backbone that enables flexibility and provides cushioning of loads.

intracellular digestion The process in single-celled protists in which food is broken down within cells.

intrasexual selection A form of sexual selection involving interactions between individuals of one sex, as when members of one sex compete with one another for access to the other sex.

intraspecific competition Competition within species.

intrinsic growth rate The per capita growth rate; the maximum rate of growth when no environmental factors limit population increase.

intron A sequence that is excised from the primary transcript and degraded during RNA splicing.

invasive species Non-native species; since they are removed from natural constraints on population growth, invasive species can expand dramatically when introduced into new areas, sometimes with devastating consequences for native species and ecosystems.

G-20

inversion The reversal of the normal order of a block of genes.

involuntary Describes the component of the nervous system that regulates internal bodily functions.

ion An electrically charged atom or molecule.

ion channels Cell-surface receptors that open and close, thereby altering the flow of ions across the plasma membrane.

ionic bond The association of two atoms resulting from the attraction of opposite charges.

iris A structure found at the front of the vertebrate eye, surrounding the pupil, that opens and closes to adjust the amount of light that enters the eye.

island population An isolated population.

isomers Molecules that have the same chemical formula but different structures.

isometric The generation of force without muscle movement.

isotopes Atoms of the same element that have different numbers of neutrons.

jejunum A section of the vertebrate small intestine that, with the ileum, carries out most nutrient absorption.

juxtaglomerular apparatus The structure formed by specialized cells of the efferent arteriole leaving the glomerulus of each nephron, which secretes the hormone rennin into the bloodstream.

karyogamy The fusion of two nuclei following plasmogamy.

karyotype A standard arrangement of chromosomes, showing the number and shapes of the chromosomes representative of a species.

key stimulus A stimulus that initiates a fixed action pattern.

keystone species Pivotal populations that affect other members of the community in ways that are disproportionate to their abundance or biomass.

kidneys In vertebrates, paired organs of the renal system that remove waste products and excess fluid; their action contributes to homeostasis.

kin selection A form of natural selection that favors the spread of alleles promoting behaviors that help close relatives.

kinesin A motor protein, similar in structure to myosin, that transports cargo toward the plus end of microtubules.

kinesis (plural, kineses) A random, undirected movement in response to a stimulus.

kinetic energy The energy of motion.

kinetochore The protein complexes on a chromatid where spindle fibers attach.

kingdom A group of closely related phyla.

Klinefelter syndrome A sex-chromosomal abnormality in which an individual has 47 chromosomes, including two X chromosomes and one Y chromosome.

knee-extension reflex A reflex commonly tested by physicians to evaluate peripheral nervous and muscular system function.

K-strategist A species that produces relatively few young but invests considerable resources into their support

labia majora Outer folds of skin in the vulva.

labia minora Inner folds of skin in the vulva that meet at the clitoris.

lactic acid fermentation The fermentation pathway in animals and bacteria during which electrons from NADH are transferred to pyruvate to produce lactic acid and NAD+.

lagging strand A daughter strand that has its 5′ end pointed toward the replication fork, so as the parental double helix unwinds, a new DNA piece is initiated at intervals, and each new piece is elongated at its 3′ end until it reaches the piece in front of it.

lamellae (singular, lamella) The many thin, sheetlike structures spread along the length of each gill filament, giving gills an enormous surface area relative to their size.

lamprey One of the earliest-branching craniates, with a cranium and vertebral column built of cartilage but no jaws; many lampreys live parasitically, sucking body fluids from fish prey.

large intestine Part of the hindgut and the site of reabsorption of water and minerals; also known as the colon.

lariat A loop and tail of RNA formed after RNA splicing.

larynx The structure, above the trachea, that contains the vocal cords.

lateral inhibition Inhibition of a process in cells adjacent to the cell receiving a signal inducing that process, enhancing the strength of a signal locally but diminishing it peripherally.

lateral line system In fish and sharks, a sensory organ along both sides of the body that uses hair cells to detect movement of the surrounding water.

lateral meristem The source of new cells that allows plants to grow in diameter.

latex A white sticky liquid produced in some plants.

latitudinal diversity gradient The increase in species diversity from the poles to the equator.

leading strand A daughter strand that has its 3′ end pointed toward the replication fork, so as the parental double helix unwinds, this daughter strand can be synthesized as one long, continuous polymer.

leaf The principal site of photosynthesis in vascular plants.

leaf primordia A lateral outgrowth of the apical meristem that will eventually develop into a leaf.

learned Describes a behavior that depends on an individual’s experience.

learning The process in which experience leads to changes in behavior.

lengthening contraction The contraction of a muscle against a load greater than the muscle’s force output, leading to a lengthening of the muscle.

lens A flexible structure in the vertebrate eye through which light passes after entering through the pupil; it is controlled by ciliary muscles that contract or relax to the adjust the shape of the lens to focus light images.

G-21

lenticel A region of less tightly packed cells in the outer bark that allows oxygen to diffuse into the stem.

Leydig cell A type of cell in the testes that secretes testosterone.

Liebig's Law of the Minimum The principle that primary production is limited by the nutrient that is least available relative to its use by primary producers.

lichens Stable associations between a fungus and a photosynthetic microorganism, usually a green alga but sometimes a cyanobacterium.

life history The typical pattern of resource investment in each stage of a given species’ lifetime.

life table A table that presents information about how many individuals of a cohort are alive at different points in time.

ligand Alternative term for a signaling molecule that binds with a receptor, usually a protein.

ligand-binding site The specific location on the receptor protein where a signaling molecule binds.

ligand-gated ion channel A receptor that alters the flow of ions across the plasma membrane when bound by its ligand.

light (L) chains Two of the four polypeptide chains that make up the simplest antibody molecule.

limbic system Inner components of the forebrain that control physiological drives, instincts, emotions, motivation, and the sense of reward.

linked Describes genes that are sufficiently close together in the same chromosome that they do not assort independently.

lipase A type of enzyme produced by the pancreas that breaks apart lipids, thus enabling their more effective digestion.

lipid An organic molecule that stores energy, acts as a signaling molecule, and is a component of cell membranes.

lipid raft Lipids assembled in a defined patch in the cell membrane.

liposome An enclosed bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water.

liver A vertebrate organ that aids in the digestion of proteins, carbohydrates, and fats in the duodenum by producing bile, which breaks down fat.

lobe-finned fish Species of fish with paired pectoral and pelvic fins that have a bone structure similar to that of tetrapod limbs.

logistic growth The pattern of population growth that results as growth potential slows down as the population size approaches K, its maximum sustainable size.

long-day plant A plant that flowers only when the light period exceeds a critical value.

longitudinal muscle Smooth muscle that runs lengthwise along a body or organ; in the digestive tract, a longitudinal muscle layer contracts to shorten small sections of the gut. A longitudinal muscle layer contracts alternately with circular muscle to move contents through the digestive tract and to enable locomotion in animals with hydrostatic skeletons.

loop of Henle The middle portion of the vertebrate renal tubule, which creates a concentration gradient that allows water passing through the collecting duct to be reabsorbed.

loss-of-function mutation A genetic change that inactivates the normal function of a gene.

Lucy An unusually complete early hominin fossil, Australopithecus afarensis, found in 1974 in Ethiopia.

lumen In eukaryotes, the continuous interior of the endoplasmic reticulum; in plants, a fluid-filled compartment enclosed by the thylakoid membrane; generally, the interior of any tubelike structure.

lunar clock A moon-based biological clock that times activities in some species, especially those living in habitats where tides are important.

lungfish Several species of lobe-finned fish that use a simple lung to survive periods when their watery habitat dries by burying themselves in moist mud and breathing air; along with coelacanths, the nearest relative of tetrapods.

lungs The internal organs for gas exchange in many terrestrial animals.

luteal phase The phase of the menstrual cycle beginning with ovulation.

luteinizing hormone (LH) A hormone secreted by the anterior pituitary gland that stimulates the male and female gonads to secrete testosterone in males and estrogen and progesterone in females.

lycophytes A monophyletic major group of spore-dispersing vascular plants that are the sister group to all other vascular plants.

lymph The fluid in the lymphatic system in which T and B cells circulate.

lymphatic system A network of vessels distributed through the body with important functions in the immune system.

lysis The breakage, or bursting, of a cell.

lysogenic pathway The alternative to the lytic pathway; in the lysogenic pathway, a virus integrates its DNA into the host cell’s DNA, which is then transmitted to offspring cells.

lysogeny The integration of viral DNA into a host cell’s DNA, which is then transmitted to offspring cells.

lysosome A vesicle derived from the Golgi apparatus that contains enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates.

lytic pathway The alternative to the lysogenic pathway; in the lytic pathway, a virus bursts, or lyses, the cell it infects, releasing new virus particles.

M phase The stage of the cell cycle consisting of mitosis and cytokinesis, in which the parent cell divides into two daughter cells.

G-22

macrophage A type of large phagocytic cell that is able to engulf other cells.

mainland population The central population of a species.

major groove The larger of two uneven grooves on the outside of a DNA duplex.

major histocompatibility complex (MHC) A group of proteins that appear on the surface of most mammalian cells; only the antigen associated with MHC proteins is recognized by T cell receptors.

malleus A small bone in the middle ear that helps amplify the waves that strike the tympanic membrane.

Malpighian tubule One of the tubes in the main body cavity of insects and other terrestrial arthropods through which fluid passes and which empties into the hindgut.

mammals A class of vertebrates distinguished by body hair and mammary glands from which they feed their young.

map information The knowledge of where an individual is in respect to the goal.

map unit A unit of distance in a genetic map equal to the distance between genes resulting in 1% recombination.

mark-and-recapture A method in which individuals are captured, marked in way that doesn’t affect their function or behavior, and then released. The percentage of marked individuals in a later exercise of capture enables ecologists to estimate population size.

marsupials A group of mammals that includes kangaroos, koalas, and opossums; their young are born at an early stage of development and must crawl to a pouch where mammary glands equipped with nipples provide them with milk.

mass extinction A catastrophic drop in recorded diversity, which has occurred five or more times in the past 541 million years.

mast cell A cell that releases histamine.

maternal-effect gene A gene that is expressed by the mother that affects the phenotype of the offspring, typically through the composition or organization of the oocyte.

maternal inheritance A type of inheritance in which the organelles in the offspring cells derive from those in the mother.

mating types Genetically distinct forms of individuals of a fungus species that, by enabling fertilization only between different types, prevent self-fertilization and promote out-crossing.

mechanical incompatibility Structural configuration of the genitalia that prevents mating with another species.

mechanoreceptor A sensory receptor that responds to physical deformations of its membrane produced by touch, stretch, pressure, motion, or sound.

mediator complex A complex of proteins that interacts with the Pol II complex and allows transcription to begin.

medulla A part of the brainstem; also, the inner layer of the mammalian kidney.

meiosis I Reductional division, the first stage of meiotic cell division, in which the number of chromosomes is halved.

meiosis II Equational division, the second stage of meiotic cell division, in which the number of chromosomes is unchanged.

meiotic cell division A form of cell division that includes only one round of DNA replication but two rounds of nuclear division; meiotic cell division makes sexual reproduction possible.

membrane attack complex (MAC) A complex of complement proteins that makes holes in bacterial cells, leading to cell lysis.

membrane potential A difference in electrical charge across the plasma membrane.

memory cell A type of long-lived cell that contains membrane-bound antibodies having the same specificity as the parent cell.

menopause The cessation of menstrual cycles resulting from decreased production of estradiol and progesterone by the ovaries.

menstrual cycle A monthly cycle in females in which oocytes mature and are released from the ovary under the influence of hormones.

menstruation The monthly shedding of the uterine lining.

meristem A discrete population of actively dividing, totipotent cells; apical meristems are located at the tip of stems and roots and produce cells that allow plants to grow in length, while lateral meristems surround stems and roots and produce cells that allow growth in diameter.

meristem identity gene A gene that contributes to meristem stability and function.

mesentery A membrane in the abdominal cavity through which blood vessels, nerves, and lymph travel to supply the gut.

mesoderm The intermediate germ layer, which differentiates into cells that make up connective tissue, muscle cells, red blood cells, bone cells, kidney cells, and gonad cells.

mesoglea In cnidarians, a gelatinous mass enclosed by the epidermis and endodermis.

mesohyl A gelatinous mass that lies between the interior and exterior cell layers of a sponge that contains some amoeba-like cells that function in skeleton formation and the dispersal of nutrients.

mesophyll A leaf tissue of loosely packed photosynthetic cells.

