Bacillus (buh sil’ us) FL.: little rod] • Any of various rod-shaped bacteria.

Bacteriophage (bak teer’ ee o fayj) [Gr. bakterion: little rod + phagein: to eat] • One of a group of viruses that infect bacteria and ultimately cause their disintegration.

Basic • having a pH greater than 7.0 (having a hydrogen ion concentration lower than 10¯7 molar).

Basidium (bass id’ ee yum) • In fungi of the class Basidiomycetes, the characteristic sporangium in which four spores are formed by meiosis and then borne externally before being shed.

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

 C₃ photosynthesis • The form of photosynthesis in which 3-phosphoglycerate is the first stable product, and ribulose bisphosphate is the CO₂ receptor.

C₄ photosynthesis • The form of photosynthesis in which oxaloacetate is the first stable product, and phosphoenolpyruvate is the CO₂ acceptor. C₄    plants also perform the reactions of C₃ photosynthesis.

Calmodulin (cal mod’ joo lin) • A calcium-binding protein found in all animal and plant cells; mediates many calcium-regulated processes.

calorie [L. calor: heat] • The amount of heat required to raise the temperature of one gram of water by one degree Celsius (1ºC) from 14.5ºC to 15.5ºC. In nutrition studies, “Calorie” (spelled with a capital C) refers to the kilocalorie (1 kcal = 1,000 cal).

Calvin-Benson cycle • The stage of photosynthesis in which CO₂ reacts with RuBP to form 3PG, 3PG is reduced to a sugar, and RuBP is regenerated, while other products are released to the rest of the plant.

Calyx (kay’ licks) [Gr. kalyx: cup] • All of the sepals of a flower, collectively. 

CAM • See crassulacean acid metabolism.

Cambium (kam’ bee urn) [L. cambiare: to exchange] • A meristem that gives rise to radial rows of cells in stem and root, increasing them in girth; commonly applied to the vascular cambium which produces wood and phloem, and the cork cambium, which produces bark.

cAMP (cyclic AMP) • A compound, formed from ATP, that mediates the effects of numerous animal hormones. Also needed for the transcription of catabolite­repressible operons in bacteria. Used for communication by cellular slime molds.

Canopy • The leaf-bearing part of a tree. Collectively the aggregate of the leave~ and branches of the larger woody plants of an ecological community.

Capillaries [L. caplllaris: hair] • Very small tubes, especially the smallest blood-carrying vessels of animals between the termination of the arteries and the beginnings of the veins.

Capsid • The protein coat of a virus.

Carbohydrates • Organic compounds with the general formula CnH₂mOm Common examples are sugars, starch, and cellulose.

Carboxylic acid (kar box sill’ 1k) • An organic acid containing the carboxyl group, -COOH, which dissociates to the carboxylate ion, -C00.

Carcinogen (car sin’ oh jen) • A substance that causes cancer.

Cardiac (kar’ dee ak) [Gr. kardia: heart] • Pertaining to the heart and its functions.

Carnivore [L. cam: flesh + vovare: to devour] • An organism that feeds on animal tissue. (Contrast with detritivore, herbivore, omnivore.) 

Carpel (kar’pel) [Gr. karpos: fruit] • The organ of the flower that contains one or more ovules.

Carrier • (1) In facilitated diffusion, a membrane protein that binds a specific molecule and transports it through the membrane. (2) In respiratory and photosynthetic electron transport, a participating substance such as NAD that exists in both oxidised and reduced forms. (3) In genetics, a person heterozygou: for a recessive trait.

Carrying capacity • In ecology, the largest number of organisms of a particular species that can be maintained indefinitely in a given part of the environment.

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

Casparian strip • A band of cell wall containing suberin and lignin, found in the endodermis. Restricts the movement of water across the endodermis.

Catabolism [Ge. kata: down + ballein: to throw] • Degradational reactions of metabolism, in which complex molecules are broken down. (Contrast with anabolism.)

Catalyst (cat’ a list) [Gr. kata-, implying the breaking down of a compound] • A chemical substance that accelerates a reaction without itself being consumed in the overall course of the reaction. Catalysts lower the activation energy of a reaction. Enzymes are biological catalysts.

Cation (cat’ eye on) • An ion with one or more positive charges. (Contrast with anion.)

cDNA • See complementary DNA.

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

Cell adhesion molecules • Molecules on animal cell surfaces that affect the selective association of cells during development of the embryo.

Cell cycle • The stages through which a cell passes between one division and the next. Includes all stages of interphase and mitosis.

Cell division • The reproduction of a cell to produce two new cells. In eukaryotes, this process involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).

Cell theory • The theory, well established, that organisms consist of cells, and that all cells come from preexisting cells.