Messel Shale A sedimentary rock formation in Germany, preserving fossils that document fish, birds, mammals, and reptiles from the beginning of the age of mammals.

messenger RNA (mRNA) The RNA molecule that combines with a ribosome to direct protein synthesis; it carries the genetic “message” from the DNA to the ribosome.

metabolic rate An animal’s overall rate of energy use.

metabolism The chemical reactions occurring within cells that convert molecules into other molecules and transfer energy in living organisms.

metamorphosis The process in some animals in which the body changes dramatically at key stages in development.

metanephridia A pair of excretory organs in each body segment of annelid worms that filters the body fluid.

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metaphase The stage of mitosis in which the chromosomes are aligned in the middle of the dividing cell.

metaphase I The stage of meiosis I in which the meiotic spindle is completed and the bivalents move to lie on an imaginary plane cutting transverely across the spindle.

metaphase II The stage of meiosis II in which the chromosomes line up so that their centromeres lie on an imaginary plane cutting across the spindle.

metapopulation A large population made up of smaller populations linked by migration.

methanogens Euryarchaeotes that generate natural gas (methane, CH₄).

MHC class I (genes and proteins) MHC genes and proteins in vertebrates that are expressed on the surface of all nucleated cells.

MHC class II (genes and proteins) MHC genes and proteins in vertebrates that are expressed on the surface of macrophages, dendritic cells, and B cells.

MHC class III (genes and proteins) MHC genes and proteins in vertebrates that encode several proteins of the complement system and proteins involved in inflammation.

micelle A spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed.

microfilament A helical polymer of actin monomers, present in various locations in the cytoplasm, that helps make up the cytoskeleton.

microfossil A microscopic fossil, including fossils of bacteria and protists.

microRNA (miRNA) Small, regulatory RNA molecules that can cleave or destabilize RNA or inhibit its translation.

microtubule A hollow, tubelike polymer of tubulin dimers that helps make up the cytoskeleton.

microvilli Highly folded surfaces of villi, formed by fingerlike projections on the surfaces of epithelial cells.

midbrain Along with the hindbrain, the region of the vertebrate brain that controls basic body functions and behaviors; it is a part of the brainstem.

middle ear The part of the mammalian ear containing three small bones, the malleus, incus, and stapes, which amplify the waves that strike the tympanic membrane.

midgut The middle part of an animal’s digestive tract, including the small intestine.

migration The movement of individuals from one population to another.

mineral nutrients Elements that come from the soil or water that play essential roles in all metabolic and structural processes in plants and algae; see also dietary minerals.

minor groove The smaller of two unequal grooves on the outside of a DNA duplex.

mitochondria (singular, mitochondrion) Specialized organelles that harness energy for the cell from chemical compounds like sugars and convert it into ATP.

mitochondrial DNA (mtDNA) A small circle of DNA, about 17,000 base pairs long in humans, found in every mitochondrion.

mitochondrial genome In eukaryotic cells, the DNA in the mitochondria.

mitochondrial matrix The space enclosed by the inner membrane of the mitochondria.

mitosis In eukaryotic cells, the division of the nucleus, in which the chromosomes are separated into two nuclei.

mitotic spindle A structure in the cytosol made up predominantly of microtubules that pull the chromosomes into separate daughter cells.

Modern Synthesis The current theory of evolution, which combines Darwin’s theory of natural selection and Mendelian genetics.

molar One of the teeth in the back of the mouth of mammals specialized for crushing and shredding tough foods such as meat and fibrous plant material.

molecular clock Estimates of the time when different taxa diverged, based on the amount of genetic divergence between them.

molecular evolution Evolution at the level of DNA, which in time results in the genetic divergence of populations.

molecular fossils Sterols, bacterial lipids, and some pigment molecules, which are relatively resistant to decomposition, that accumulate in sedimentary rocks and document organisms that rarely form conventional fossils.

molecular orbital A merged orbital traversed by a pair of shared electrons.

molecular self-assembly The process by which, when conditions and relative amounts are suitable, viral components spontaneously interact and assemble into mature virus particles.

molecule A substance made up of two or more atoms.

mollusks A monophyletic group distinguished by a mantle, which plays a major role in movement, skeleton-building, breathing, and excretion; includes clams, snails, and squid.

molting Periodic shedding, as of an exoskeleton.

monophyletic Describes groupings in which all members share a single common ancestor not shared with any other species or group of species.

monosaccharide A simple sugar.

morphospecies concept The idea that members of the same species usually look like each other more than like other species.

morula The solid ball of cells resulting from early cell divisions of the fertilized egg.

motor endplate The region on a muscle cell where acetylcholine binds with receptors.

motor neuron A neuron that, on receiving information from interneurons, effects a response in the body.

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motor protein Any of various proteins that are involved in intracellular transport or cause muscle contraction by moving the actin microfilaments inside muscle cells.

motor unit A vertebrate motor neuron and the population of muscle fibers that it innervates.

mouth The first part of the foregut, which receives food. 

mucosa An inner tissue layer with secretory and absorptive functions surrounding the lumen of the digestive tract.

multiple alleles Two or more different alleles of the same gene occurring in a population of organisms.

multiplication rule The principle that the probability of two independent events occurring together is the product of their respective probabilities.

multipotent Describes cells that can form a limited number of types of specialized cell.

multiregional hypothesis The idea that modern humans derive from the Homo ergaster populations that spread around the world starting 2 million years ago.

mutagen An agent that increases the probability of mutation.

mutation Any heritable change in the genetic material, usually a change in the nucleotide sequence of a gene.

mutualism An interaction between two or more species that benefits all.

mycelium A network of branching hyphae.

mycorrhizae Symbioses between roots and fungi that enhance nutrient uptake.

myelin Lipid-rich layers or sheaths formed by glial cells that wrap around the axons of vertebrate neurons and provide electrical insulation.

myofibril A long rodlike structure in muscle fibers that contains parallel arrays of the actin and myosin filaments.

myoglobin An oxygen-binding protein in the cells of vertebrate muscles, related to hemoglobin, that facilitates oxygen delivery to mitochondria.

myosin A motor protein found in cells that carries cargo to the plus ends of microfilaments and is also used by both striated and smooth muscles to contract and generate force.

myotome In chordates, any one of a series of segments that organizes the body musculature.

myriapods One of the four main groups of arthropods, including centipedes and millipedes; distinguished by their many pairs of legs.

natural killer cell A cell type of the innate immune system that does not recognize foreign cells but instead recognizes and kills host cells that are infected by a virus or have become cancerous or otherwise abnormal.

natural selection The process in which, when there is inherited variation in a population of organisms, the variants best suited for growth and reproduction in a given environment contribute disproportionately to future generations. Of all the evolutionary mechanisms, natural selection is the only one that leads to adaptations.

Neanderthal Homo neanderthalensis, a species similar to humans, but with thicker bones and flatter heads that contained brains about the same size as humans’; present in the fossil record 600,000–30,000 years ago.

necrotrophic pathogen A plant pathogen that kills cells before drawing resources from them.

negative feedback Describes the effect in which the final product of a biochemical pathway inhibits the first step; the process in which a stimulus acts on a sensor that communicates with an effector, producing a response that opposes the initial stimulus. Negative feedback is used to maintain steady conditions, or homeostasis.

negative regulation The process in which a regulatory molecule must bind to the DNA at a site near the gene to prevent transcription.

negative selection Natural selection that reduces the frequency of a deleterious allele.

negatively selected Describes T cells that react too strongly to self antigens in association with MHC and are eliminated through cell death.

nematodes Roundworms, the most numerous of all animals; a phylum of the Ecdysozoa.

neoteny The process in which the timing of development is altered so that a sexually mature organism retains the physical characteristics of the juvenile form.

nephron The functional unit of the kidney, consisting of the glomerulus, capsule, renal tubules, and collecting ducts.

nerve A bundle of long fiberlike extensions from multiple nerve cells.

nerve cord A bundle of long fiberlike extensions from multiple nerve cells that serves as the central nervous system of invertebrates such as flatworms and earthworms.

nervous system A network of many interconnected nerve cells.

neural tube In chordates, a cylinder of embryological tissue that develops into a dorsal nerve cord.

neuron Nerve cell; the basic fundamental unit of nervous systems.

neurosecretory cell A neuron in the hypothalamus that secretes hormones into the bloodstream.

neurotransmitter A molecule that conveys a signal from the end of the axon to the postsynaptic target cell.

neutral mutations Genetic changes that have no effect or negligible effects on the organism, or whose effects are not associated with differences in survival or reproduction.

neutron An electrically neutral particle in the atomic nucleus.

neutrophil A type of phagocytic cell that is very abundant in the blood and is often one of the first cells to respond to infection.

niche The combination of traits and habitat in which a species exists.

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nicotinamide adenine dinucleotide phosphate (NADPH) An important cofactor in many biosynthetic reactions; the reducing agent used in the Calvin cycle.

nitrification The process by which chemoautotrophic bacteria oxidize ammonia (NH₃) to nitrite (NO₂⁻) and then nitrate (NO₃⁻).

nitrogen fixation The process in which nitrogen gas (N₂) is converted into ammonia (NH₃), a form biologically useful to primary producers.

nitrogenous waste Waste in the form of ammonia, urea, and uric acid, which are toxic to organisms in varying degrees.

nociceptor A type of nerve cell with dendrites in the skin and connective tissues of the body that responds to excessive mechanical, thermal, or chemical stimuli by withdrawal from the stimulus and by the sensation of pain.

node In phylogenetic trees, the point where a branch splits, representing the common ancestor from which the descendant species diverged. In plants, the point on a shoot where one or more leaves are attached.

nodes of Ranvier Sites on an axon that lie between adjacent myelin-wrapped segments, where the axon membrane is exposed.

non-associative learning Learning that occurs in the absence of any particular outcome, such as a reward or punishment.

nondisjunction The failure of a pair of chromosomes to separate normally during anaphase of cell division.

nonpolar Describes compounds that do not have regions of positive and negative charge.

nonpolar covalent bond A covalent bond between atoms that have the same, or nearly the same, electronegativity.

non-random mating Mate selection biased by genotype or relatedness.

nonrecombinants Progeny in which the alleles are present in the same combination as that present in a parent.

nonself From the point of view of a given organism, describes the molecules and cells of another organism.

nonsense mutation A mutation that creates a stop codon, terminating translation.

non-sister chromatids Chromatids of different members of a pair of homologous chromosomes; although they carry the same complement of genes, they are not genetically identical.

nonsynonymous (missense) mutation A point mutation (nucleotide substitution) that causes an amino acid replacement.

nontemplate strand The untranscribed partner of the template strand of DNA used in transcription.

norm of reaction A graphical depiction of the change in phenotype across a range of environments.

normal distribution A distribution whose plot is a bell-shaped curve.

notochord In chordates, a stiff rod of collagen and other proteins that runs along the back and provides support for the axis of the body.

nuclear envelope The cell structure, composed of two membranes, inner and outer, that defines the boundary of the nucleus.

nuclear genome In eukaryotic cells, the DNA in the chromosomes.

nuclear localization signal The signal sequence for the nucleus that enables proteins to move through pores in the nuclear envelope.

nuclear pore One of many protein channels in the nuclear envelope that act as gateways that allow molecules to move into and out of the nucleus and are thus essential for the nucleus to communicate with the rest of the cell.

nuclear transfer A procedure in which a hollow glass needle is used to insert the nucleus of a cell into the cytoplasm of an egg whose own nucleus has been destroyed or removed.

nucleic acid A polymer of nucleotides that encodes and transmits genetic information.

nucleoid In prokaryotes, a cell structure with multiple loops formed from supercoils of DNA.

nucleolus A distinct, dense, non–membrane-bound spherical structure within the nucleus that contains the genes and transcripts for ribosomal RNA.

nucleoside A molecule consisting of a 5-carbon sugar and a base.

nucleosome A beadlike repeating unit of histone proteins wrapped with DNA making up the 10-nm chromatin fiber.

nucleotide A constituent of nucleic acids, consisting of a 5-carbon sugar, a nitrogen-containing base, and one or more phosphate groups.

nucleotide excision repair The repair of multiple mismatched or damaged bases across a region; a process similar to mismatch repair, but over a much longer piece of DNA, sometimes thousands of nucleotides.

nucleotide substitution A mutation in which a base pair is replaced by a different base pair; this is the most frequent type of mutation; also known as a point mutation.

nucleus (of an atom) The dense central part of an atom containing protons and neutrons.

nucleus (of a cell) The compartment of the cell that houses the DNA in chromosomes.

obligate Describes a mutualism in which one or both sides cannot survive without the other.

observation The act of viewing the world around us.

occipital lobe The region of the brain that processes visual information from the eyes.

ocean acidification An increase in the abundance of carbon dioxide in the oceans that causes the pH of seawater to go down.

Okazaki fragment In DNA replication, any of the many short DNA pieces in the lagging strand.

olfaction The sense of smell.

oligodendrocyte A type of glial cell that insulates cells in the brain and spinal cord by forming a myelin sheath.

oligonucleotide A short (typically 20 to 30 nucleotides), single-stranded molecule of known sequence produced by chemical synthesis; oligonucleotides are often used as primer sequences in the polymerase chain reaction.

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omasum The third chamber in the stomach of ruminants, into which the mixture of food and bacteria passes and where water is reabsorbed.

ommatidia (singular, ommatidium) Individual light-focusing elements that make up the compound eyes of insects and crustaceans; the number of ommatidia determines the resolution of the image.

oncogene A cancer-causing gene.

oocyte The unfertilized egg cell produced by the mother; the developing female gamete.

oogenesis The formation of ova or eggs.

open circulatory system A circulatory system found in many smaller animals that contains few blood vessels and in which most of the circulating fluid is contained within the animal’s body cavity.

open reading frame (ORF) A stretch of DNA or RNA consisting of codons for amino acids uninterrupted by a stop codon. In genome annotation, this sequence motif identifies the region as potentially protein coding.

operant conditioning Associative learning in which a novel behavior that was initially undirected has become paired with a particular stimulus through reinforcement.

operator The binding site for a repressor protein.

operon A group of functionally related genes located in tandem along the DNA and transcribed as a single unit from one promoter; the region of DNA consisting of the promoter, the operator, and the coding sequence for the structural genes.

Opisthokonta A superkingdom that encompasses animals, fungi, and related protists.

opsin A photosensitive protein that converts the energy of light photons into electrical signals in the receptor cell.

opsonization The binding of a molecule to a pathogen to facilitate uptake by a phagocyte.

optic nerve A cranial nerve that transmits action potentials from ganglion cells in the retina to the visual cortex of the brain.

orbital A region in space where an electron is present most of the time.

order A group of closely related families.

organ Two or more tissues that combine and function together.

organ of Corti A structure in the cochlear duct, supported by the basilar membrane, with specialized hair cells with stereocilia, that functions to convert mechanical vibrations to electrical impulses.

organelle Any one of several compartments in eukaryotes that divide the cell contents into smaller spaces specialized for different functions.

organic molecule A carbon-containing molecule.

organogenesis The transformation of the three germ layers into all the organ systems of the body.

origin of replication Any point on a DNA molecule at which DNA synthesis is initiated.

osmoconformer An animal that matches its internal osmotic pressure to that of its external environment.

osmoregulation The regulation of water and solute levels to control osmotic pressure.

osmoregulator An animal that maintains internal solute concentrations that differ from that of its environment.

osmosis The net movement of a solvent, such as water, across a selectively permeable membrane toward the side of higher solute concentration.

osmotic pressure The pressure needed to prevent water from moving from one solution into another by osmosis.

Osteichthyes Bony fish that have a cranium, jaws, and mineralized bones; there are about 20,000–25,000 species.

osteoblast A type of cell that forms bone tissue.

osteoclast A type of cell that secretes digestive enzymes and acid that dissolves the calcium mineral and collagen in bone.

outer ear The part of the human ear that includes the pinna, the ear canal, and tympanic membrane.

out-of-Africa hypothesis The idea that modern humans arose from Homo ergaster descendants in Africa before dispersing beyond Africa around 60,000 years ago.

oval window The thin membrane between the stapes of the middle ear and the cochlea of the inner ear.

ovary In plants, a hollow structure at the base of the carpel in which the ovules develop and which protects the ovules from being eaten or damaged by animals; in animals, the female gonad where eggs are produced.

oviparity Laying eggs.

ovoviviparity Giving birth to live young, with nutritional support of the embryo from the yolk.

ovule A reproductive structure in seed plants consisting of the female gametophyte developing within a sporangium and surrounded by outer protective layers. Ovules, when fertilized, develop into seeds.

ovule cone A reproductive shoot in gymnosperms that produces ovules.

ovum (egg) (plural, ova) The larger, female gamete.

oxidation reaction Describes a reaction in which a molecule loses electrons.

oxidation–reduction reaction A reaction involving the loss and gain of electrons between reactants. In biological systems these reactions are often used to store or release chemical energy.

oxidative phosphorylation A set of metabolic reactions that occurs by passing electrons along an electron transport chain to the final electron acceptor, oxygen, pumping protons across a membrane, and using the proton electrochemical gradient to drive synthesis of ATP.

oxygen dissociation curve The curve that results when blood pO₂ is plotted against the percentage of O₂ bound to hemoglobin.

oxygenic Producing oxygen.

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oxytocin A posterior pituitary gland hormone that causes uterine contraction during labor and stimulates the release of milk during breastfeeding.

P₁ generation The parental generation in a series of crosses.

pacemaker Describes cardiac muscle cells that function as a regulator of heart rhythm.

palindromic Reading the same in both directions; describes sequence identity along the paired strands of a duplex DNA molecule; a symmetry typical of restriction sites.

pancreas A secretory gland that has both endocrine function, secreting hormones, including insulin, directly into the blood, and exocrine function, aiding the digestion of proteins, carbohydrates, and fats by secreting digestive enzymes into ducts that connect to the duodenum.

paracrine signaling Signaling by a molecule that travels a short distance to the nearest neighboring cell to bind its receptor and deliver its message.

paraphyletic Describes groupings that include some, but not all, the descendants of a common ancestor.

parasexual Describes asexual species that generate genetic diversity by the crossing over of DNA during mitosis.

parasites Organisms that live in close association with another species, consuming or gaining nutrition from their hosts’ tissues, generally without killing them.

parasympathetic division The division of the autonomic nervous system that slows the heart and stimulates digestion and metabolic processes that store energy, enabling the body to “rest and digest.”

parathyroid gland A gland adjacent to the thyroid gland that secretes parathyroid hormone (PTH), which, with calcitonin, regulates the actions of bone cells.

parenchyma In plants, describes thin-walled, undifferentiated cells.

parietal lobe The brain region, posterior to the frontal lobe, that controls body awareness and the ability to perform complex tasks.

parsimony Choosing the simplest hypothesis to account for a given set of observations.

parthenogenesis A form of asexual reproduction in which females produce eggs that are not fertilized by males but divide by mitosis and develop into new individuals.

partial pressure (p) The fractional concentration of a gas relative to other gases present multiplied by the atmospheric pressure exerted on the gases.

partially reproductively isolated Describes populations that have not yet diverged as separate species but whose genetic differences are extensive enough that the hybrid offspring they produce have reduced fertility or viability compared with offspring produced by crosses between individuals within each population.

patch A bit of habitat that is separated from other bits by inhospitable environments that are difficult or risky for individuals to cross.

paternal inheritance A type of inheritance in which the organelles in the offspring cells derive from those in the father.

pathogens Organisms and other agents, such as viruses, that cause disease.

peat bog Wetland in which dead organic matter accumulates.

pedigree A diagram of family history that summarizes the record of the ancestor-descendent relationships among individuals.

penis The male copulatory organ.

pepsin An enzyme produced in the stomach that breaks down proteins.

peptide bond A covalent bond that links the carbon atom in the carboxyl group of one amino acid to the nitrogen atom in the amino group of another amino acid.

peptide hormone A hormone that is a short chain of linked amino acids.

peptidoglycan A complex polymer of sugars and amino acids that makes up the cell wall.

peptidyl (P) site One of three binding sites for tRNA on the large subunit of a ribosome.

pericycle In roots, a single layer of cells just to the inside of the endodermis from which new root apical meristems are formed, allowing roots to branch.

periodic selection The episodic loss of diversity as a successful variant outcompetes others.

periodic table of the elements The arrangement of the chemical elements in tabular form, organized by their chemical properties.

peripatric speciation A specific kind of allopatric speciation in which a few individuals from a mainland population disperse to a new location remote from the original population and evolve separately.

peripheral membrane protein A protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions.

peripheral nervous system (PNS) Collectively, the sensory and motor nerves, including the cranial and spinal nerves, and interneurons and ganglia.

peristalsis Waves of muscular contraction that move food toward the base of the stomach.

personalized medicine An approach in which the treatment is matched to the patient, not the disease; examination of an individual’s genome sequence, by revealing his or her disease susceptibilities and drug sensitivities, allows treatments to be tailored to that individual.

peroxisomes Organelles in eukaryotic cells that contain many different enzymes and are involved in metabolic reactions.

petal A structure, often brightly colored and distinctively shaped, occurring in the next-to-outermost whorl of a flower; petals attract and orient animal pollinators.

phagocyte A type of immune cell that engulfs and destroys foreign cells or particles.

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phagocytosis The engulfing of a cell or particle by another cell.

pharyngeal slit A vertical opening separated from other slits by stiff rods of protein in the pharynx of hemichordates.

pharynx The region of the throat that connects the nasal and mouth cavities; in hemichordates, a tube that connects the mouth and the digestive tract.

phenol Any one of a class of compounds, produced by some plants as a defensive mechanism.

phenotype The expression of a physical, behavioral, or biochemical trait; an individual’s observable phenotypes include height, weight, eye color, and so forth.

pheromone A water- or airborne chemical compound released by animals into the environment that signals and influences the behavior of other members of their species.

phloem The vascular tissue that transports carbohydrates from leaves to the rest of the plant body.

phloem sap The sugar-rich solution that flows through both the lumen of the sieve tubes and the sieve plate pores.

phosphatase An enzyme that removes a phosphate group from another molecule.

phosphate group A chemical group consisting of a phosphorus atom bonded to four oxygen atoms.

phosphodiester bond A bond that forms when a phosphate group in one nucleotide is covalently joined to the sugar unit in another nucleotide. Phosophodiester bonds are relatively stable and form the backbone of a DNA strand.

phospholipid A type of lipid and a major component of the cell membrane.

photic zone The surface layer of the ocean through which enough sunlight penetrates to enable photosynthesis.

photoheterotroph An organism that uses the energy from sunlight to make ATP and relies on organic molecules obtained from the environment as the source of carbon for growth and other vital functions.

photoperiod Day length.

photoperiodism The effect of the photoperiod, or day length, on flowering.

photoreceptor A molecule whose chemical properties are altered when it absorbs light; also, photoreceptors are the sensory receptors in the eye.

photorespiration A process in which rubisco acts as an oxygenase, resulting in a release of CO₂ and a net loss of energy.

photosynthesis The biochemical process in which carbohydrates are built from carbon dioxide and the energy of sunlight.

photosynthetic electron transport chain A series of redox reactions in which light energy absorbed by chlorophyll is used to power the movement of electrons; in oxygenic photosynthesis, the electrons ultimately come from water and the terminal electron acceptor is NADP+.

photosystem A protein-pigment complex that absorbs light energy to drive redox reactions and thereby sets the photosynthetic electron transport chain in motion.

photosystem I The photosystem that energizes electrons with a second input of light energy so they have enough energy to reduce NADP+.

photosystem II The photosystem that supplies electrons to the beginning of the electron transport chain. When photosystem II loses an electron, it can pull electrons from water.

phototroph An organism that captures energy from sunlight.

phototropic Bending in response to light. A positive phototropic response, as in stems, is toward light; a negative phototropic response, as in roots, is away from light.

phragmoplast In dividing plant cells, a structure formed by overlapping microtubules that guide vesicles containing cell wall components to the middle of the cell.

phylogenetic niche conservatism The observed similarity in closely related species of some aspect of their niches, indicating its presence in their common ancestor.

phylogenetic species concept (PSC) The idea that members of a species all share a common ancestry and a common fate.

phylogenetic tree A diagrammed hypothesis about the evolutionary history, or phylogeny, of a species.

phylogeny The history of descent with modification and the accumulation of change over time.

phylum (plural, phyla) A group of closely related classes, defined by having one of a number of distinctive body plans.

phytochrome A photoreceptor for red and far-red light that switches back and forth between two stable forms, active and inactive, depending on its exposure to light.

pilus (plural, pili) A threadlike structure that connects bacteria, allowing plasmids to be transferred between them.

pineal gland A gland located in the thalamic region of the brain that responds to autonomic nervous system input by secreting melatonin, which controls wakefulness.

pinna (plural, pinnae) Small unit of the photosynthetic surface on fern leaves; also, the external structure of mammalian ears that enhance the reception of sound waves contacting the ear.

pit A circular or ovoid region in the walls of xylem cells where the lignified cell wall layer is not produced.

pith In a stem, the region inside the ring of vascular bundles.

pituitary gland A gland beneath the brain that produces a number of hormones, including growth hormone.

placenta In placental mammals, an organ formed by the fusion of the chorion and allantois that allows the embryo to obtain nutrients directly from the mother.

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placental mammal A mammal that provides nutrition to the embryo through the placenta, a temporary organ that develops in the uterus; placental mammals include carnivores, primates, hooved mammals, and whales.

placozoans Possibly the simplest of all animals; each contains only a few thousand cells arranged into upper and lower epithelia that sandwich an interior fluid crisscrossed by a network of multinucleate fiber cells.

plasma (effector) cell A cell that secretes antibodies.

plasma membrane The membrane that defines the space of the cell, separating the living material within the cell from the nonliving environment around it.

plasmid In bacteria, a small circular molecule of DNA carrying a small number of genes that can replicate independently of the bacterial genomic DNA.

plasmodesmata (singular, plasmodesma) Connections between the plasma membranes of adjacent plant cells that permit molecules to pass directly from the cytoplasm of one cell to the cytoplasm of another.

plasmogamy The cytoplasmic union of two cells.

plate tectonics The dynamic movement of Earth’s crust, the outer layer of Earth.

pleiotropy The phenomenon in which a single gene has multiple effects on seemingly unrelated traits.

pluripotent Describes embryonic stem cells (cells of the inner mass), which can give rise to any of the three germ layers and therefore to any cell of the body.

podocytes Cells with footlike processes that make up one of the three layers of the filtration barrier of the glomerulus.

point mutation A mutation in which a base pair is replaced by a different base pair; this is the most frequent type of mutation; also known as a nucleotide substitution.

Pol II The RNA polymerase complex responsible for transcription of protein-coding genes.

polar (molecule) A molecule that has regions of positive and negative charge.

polar body A small cell produced by the asymmetric first meiotic division of the primary oocyte.

polar covalent bond Bonds that do not share electrons equally.

polar transport The coordinated movement of auxin across many cells.

polarity An asymmetry such that one end of a structure differs from the other.

polarized Having opposite properties in opposite parts; describes a resting membrane potential in which there is a buildup of negatively charged ions on the inside surface of the cell’s plasma membrane and positively charged ions on its outer surface.

pollen In seed plants, the multicellular male gametophyte surrounded by a sporopollenin-containing outer wall.

pollen cone A reproductive shoot in gymnosperms that produces pollen.

pollen tube A structure produced by the male gametophyte that grows through an opening in the sporopollenin-containing pollen wall and eventually conveys the sperm to the ovule.

pollination The process in which pollen is carried to an ovule.

poly(A) tail The nucleotides added to the 3′ end of the primary transcript by polyadenylation.

polyadenylation The addition of a long string of consecutive A-bearing ribonucleotides to the 3′ end of the primary transcript.

polycistronic mRNA A single molecule of messenger RNA that is formed by the transcription of a group of functionally related genes located next to one another along bacterial DNA.

polymer A complex organic molecule made up of repeated simpler units connected by covalent bonds.

polymerase chain reaction (PCR) A selective and highly sensitive method for making copies of a piece of DNA, which allows a targeted region of a DNA molecule to be replicated into as many copies as desired.

polymorphism Any genetic difference among individuals sufficiently common that it is likely to be present in a group of 50 randomly chosen individuals.

polypeptide A polymer of amino acids connected by peptide bonds.

polyphyletic Describes groupings that do not include the last common ancestor of all members.

polyploidy The condition of having more than two complete sets of chromosomes in the genome.

polysaccharide A polymer of simple sugars. Polysaccharides provide long-term energy storage or structural support.

polyspermy In animals, fertilization by more than one sperm.

pons A part of the brainstem.

population All the individuals of a given species that live and reproduce in a particular place; one of several interbreeding groups of organisms of the same species living in the same geographical area.

population density The size of a population divided by its range.

population size The number of individuals of all ages alive at a particular time in a particular place.

positive feedback In the nervous system, the type of feedback in which a stimulus causes a response that leads to an enhancement of the original stimulus that leads to a larger response. In the endocrine system, the type of feedback in which a stimulus causes a response, and that response causes a further response in the same direction. In both cases, the process reinforces itself until interrupted.

positive regulation The process in which a regulatory molecule must bind to the DNA at a site near the gene in order for transcription to take place.

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positive selection Natural selection that increases the frequency of a favorable allele.

positively selected Describes T cells that recognize self MHC molecules on epithelium cells and continue to mature.

posterior pituitary gland The region of the pituitary gland that develops from neural tissue at the base of the brain and into which neurosecretory cells of the hypothalamus extend that secrete releasing factors.

postreplication mismatch repair The correction of a mismatched base in a DNA strand by cleaving one of the strand backbones, degrading the sequence with the mismatch, and resynthesizing from the intact DNA strand.

posttranslational modification The modification, after translation, of proteins in ways that regulate their structure and function.

post-zygotic isolation Describes factors that cause the failure of the fertilized egg to develop into a fertile individual.

potential energy Stored energy that is released by a change in an object’s structure or position.

potential evapotranspiration The amount of evapotranspiration that temperature, humidity, and wind would cause if water supply weren’t limiting; the demand on the water resources of an ecosystem.

potential evapotranspiration ratio In an ecosystem, the ratio of water demand to supply.

power stroke The stage in the cross-bridge cycle in which the myosin head pivots and generates a force, causing the myosin and actin filaments to slide relative to each other.

predation An interaction between organisms in which one (the predator) consumes the other (the prey).

pregnancy The carrying of one or more embryos in the mammalian uterus.

premolar One of the teeth of mammals between the canines and molars that are specialized for shearing tough foods.

pre-zygotic isolation Describes factors that prevent the fertilization of an egg.

primary active transport Active transport that uses the energy of ATP directly.

primary bronchi The two divided airways from the trachea, supported by cartilage rings, each airway leading to a lung.

primary consumers Herbivores, which consume primary (plant or algae) producers. Sometimes called grazers.

primary growth The increase in plant length made possible by apical meristems.

primary motor cortex The part of the frontal lobe of the brain that produces complex coordinated behaviors by controlling skeletal muscle movements.

primary oocyte A diploid cell formed by mitotic division of oogonia during fetal development.

primary producer An organism that takes up inorganic carbon, nitrogen, phosphorus, and other compounds from the environment and converts them into organic compounds that will provide food for other organisms in the local environment.

primary response The response to the first encounter with an antigen, during which there is a short lag before antibody is produced.

primary somatosensory cortex The part of the parietal lobe that integrates tactile information from specific body regions and relays it to the motor cortex.

primary spermatocyte A diploid cell formed by mitotic division of spermatogonia at the beginning of spermatogenesis.

primary structure The sequence of amino acids in a protein.

primary transcript The initial RNA transcript that comes off the template DNA strand.

primate A member of an order of mammals that share a number of general features that distinguish them from other mammals, including nails rather than claws, front-facing eyes, and an opposable thumb.

primer A short stretch of RNA at the beginning of each new DNA strand that serves as a starter for DNA synthesis; an oligonucleotide that serves as a starter in the polymerase chain reaction.

primordia (singular, primordium) An organ in its earliest stage of development; leaf primordia form near the tips of shoot apical meristems and develop into leaves.

principle of independent assortment The principle that segregation of one set of alleles of a gene pair is independent of the segregation of another set of alleles of a different gene pair.

principle of segregation The principle by which half the gametes receive one allele of a gene and half receive the other allele.

probability Among a very large number of observations, the expected proportion of observations that are of a specified type.

probe A labeled DNA fragment that can be tracked in a procedure such as a Southern blot.

procambial cell A cell that retains the capacity for cell division and gives rise to both xylem and phloem.

product Any one of the transformed molecules that result from a chemical reaction.

progesterone A hormone secreted by the vertebrate ovaries that maintains the thickened and vascularized uterine lining.

prokaryote An organism whose cell or cells does not have a nucleus. Often used to refer collectively to archaeons and bacteria.

prometaphase The stage of mitosis in which the nuclear envelope breaks down and the microtubules of the mitotic spindle attach to chromosomes.

prometaphase I The stage of meiosis I in which the nuclear envelope breaks down and the meiotic spindles attach to kinetochores on chromosomes.

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prometaphase II The stage of meiosis II in which the meiotic spindles attach to kinetochores on chromosomes.

promoter A regulatory region where RNA polymerase and associated proteins bind to the DNA duplex.

proofreading The process in which DNA polymerases can immediately correct their own errors by excising and replacing a mismatched base.

prophase The stage of mitosis characterized by the appearance of visible chromosomes.

prophase I The beginning of meiosis I, marked by the visible manifestation of chromosome condensation.

prophase II The stage of meiosis II in which the chromosomes in the now-haploid nuclei recondense to their maximum extent.

prostate gland An exocrine gland that produces a thin, slightly alkaline fluid that helps maintain sperm motility and counteracts the acidity of the female reproductive tract.

protease inhibitor An antidigestive protein that binds to the active site of enzymes that break down proteins in a herbivore’s digestive system.

protein family A group of proteins that are structurally and functionally related.

protein sorting The process by which proteins end up where they need to be in the cell to perform their function.

proteins The key structural and functional molecules that do the work of the cell, providing structural support and catalyzing chemical reactions. The term “protein” is often used as a synonym for “polypeptide.”

proteobacteria The most diverse bacterial group, defined largely by similarities in rRNA gene sequences; it includes many of the organisms that populate the expanded carbon cycle and other biogeochemical cycles.

protist An organism having a nucleus but lacking other features specific to plants, animals, or fungi.

proton A positively charged particle in the atomic nucleus.

protonephridia Excretory organs in flatworms that isolate waste from the body cavity.

proto-oncogene The normal cellular gene counterpart to an oncogene, which is similar to a viral oncogene but can cause cancer only when mutated.

protostome A bilaterian in which the blastopore, the first opening to the internal cavity of the developing embryo, becomes the mouth.

protozoan (plural, protozoa) A heterotrophic protist.

proximal convoluted tubule The first portion of the renal tubule from which electrolytes and other nutrients are reabsorbed into the blood.

pseudogene A gene that is no longer functional.

pulmonary artery One of two arteries, left and right, that carries deoxygenated blood from the right ventricle to the lungs.

pulmonary capillary A small blood vessel that supplies the alveolar wall.

pulmonary circulation Circulation of the blood to the lungs.

pulmonary valve A valve between the right ventrical and the pulmonary trunk, which divides into the pulmonary arteries.

pulmonary vein A vein that returns oxygenated blood from the lungs to the left ventricle.

Punnett square A worksheet in the form of a checkerboard used to predict the consequences of a random union of gametes.

pupa The quiescent stage of metamorphosis in insects, during which the body tissues undergo a transformation from larva to an adult.

pupil An opening in the vertebrate eye through which light enters.

purine In nucleic acids, either of the bases adenine and gunanine, which have a double-ring structure.

pyloric sphincter A band of muscle at the base of the stomach that opens to allow small amounts of digested food to enter the small intestine.

pyrimidine In nucleic acids, any of the bases thymine, cytosine, and uracil, which have a single-ring structure.

quantitative trait A complex trait in which the phenotype is measured along a continuum with only small intervals between similar individuals.

quaternary structure The structure that results from the interactions of several polypeptide chains.

R gene Any one of the group of genes that express the R proteins in plants.

R group A chemical group attached to the central carbon atom of an amino acid, whose structure and composition determine the identity of the amino acid; also known as a side chain.

R protein Any one of a group of receptors in plant cells, each expressed by a different gene, that function as part of the plant’s immune system by each binding to a specific pathogen-derived protein.

radiometric dating Dating by using the decay of radioisotopes as a yardstick, including (for time intervals up to a few tens of thousands of years) the decay of radioactive ¹⁴C to nitrogen and (for most of Earth history) the decay of radioactive uranium to lead.

rain shadow The area on the lee side of mountains, where air masses descend, warming and taking up water vapor; as a result, lands in the rain shadow are arid.

range The extent of the geographic area over which the populations of a species are distributed.

reabsorption In renal systems, an active or passive process in which substances that are important for an animal to retain are taken up by cells of the excretory tubule and returned to the bloodstream.

reactant Any of the starting molecules in a chemical reaction.

reaction center Specially configured chlorophyll molecules where light energy is converted into electron transport.

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reactive oxygen species Highly reactive forms of oxygen.

reading frame Following a start codon, a consecutive sequence of codons for amino acids.

realized niche The actual range of habitats occupied by a species.

receiver The individual who, during communication, receives from the sender a signal that elicits a response.

receptor A molecule on cell membranes that detects critical features of the environment. Receptors detecting signals that easily cross the cell membrane are sometimes found in the cytoplasm.

receptor activation The “turning on” of a receptor, which often occurs when a signaling molecule binds to a receptor on a responding cell.

receptor kinase A receptor that is an enzyme that adds a phosphate group to another molecule.

receptor protein The molecule on the responding cell that binds to the signaling molecule.

recessive The trait that fails to appear in heterozygous genotypes from a cross between the corresponding homozygous genotypes.

reciprocal altruism The exchange of favors between individuals.

reciprocal cross A cross in which the female and male parents are interchanged.

reciprocal inhibition The activation of opposing sets of muscles so that one set is inhibited as the other is activated, allowing the movement of joints such as the knee.

reciprocal translocation Interchange of parts between nonhomologous chromosomes.

recombinant An offspring with a different combination of alleles from that of either parent, resulting from one or more crossovers in prophase I of meiosis.

recombinant DNA DNA molecules from two (or more) different sources combined into a single molecule.

recovery metabolism An animal’s elevated consumption of oxygen following activity.

rectum The part of the hindgut where feces are stored until elimination.

Redfield ratio On average, cells contain atoms in the ratio 1 P:16 N:100 C, a correspondence first described by the American biologist Alfred Redfield.

reduction A reaction in which a molecule gains electrons.

reductional division An alternative name for meiosis I, since this division reduces the number of chromosomes by half.

refractory period The period following an action potential during which the inner membrane voltage falls below and then returns to the resting potential.

regeneration In the context of photosynthesis, the third step of the Calvin cycle, in which the 5-carbon molecule needed for carboxylation is produced.

regenerative medicine A discipline that aims to use the natural processes of cell growth and development to replace diseased or damaged tissues.

regression toward the mean With regard to complex traits, the principle that offspring exhibit an average phenotype that is intermediate between that of the parents and that of the population as a whole.

regulatory transcription factor A protein that recruits the components of the transcription complex to the gene.

reinforcement of reproductive isolation (reinforcement) The process by which diverging populations undergo natural selection in favor of enhanced pre-zygotic isolation to prevent the production of inferior hybrid offspring.

release factor A protein that causes a finished polypeptide chain to be freed from the ribosome.

releasing factor A peptide hormone that signals to the anterior pituitary gland through blood vessels, leading to a much larger release of associated hormones from that organ.

renal tubules Tubes in the vertebrate kidney that process the filtrate from the glomerulus by reabsorption and secretion.

renaturation The base pairing of complementary single-stranded nucleic acids to form a duplex; also known as hybridization, it is the opposite of denaturation.

renin An enzyme produced by the kidneys that converts angiotensinogen to angiotensin I and is involved in the regulation of blood volume and pressure.

repetitive DNA The collective term for repeated sequences of various types in eukaryotic genomes.

replication The process of copying DNA so genetic information can be passed from cell to cell or from an organism to its progeny.

replication bubble A region formed by the opening of a DNA duplex at an origin of replication, which has a replication fork at each end.

replication fork The site where the parental DNA strands separate as the DNA duplex unwinds.

repressor A protein that, when bound with a sequence in DNA, can inhibit transcription.

reservoir A supply or source of a substance. Reservoirs of carbon, for example, include organisms, the atmosphere, soil, the oceans, and sedimentary rocks.

residue In the context of protein synthesis, any of the amino acids that is incorporated into a protein.

resource partitioning A pattern in which species whose niches overlap may diverge to minimize the overlap.

responding cell The cell that receives information from the signaling molecule.

response A change in cellular behavior, such as activation of enzymes or genes, following a signal.

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resting membrane potential The negative voltage across the membrane at rest.

restriction enzyme Any one of a class of enzymes that recognizes specific, short nucleotide sequences in double-stranded DNA and cleaves DNA at or near these sites.

restriction fragment length polymorphism (RFLP) A polymorphism in which the length of the restriction fragments is different in the two alleles.

restriction site A recognition sequence in DNA cutting, which is typically four or six base pairs long; most restriction enzymes cleave double-stranded DNA at or near these restriction sites.

reticulum The second chamber in the stomach of ruminants, which, along with the rumen, harbors large populations of anaerobic bacteria that break down cellulose.

retina A thin tissue in the posterior of the vertebrate eye that contains the photoreceptors and other nerve cells that sense and initially process light stimuli.

retinal A derivative of vitamin A that absorbs light and binds to rhodopsin, a transmembrane protein in the photosensitive cells of vertebrates.

retrotransposons Transposable elements in DNA sequences in which RNA is used as a template to synthesize complementary strands of DNA, a reversal of the usual flow of genetic information from DNA into RNA.

reverse transcriptase An RNA-dependent DNA polymerase that uses a single-stranded RNA as a template to synthesize a DNA strand that is complementary in sequence to the RNA.

rhizosphere The soil layer that surrounds actively growing roots.

ribonucleic acid (RNA) A molecule chemically related to DNA that is synthesized by proteins from a DNA template.

ribose The sugar in RNA.

ribosomal RNA (rRNA) Noncoding RNA found in all ribosomes that aid in translation.

ribosome A complex structure of RNA and protein, bound to the cytosolic face of the RER in the cytoplasm, on which proteins are synthesized.

ribulose bisphosphate carboxylase oxygenase (rubisco) The enzyme that catalyzes the carboxylation reaction in the Calvin cycle.

ribulose 1,5-bisphosphate (RuBP) The 5-carbon sugar to which carbon dioxide is added by the enzyme rubisco.

ring species Species that contain populations that are reproductively isolated from each other but can exchange genetic material through other, linking populations.

RISC (RNA-induced silencing complex) A protein complex that is targeted to specific mRNA molecules by base pairing with short regions on the target mRNA, inhibiting translation or degrading the RNA.

ritualization The process of co-opting and modifying behaviors used in another context by increasing the conspicuousness of the behavior, reducing the amount of variation in the behavior so that it can be immediately recognized, and increasing its separation from the original function.

RNA editing The process in which some RNA molecules become a substrate for enzymes that modify particular bases in the RNA, thereby changing its sequence and sometimes what it codes for.

RNA polymerase The enzyme that carries out polymerization of ribonucleoside triphosphates from a DNA template to produce an RNA transcript.

RNA polymerase complex An aggregate of proteins that synthesizes the RNA transcript complementary to the template strand of DNA.

RNA primase An RNA polymerase that synthesizes a short piece of RNA complementary to the DNA template and does not require a primer.

RNA processing Chemical modification that converts the primary transcript into finished mRNA, enabling the RNA molecule to be transported to the cytoplasm and recognized by the translational machinery.

RNA splicing The process of intron removal from the primary transcript.

RNA transcript The RNA sequence synthesized from a DNA template.

RNA world hypothesis The hypothesis that the earliest organisms relied on RNA for both catalysis and information storage.

rod cell A type of photoreceptor cell on the retina that detects light and shades ranging from white to shades of gray and black, but not color.

root apical meristem A group of totipotent cells near the tip of a root that is the source of new root cells.

root cap A structure that covers and protects the root apical meristem as it grows through the soil.

root hair A slender outgrowth produced by epidermal cells that greatly increases the surface area of the root.

root nodule A structure, formed by dividing root cells, in which nitrogen-fixing bacteria live.

roots A major organ system of vascular plants, generally belowground.

rough endoplasmic reticulum (RER) The part of the endoplasmic reticulum with attached ribosomes.

r-strategist A species that produces large numbers of offspring but provides few resources for their support.

rumen The first chamber in the stomach of ruminants, which, along with the reticulum, harbors large populations of anaerobic bacteria that break down cellulose.

S phase The phase of interphase in which the entire DNA content of the nucleus is replicated.

saccharide The simplest carbohydrate molecule, also called a sugar.

saltatory propagation The movement of an action potential along a myelinated axon, “jumping” from node to node.

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Sanger sequencing A procedure in which chemical termination of daughter strands help in determining the DNA sequence.

sapwood In long-lived trees, the layer adjacent to the vascular cambium that contains the functional xylem.

sarcomere The region from one Z disc to the next, the basic contractile unit of a muscle.

sarcoplasmic reticulum (SR) A modified form of the endoplasmic reticulum surrounding the myofibrils of muscle cells.

saturated Describes fatty acids that do not contain double bonds; the maximum number of hydrogen atoms is attached to each carbon atom, “saturating” the carbons with hydrogen atoms.

scaffold A supporting protein structure in a metaphase chromosome.

scientific method A deliberate, careful, and unbiased way of learning about the natural world.

sclera A tough, white outer layer surrounding the vertebrate eye.

scrotum A sac outside the abdominal cavity of the male that holds the testes.

second law of thermodynamics The principle that the transformation of energy is associated with an increase in the degree of disorder in the universe.

secondary consumers Predators or scavengers that feed on primary consumers.

second messenger An intermediate cytosolic signaling molecule that transmits signals from a receptor to a target within the cell. (First messengers transmit signals from outside the cell to a receptor.)

secondary active transport Active transport that uses the energy of an electrochemical gradient to drive the movement of molecules.

secondary growth The increase in plant diameter resulting from meristems that surround stems and roots.

secondary oocyte A large cell produced by the asymmetric first meiotic division of the primary oocyte.

secondary phloem New phloem cells produced by the vascular cambium, which are located to the outside of the vascular cambium.

secondary response The response to re-exposure to an antigen, which is quicker, stronger, and longer than the primary response.

secondary sexual characteristic A trait that characterizes and differentiates the two sexes but that does not relate directly to reproduction.

secondary spermatocyte A diploid cell formed during the first meiotic division of the primary spermatocyte.

secondary structure The structure formed by interactions between stretches of amino acids in a protein.

secondary xylem New xylem cells produced by vascular cambium, which are located to the inside of the vascular cambium.

second-division nondisjunction Disjunction in the second meiotic division.

secretin A hormone released by cells lining the duodenum in response to the acidic pH of the stomach contents entering the small intestine and that stimulates the pancreas to secrete bicarbonate ions.

secretion In renal systems, an active process that eliminates substances that were not previously filtered from the blood.

seed A fertilized ovule; seeds are multicellular structures that allow offspring to disperse away from the parent plant.

seed coat A protective outer structure surrounding the seed.

segmentation The formation of discrete parts or segments in the insect embryo.

segregate Separate; applies to chromosomes or members of a gene pair moving into different gametes.

selection The retention or elimination of mutations in a population of organisms.

selectively permeable Describes the properties of some membranes, including the plasma membrane, which lets some molecules in and out freely, lets others in and out only under certain conditions, and prevents other molecules from passing through at all.

self Describes an organism’s own molecules and cells.

self-compatible Describes species in which pollen and ovules produced by flowers on the same plant can produce viable offspring.

self-incompatible Describes species in which pollination by the same or a closely related individual does not lead to fertilization.

self-propagating Continuing without input from an outside source; action potentials are self-propagating in that they move along axons by sequentially opening and closing adjacent ion channels.

semen A fluid that nourishes and sustains sperm as they travel in the male and then the female reproductive tracts.

semicircular canal One of three connected fluid-filled tubes in the mammalian inner ear that contains hair cells that sense angular motions of the head in three perpendicular planes.

semiconservative replication The mechanism of DNA replication in which each strand of a parental DNA duplex serves as a template for the synthesis of a new daughter strand.

seminal vesicles Two glands at the junction of the vas deferens and the prostate gland that secrete a protein- and sugar-rich fluid that makes up most of the semen and provides energy for sperm motility.

seminiferous tubules A series of tubes in the testes where sperm are produced.

sender The indivdual who, during communication, supplies a signal that elicits a response from the receiver.

sensitization The enhancement of a response to a stimulus that is achieved by first presenting a strong or novel stimulus.

sensory neuron A neuron that receives and transmits information about an animal’s environment or its internal physiological state.

sensory organ A group of sensory receptors that converts particular physical and chemical stimuli into nerve impulses that are processed by a nervous system and sent to a brain.

sensory receptor cell A sensory neuron with specialized membranes in which receptor proteins are embedded.

sensory transduction The conversion of physical or chemical stimuli into nerve impulses.

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sepal A structure, often green, that forms the outermost whorl of a flower with other sepals and encases and protects the flower during its development.

septum (plural, septa) In fungi, a wall that partially divides the cytoplasm into separate cells in hyphae.

sequence assembly The process in which short nucleotide sequences of a long DNA molecule are arranged in the correct order to generate the complete sequence.

sequence motif Any of a number of sequences or sequence arrangements that indicate the likely function of a segment of DNA.

serosa An outer layer of cells and connective tissue that covers and protects the gut.

Sertoli cell A type of cell in the seminiferous tubules that supports sperm production.

sex chromosome Any of the chromosomes associated with sex.

sex determination The factors that trigger development of male or female characteristics; in humans, sex determination occurs because of the Y chromosome.

sexual reproduction The process of producing offspring that receive genetic material from two parents; in eukaryotes, the process occurs through meiosis and fertilization.

sexual selection A form of selection that promotes traits that increase an individual’s access to reproductive opportunities.

shell (of an atom) An energy level.

shoot The collective name for the leaves, stems, and reproductive organs, the major aboveground organ systems of vascular plants.

shoot apical meristem A group of totipotent cells near the tip of a stem or branch that gives rise to new shoot tissues in plants.

short-day plant A plant that flowers only when the day length is less than a critical value.

shotgun sequencing DNA sequencing method in which the sequenced fragments do not originate from a particular gene or region, but from sites scattered randomly across the molecule.

sickle-cell anemia A condition in which hemoglobin molecules tend to crystallize when exposed to lower-than-normal levels of oxygen, causing the red blood cells to collapse and block capillary blood vessels.

side chain A chemical group attached to the central carbon atom of an amino acid, whose structure and composition determine the identity of the amino acid; also known as an R group.

sieve plate A modified end wall with large pores that links sieve elements.

sieve tube A multicellular unit composed of sieve elements that are connected end to end, through which phloem transport takes place.

sigma factor A protein that associates with RNA polymerase that facilitates its binding to specific promoters.

signal sequence An amino acid sequence that directs a protein to its proper cellular compartment.

signal transduction The process in which an extracellular molecule acts as a signal to activate a receptor, which transmits information through the cytoplasm.

signal-anchor sequence In protein sorting, an amino acid sequence in a polypeptide chain that embeds the chain in the membrane.

signaling cell The source of the signaling molecule.

signaling molecule The carrier of information transmitted when the signaling molecule binds to a receptor; also referred to as a ligand.

signal-recognition particle (SRP) An RNA–protein complex that binds with part of a polypeptide chain and marks the molecule for incorporation into the endoplasmic reticulum (eukaryotes) or the plasma membrane (prokaryotes).

silencers DNA sequences that bind with regulatory transcription factors and repress transcription.

single-gene trait A trait determined by Mendelian alleles of a single gene with little influence from the environment.

single-lens eye An eye structure found in vertebrates and cephalopod mollusks that works like a camera to produce a sharply defined image of the animal’s visual field.

single-nucleotide polymorphism (SNP) A site in the genome where the base pair that is present differs among individuals in a population.

single-strand binding protein A protein that binds single-stranded nucleic acids.

sink In plants, any portion of the plant that needs carbohydrates to fuel growth and respiration, such as a root, young leaf, or developing fruit.

sinoatrial (SA) node A specialized region of the heart containing pacemaker cells where the heartbeat is initiated.

sister chromatids The two copies of a chromosome produced by DNA replication.

sister groups Groups that are more closely related to each other than either of them is to any other group.

skeletal muscle Muscle that connects to the body skeleton to move an animal’s limbs and torso.

sliding filament model The hypothesis that muscles produce force and change length by the sliding of actin filaments relative to myosin filaments.

slow-twitch Describes muscle fibers that contract slowly and consume less ATP than do fast-twitch fibers to produce force.

small interfering RNA (siRNA) A type of small double-stranded regulatory RNA that becomes part of a complex able to cleave and destroy single-stranded RNA with a complementary sequence.

small intestine Part of the midgut; the site of the last part of digestion and most nutrient absorption.

small nuclear RNA (snRNA) Noncoding RNA found in eukaryotes and involved in splicing, polyadenylation, and other processes in the nucleus.

small regulatory RNA A short RNA molecule that can block transcription, cleave or destabilize RNAA, or inhibit mRNA translation.

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smooth endoplasmic reticulum (SER) The portion of the endoplasmic reticulum that lacks ribosomes.

smooth muscle The muscle in the walls of arteries, the respiratory system, and the digestive and excretory systems; smooth muscle appears uniform under the light microscope.

solubility The ability of a substance to dissolve.

solute A dissolved molecule such as the electrolytes, amino acids, and sugars often found in water, a solvent.

solvent A liquid capable of dissolving a substance.

somatic cell A nonreproductive cell, the most common type of cell in body.

somatic mutation A mutation that occurs in somatic cells.

somatic nervous system The voluntary component of the peripheral nervous system, which is made up of sensory neurons that respond to external stimuli and motor neurons that synapse with voluntary muscles.

source In plants, a region that supplies carbohydrates to other parts of the plant.

Southern blot A method for determining the size and number of copies of a DNA sequence of interest by means of a labeled probe hybridized to DNA fragments separated by size by means of electrophoresis.

spatial summation The converging of multiple receptors onto a neighboring neuron, increasing its firing rate proportionally to the number of signals received.

speciation The process whereby new species are produced.

species A group of individuals that can exchange genetic material through interbreeding to produce fertile offspring.

species–area relationship The relationship between island size and equilibrium species diversity.

spermatogenesis The formation of sperm.

spermatozoa (sperm) The smaller, male gametes.

spinal cord In vertebrates, a central tract of neurons that passes through the vertebrae to transmit information between the brain and the periphery of the body.

spinal nerve In vertebrates, a nerve running from the spinal cord to the periphery containing axons of both sensory and motor neurons.

spiracle An opening in the exoskeleton on either side of an insect’s abdomen through which gases are exchanged.

spliceosome A complex of RNA and protein that catalyzes RNA splicing.

spongy bone Vertebrate bone tissue consisting of trabeculae, and thus lighter than compact bone, found in the ends of limb bones and within vertebrae.

spontaneous Occurring in the absence of any assignable cause; most mutations are spontaneous.

sporangium In plants, a multicellular structure in which haploid spores are formed by meiosis.

spore A reproductive cell capable of growing into a new individual without fusion with another cell.

sporophyte Describes the diploid multicellular generation in plants that produces spores.

sporopollenin A complex mixture of polymers that is remarkably resistant to environmental stresses such as ultraviolet radiation and desiccation.

stabilizing selection A form of selection that selects against extremes and so maintains the status quo.

stamen A pollen-producing floral organ.

stapes A small bone in the middle ear that helps amplify the waves that strike the tympanic membrane; the stapes connects to the oval window of the cochlea.

starch The form in which glucose is stored in plants.

Starling's Law The correspondence between change in stroke volume and change in the volume of blood filling the heart.

statocyst A type of gravity-sensing organ found in most invertebrates.

statolith In plants, a large starch-filled organelle in the root cap that senses gravity; in animals, a dense particle that moves freely within a statocyst, enabling it to sense gravity.

stem cell An undifferentiated cell that can undergo an unlimited number of mitotic divisions and differentiate into any of a large number of specialized cell types.

stereocilia Nonmotile cell-surface projections on hair cells whose movement causes a depolarization of the cell’s membrane.

steroid A type of lipid.

steroid hormone A hormone that is derived from cholesterol.

stigma The surface at the top of the carpel, to which pollen adheres.

stomach The last part of the foregut, which serves as a storage and digestive chamber; also known as the crop.

stomata (singular, stoma) Pores in the epidermis of a leaf that regulate the diffusion of gases between the interior of the leaf and the atmosphere.

Stramenopila A eukaryotic superkingdom including unicellular organisms, giant kelps, algae, protozoa, free-living cells, and parasites; distinguished by a flagellum with two rows of stiff hairs and, usually, a second, smooth flagellum.

striated muscle Skeletal muscle and cardiac muscle, which appear striped under a light microscope.

strigolactone A hormone, produced in roots and transported upward in the xylem, that inhibits the outgrowth of axillary buds.

stroke volume (SV) The volume of blood pumped during each heart beat.

stroma The region surrounding the thylakoid, where carbohydrate synthesis takes place.

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stromatolite A layered structure that records sediment accumulation by microbial communities.

structural gene A gene that codes for the sequence of amino acids in a polypeptide chain.

style A cylindrical stalk between the ovary and the stigma, through which the pollen tube grows.

suberin A waxy compound coating cork cells that protects against mechanical damage, the entry of pathogens, and water loss.

submucosa A tissue layer surrounding the mucosa that contains blood vessels, lymph vessels, and nerves.

subspecies Allopatric populations that have yet to evolve even partial reproductive isolation but which have acquired population-specific traits.

substrate (S) A molecule acted upon by an enzyme.

substrate-level phosphorylation A way of generating ATP in which a phosphate group is transferred to ADP from an organic molecule, which acts as a phosphate donor or substrate.

succession The replacement of species by other species over time.

sugar The simplest carbohydrate molecule; also called a saccharide.

sulci Deep crevices in the brain that separate the lobes of the cerebral hemispheres.

supercoil A coil of coils; a circular molecule of DNA can coil upon itself to form a supercoil.

superkingdom One of seven major groups of eukaryotic organisms, classified by molecular sequence comparisons.

supernormal stimulus An exaggerated stimulus that elicits a response more strongly than the normal stimulus.

surfactant A compound that reduces the surface tension of a fluid film.

survivorship The proportion of individuals from an initial cohort that survive to each successive stage of the life cycle.

suspension filter feeding The most common form of food capture by animals, in which water with food suspended in it passes through a sievelike structure.

sustainable development Use of natural resources at rates no higher than the rate at which they can be replenished.

symbiont An organism that lives in closely evolved association with another species.

symbiosis (plural, symbioses) Close interaction between species that live together, often interdependently.

sympathetic division The division of the autonomic nervous system that generally produces arousal and increased activity; active in the fight-or-flight response.

sympatric Describes populations that are in the same geographic location.

synapomorphy A shared derived character; a homology shared by some, but not all, members of a group.

synapse A junction through which the axon terminal communicates with a neighboring cell.

synapsis The gene-for-gene pairing of homologous chromosomes in prophase I of meiosis.

synaptic cleft The space between the axon of the presynaptic cell and the neighboring postsynaptic cell.

synaptic plasticity The ability to adjust synaptic connections between neurons.

synonymous (silent) mutation A mutation resulting in a codon that does not alter the corresponding amino acid in the polypeptide.

systemic acquired resistance (SAR) The ability of a plant to resist future infections, occurring in response to a wide range of pathogens.

systemic circulation Circulation of the blood to the body, excluding the lungs.

systole The phase of the vertebrate cardiac cycle when the ventricles contract to eject blood from the heart.

T cell receptor (TCR) A protein receptor on a T cell that recognizes and binds to an antigen.

T lymphocyte (T cell) A cell type that matures in the thymus and includes helper and cytotoxic cells.

tannin Any one of a group of phenols found widely in plant tissues that bind with proteins and reduce their digestibility.

taste bud One of the sensory organs for taste.

TATA box A DNA sequence present in many promoters in eukaryotes and archaeons that serves as a protein-binding site for a key general transcription factor.

taxis (plural, taxes) Movement in a specific direction in response to a stimulus.

taxon (plural, taxa) All the species in a taxonomic entity such as family or genus.

tectorial membrane A rigid membrane in the cochlear duct, against which the stereocilia of hair cells in the organ of Corti bend when stimulated by vibration, setting off an action potential.

telomerase An enzyme containing an RNA template from which complementary telomere repeats are synthesized.

telomere A repeating sequence at each end of a eukaryotic chromosome.

telophase The stage of mitosis in which the nuclei of the daughter cells are formed and the chromosomes uncoil to their original state.

telophase I The stage of meiosis I in which the chromosomes uncoil slightly, a nuclear envelope briefly reappears, and in many species the cytoplasm divides, producing two separate cells.

telophase II The stage of meiosis II in which the chromosomes uncoil and become diffuse, a nuclear envelope forms around each set of chromosomes, and the cytoplasm divides by cytokinesis.

template A strand of DNA or RNA whose squence of nucleotides is used to synthesize a compementary strand.

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template strand In DNA replication, the parental strand whose sequence is used to synthesize a complementary daughter strand.

temporal isolation Pre-zygotic isolation between individuals that are reproductively active at different times.

temporal lobe The region of the brain involved in the processing of sound, language and reading, and object identification and naming.

temporal summation The frequency of synaptic stimuli; the integration of sensory stimuli that are received repeatedly over time by the same sensory cell.

temporomandibular joint A specialized jaw joint in mammals that allows the teeth of the lower and upper jaws to fit together precisely.

tendon A collagen structure that attaches muscles to the skeleton and transmits muscle forces over a wide range of joint motion.

termination In protein translation, the time at which the addition of amino acids stops and the completed polypeptide chain is released from the ribosome. In cell communication, the stopping of a signal.

terminator A DNA sequence at which transcription stops and the transcript is released.

terpene Any one of a group of compounds that do not contain nitrogen and are produced by some plants as a defensive mechanism.

tertiary structure The overall three-dimensional shape of a protein, formed by interactions between secondary structures.

test (of a protist) A “house” constructed of organic molecules that shelters a protist.

test group The experimental group that is exposed to the variable in an experiment.

testcross Any cross of an unknown genotype with a homozygous recessive genotype.

testis (plural, testes) The male gonad, where sperm are produced.

testosterone A steroid hormone, secreted by the testes, that plays key roles in male growth, development, and reproduction.

tetanus A muscle contraction of sustained force.

tetraploid A cell or organism with four complete sets of chromosomes; a double diploid.

Tetrapoda A monophyletic group of animals whose last common ancestor had four limbs; this group includes amphibians, lizards, turtles, crocodilians, birds, and mammals (some tetrapods, like snakes, have lost their legs in the course of evolution).

thalamus The inner brain region of the forebrain, which acts as a relay station for sensory information sent to the cerebrum.

thallus A flattened photosynthetic structure produced by some bryophytes.

Thaumarchaeota One of the three major divisions of Archaea; thaumarchaeota are chemotrophs, deriving energy from the oxidation of ammonia.

theory A general explanation of a natural phenomenon supported by a large body of experiments and observations.

theory of island biogeography A theory that states that the number of species that can occupy a habitat island depends on two factors: the size of the island and the distance of the island from a source of colonists.

thermoreceptor A sensory receptor in the skin and in specialized regions of the central nervous system that responds to heat and cold.

thick filament A parallel grouping of myosin molecules that makes up the myosin filament.

thin filament Two helically arranged actin filaments twisted together that make up the actin filament.

threshold potential The critical depolarization voltage of –50 mV required for an action potential.

thylakoid membrane A highly folded membrane in the center of the chloroplast that contains light-collecting pigments and that is the site of the photosynthetic electron transport chain.

thymine (T) A pyrimidine base.

thyroid gland A gland located in the front of the neck that leads to the release of two peptide hormones, thyroxine and triiodothyorine.

Ti plasmid A small circular DNA molecule in virulent strains of R. radiobacter containing genes that can be integrated into the host cell’s genome, as well as the genes needed to make this transfer.

tidal ventilation A breathing technique in most land vertebrates in which air is drawn into the lungs during inhalation and moved out during exhalation.

tidal volume The amount of air inhaled and exhaled in a cycle; in humans, tidal volume is 0.5 liter when breathing at rest.

tight junction A junctional complex that establishes a seal between cells so that the only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by a cellular transport mechanism.

tissue A collection of cells that work together to perform a specific function.

tolerance The ability of T cells not to respond to self antigens even though the immune system functions normally otherwise.

toll-like receptors (TLRs) A family of transmembrane receptors on phagocytes that recognize and bind to molecules on the surface of microorganisms, providing an early signal that an infection is present.

topography The physical features of Earth.

topoisomerase Any one of a class of enzymes that regulates the supercoiling of DNA by cleaving one or both strands of the DNA double helix, and later repairing the break.

topoisomerase II An enzyme that breaks a DNA double helix, rotates the ends, and seals the break.

totipotent Describes cells that have the potential to give rise to a complete organism; a fertilized egg is a totipotent cell.

trabeculae Small plates and rods with spaces between them, found in spongy bone.

trace fossil A track or trail, such as a dinosaur track or the feeding trails of snails and trilobites, left by an animal as it moves about or burrows into sediments.

trachea The central airway leading to the lungs, supported by cartilage rings.

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tracheae An internal system of tubes in insects that branch from openings along the abdominal surface into smaller airways, directing oxygen to and removing carbon dioxide from respiring tissues.

tracheid A unicellular xylem conduit.

trade-off An exchange in which something is gained at the expense of something lost.

trait A characteristic of an individual.

transcription The synthesis of RNA from a DNA template.

transcriptional activator protein A protein that binds to a sequence in DNA to enable transcription to begin.

transcriptional regulation The mechanisms that collectively regulate whether or not transcription occurs.

transduction Horizontal gene transfer by means of viruses.

transfer RNA (tRNA) Noncoding RNA that carries individual amino acids for use in translation.

transformation The conversion of cells from one state to another, as from nonvirulent to virulent, when DNA released to the environment by cell breakdown is taken up by recipient cells. In recombinant DNA technology, the introduction of recombinant DNA into a recipient cell.

transgenic organisms An alternative term for genetically modified organisms.

transition state The brief time in a chemical reaction in which chemical bonds in the reactants are broken and new bonds in the product are formed.

translation Synthesis of a polypeptide chain corresponding to the coding sequence present in a molecule of messenger RNA.

transmembrane proteins Proteins that span the entire lipid bilayer; most integral membrane proteins are transmembrane proteins

transmission genetics The discipline that deals with the manner in which genetic material is passed from generation to generation.

transpiration The loss of water vapor from leaves.

transporters Membrane proteins that move ions or other molecules across the cell membrane.

transposable element (TE) A DNA sequence that can replicate and move from one location to another in a DNA molecule; also known as transposon.

transposase The enzyme that cleaves a transposon from its original location in the genome and inserts it into a different position.

transposition The movement of a transposable element.

transposon A DNA sequence that can replicate and move from one location to another in a DNA molecule; also known as transposable element (TE).

triacylglycerol A lipid composed of a glycerol backbone and three fatty acids.

trimesters The three periods of pregnancy, each lasting about 3 months.

triose phosphate A 3-carbon carbohydrate molecule, produced by the Calvin cycle and exported from the chloroplast.

triploid A cell or organism with three complete sets of chromosomes.

trisomy 21 A condition resulting from the presence of three, rather than two, copies of chromosome 21; also known as Down syndrome.

trophic level An organism’s typical place in a food web as a producer or consumer.

trophic pyramid A diagram that traces the flow of energy through communities, showing the amount of energy available at each level to feed the next. The pyramid shape results because biomass and the energy it represents generally decrease from one trophic level to the next.

tropic hormone A hormone that controls the release of other hormones.

tropism The bending or turning of an organism in response to an external signal such as light or gravity.

tropomyosin A protein that runs in the grooves formed by the actin helices and blocks the myosin-binding sites.

troponin A protein that moves tropomyosin away from myosin-binding sites, allowing cross-bridges between actin and myosin to form and the muscle to contract.

true breeding Describes a trait whose physical appearance in each successive generation is identical to that in the previous one.

trypsin A digestive enzyme produced by the pancreas that breaks down proteins.

tube feet Small projections of the water vascular system that extend outward from the body surface and facilitate locomotion, sensory perception, food capture, and gas exchange in echinoderms.

tubulin Dimers (composed of an α tubulin and a β tubulin) that assemble into microfilaments.

tumor suppressors A family of genes that encode proteins whose normal activities inhibit cell division.

tunicates A subphylum of Chordata that includes about 3000 species of filter-feeding marine animals, such as sea squirts and salps.

turgor pressure Pressure within a cell resulting from the movement of water into the cell by osmosis and the tendency of the cell wall to resist deformation.

Turner syndrome A sex-chromosomal abnormality in which an individual has 45 chromosomes, including only one X chromosome.

twitch A muscle contraction that results from a single action potential.

twofold cost of sex Population size can increase more rapidly in asexually reproducing organisms than in sexually reproducing organisms because only female produce offspring, and sexual females have only half the fitness of asexual parents.

tympanic membrane A thin sheet of tissue at the surface of the ear that vibrates in response to sound waves, amplifying airborne vibrations; in mammals, also known as the eardrum.

unbalanced translocation Translocation in which only part of a reciprocal translocation (and one of the nontranslocated chromosomes) is inherited from one of the parents.

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unsaturated Describes fatty acids that contain carbon–carbon double bonds.

uracil (U) A pyrimidine base in RNA, where it replaces the thymine found in DNA.

urea A waste product of protein metabolism that many animals excrete.

ureter A large tube in the vertebrate kidney that brings urine from the kidneys to the bladder.

urethra A tube from the bladder that in males carries semen as well as urine from the body.

uterus A hollow organ within the reproductive tract of female mammals with thick, muscular walls that is adapted to support the developing embryo if fertilization occurs and to deliver the baby during birth.

vaccination The deliberate delivery in a vaccine of an antigen from a pathogen to induce a primary response but not the disease, thereby providing future protection from infection.

vacuole A cell structure that absorbs water and contributes to turgor pressure.

vagina A tubular channel connecting the uterus to the exterior of the body; also known as the birth canal.

valence electrons The electrons farthest from the nucleus, which are at the highest energy level.

van der Waals interactions The binding of temporarily polarized molecules because of the attraction of opposite charges.

variable (in experimentation) The feature of an experiment that is changed by the experimenter from one treatment to the next.

variable (V) region A region of the heavy (H) and light (L) chains of an antibody; the variable region distinguishes a given antibody from all others.

variable expressivity The phenomenon in which a particular phenotype is expressed with a different degree of severity in different individuals.

variable number tandem repeat (VNTR) A genetic difference in which the number of short repeated sequences of DNA differs from one chromosome to the next.

vas deferens A long, muscular tube from the scrotum, through the abdominal cavity, along the bladder, and connecting with the ejaculatory duct.

vasa recta The blood vessels in the kidneys.

vascular cambium Lateral meristem that is the source of new xylem and phloem.

vascular plant A plant that produces both xylem and phloem.

vasoconstriction The narrowing of a vessel by the contraction of smooth muscle, which in turn increases resistance and decreases blood flow.

vasodilation The widening of a vessel by the relaxation of smooth muscle, which in turn decreases resistance and increases blood flow.

vasopressin A posterior pituitary gland hormone that acts on the kidneys and controls the water permeability of the collecting ducts, thus regulating the concentration of urine that an animal excretes; also known as antidiuretic hormone (ADH).

vector In recombinant DNA, a carrier of the donor fragment, usually a plasmid.

vegetative reproduction Reproduction by growth and fragmentation. In plants, the production of upright shoots from horizontal stems, permitting new plants to be produced at a distance from the site where the parent plant originally germinated.

veins In plants, the system of vascular conduits within the leaf; in animals, the large, low-pressure vessels that return blood to the heart.

vena cava (plural, venae cavae) One of two large veins in the body that drain blood from the head and body into the heart.

ventilation The movement of an animal’s respiratory medium—water or air—past a specialized respiratory surface.

ventricle A heart chamber that pumps blood to the lungs or the rest of the body.

venule A blood vessel into which capillaries drain as blood is returned to the heart.

vernalization A prolonged period of exposure to cold temperatures necessary to induce flowering.

vertebrae (singular, vertebra) The series of hard bony segments making up the jointed skeleton that runs along the main axis of the body in vertebrates.

vertebral column A skeletal structure in vertebrates that functionally replaces the embryonic notochord that supports the body.

vertebrates A subphylum of Chordata, distinguished by a bony cranium that protects the brain and (unless lost through evolution), a vertebral column; also known as craniates.

vesicle A small membrane-enclosed sac that transports substances within the cell.

vessel A multicellular xylem conduit.

vessel element An individual cell that is part of a xylem vessel for water transport in plants.

vestibular system A system in the mammalian inner ear made up of two statocyst chambers and three semicircular canals.

vestigial structure A structure that has lost its original function over time and is now much reduced in size.

vicariance The process in which a geographic barrier arises within a single population, separating it into two or more isolated populations.

villi Highly folded inner surfaces of the jejunum and ileum of the small intestine.

virulent Describes pathogens that are able to overcome a host’s defenses and lead to disease.

virus A small infectious agent that contains a nucleic acid genome packaged inside a protein coat called a capsid.

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visible light The portion of the electromagnetic spectrum apparent to our eyes.

visual cortex The part of the brain that processes visual images.

vitamin An organic molecule that is required in very small amounts in the diet.

vitreous humor A gel-like substance filling the large cavity behind the lens that makes up most of the volume of the vertebrate eye.

viviparity Giving birth to live young, with nutritional support of the embryo from the mother.

vocal cords Twin organs in the larynx that vibrate as air passes over them, enabling speech, song, and sound production.

voltage-gated channels Ion membrane channels that open and close in response to changes in membrane potential.

voluntary Describes the component of the nervous system that handles sensing and responding to external stimuli.

vulva The external genitalia of the female.

water vascular system A series of fluid-filled canals that permit bulk transport of oxygen and nutrients in echinoderms.

white blood cell (leukocyte) A type of cell in the immune system that arises by differentiation from stem cells in the bone marrow.

white matter Collectively, the axons of cortical neurons in the interior of the brain; it is the fatty myelin produced by glial cells surrounding the axons that makes this region of the brain white.

wild type The most common allele, genotype, or phenotype present in a population; nonmutant.

X chromosome One of the sex chromosomes; a normal human female has two copies of the X chromosome; a normal male has one X and one Y chromosome.

xanthophyll Any one of several yellow-orange pigments that slow the formation of reactive oxygen species by reducing excess light energy; these pigments accept absorbed light energy directly from chlorophyll and convert this energy to heat.

X-inactivation The process in mammals in which dosage compensation occurs through the inactivation of one X chromosome in each cell in females.

X-linked gene A gene in the X chromosome.

xylem Vascular tissue consisting of lignified conduits that transports water and nutrients from the roots to the leaves.

Y chromosome One of the sex chromosomes; a normal human male has one X and one Y chromosome.

yeast A single-celled fungus found in moist, nutrient-rich environments.

Y-linked gene A gene that is present in the region of the Y chromosome that shares no homology with the X chromosome.

yolk A substance in the eggs of animals with external fertilization that provides all the nutrients that the developing embryo needs until it hatches.

Z disc A protein backbone found regularly spaced along the length of a myofibril.

Z scheme Another name for the photosynthetic electron transport chain, so called because the overall energy trajectory resembles a “Z.”

zygomycetes Fungi groups that produce hyphae undivided by septa and do not form multicellular fruiting bodies; they make up less than 1% of known fungal diversity.

zygote The diploid cell formed by the fusion of two gametes.