Cell wall • A relatively rigid structure that encloses cells of plants, fungi, many protists, and most bacteria. The cell wall gives these cells their shape and limits their expansion in hypotonic media.

Cellular immune system • That part of the immune system that is based on the activities of T cells. Directed against parasites, fungi, intracellular viruses, and foreign tissues (grafts). (Contrast with humoral immune system.)

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

Central nervous system • That part of the nervous system which is condensed and centrally located, e.g., the brain and spinal cord of vertebrates; the chain of cerebral, thoracic and abdominal ganglia of arthropods.

Centrifuge [L. fugere: to flee] • A device in which a sample can be spun around a central axis at high speed, creating a centrifugal force that mimics a very strong gravitational force. Used to separate mixtures of suspended materials.

Centriole (sen’ tree ole) • A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.

Centromere (sen’ tro meer) [Gr. centron: centre + meros: part] • The region where sister chromatids join.

Centrosome (sen’ tro soam) • The major microtubule organizing centre of an animal cell.

Cerebellum (sair’ uh bell’ urn) [L.: diminutive of cerebrum: brain] • The brain region that controls muscular coordination; located at the anterior end of the hindbrain.

Cerebral cortex • The thin layer of gray matter (neuronal cell bodies) that overlays the cerebrum.

Cerebrum (su ree’ brum) [L.: brain] • The dorsal anterior portion of the forebrain, making up the largest part of the brain of mammals. In mammals, the chief coordination centre of the nervous system; consists of two cerebral hemispheres.

Cervix (sir’ vix) [L.: neck] • The opening of the uterus into the vagina.

Channel • A membrane protein that forms an aqueous passageway though which specific solutes may pass by simple diffusion; some channels are gated: they open and close in response to binding of specific molecules.

Chemical bond • An attractive force stably linking two atoms.

Chemiosmotic mechanism • The formation of ATP in mitochondria and chloroplasts, resulting from a pumping of protons across a membrane (against a gradient of electrical charge and of pH), followed by the return of the protons through a protein channel with ATPase activity.

Chemoautotroph • An organism that uses carbon dioxide as a carbon source and obtains energy by oxidizing inorganic substances from its environment. (Contrast with chemoheterotroph, photoautotroph, photoheterotroph.)

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

Chemoreceptor • A cell or tissue that senses specific substances in its environment.

Chemosynthesis • Synthesis of food substances, using the oxidation of reduced materials from the environment as a source of energy.

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

Chitin (kye’ tin) LGr. kiton: tunic] • The characteristic tough but flexible organic component of the exoskeleton of arthropods, consisting of a complex, nitrogen-containing polysaccharide. Also found in cell walls of fungi.

Chlorophyll (klor’ o fill) [Gr. kloros: green + phyllon: leaf] • Any of a few green pigments associated with chloroplasts or with certain bacterial membranes; responsible for trapping light energy for photosynthesis.

Chloroplast [Gr. kloros: green + plast: a particle] • An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloronlasts occur only in eukaryotes.

Cholecystokinin (ko’ lee sis to kai nm) • A hormone produced and released by the lining of the duodenum when it is stimulated by undigested fats and proteins. It stimulates the gallbladder to release bile and slows stomach activity.

Chromatid (kro’ ma tid) • Each of a pair of new sister chromosomes from the time at which the molecular duplication occurs until the time at which the centromeres separate at the anaphase of nuclear division.

Chromatin • The nucleic acid-protein complex found in eukaryotic chromosomes.

Chromosome (krome’ o sowm) [Gr. kroma: color + soma: body] • In bacteria and viruses, the DNA molecule that contains most or all of the genetic information of the cell or virus. In eukaryotes, a structure composed of DNA and proteins that bears part of the genetic information of the cell.

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

Chyme (kime) [Gr. kymus, juice] • Created in the stomach; a mixture of ingested food with the digestive juices secreted by the salivary glands and the stomach lining.

Cilium (sil’ ee urn) (plural: cilia) [[. cilium: eyelash] • Hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms. Generally shorter than a flagellum.

Circadian rhythm (sir kade’ ee an) [L. circa: approximately + dies: day] • A rhythm in behaviour, growth, or some other activity that recurs about every 24 hours under constant conditions.

Citric acid cycle • A set of chemical reactions in cellular respiration, in which acetyl CoA reacts with oxaloacetate to form citric acid, and oxaloacetate is regenerated. Acetyl CoA is oxidised to carbon dioxide, and hydrogen atoms are stored as NADH and FADH₂. Also called the Krebs cycle.

Class • In taxonomy, the category below the phylum and above the order; a group of related, similar orders.

Class I MHC molecules • These cell surface proteins participate in the cellular immune response directed against virus-infected cells.

Class II MHC molecules • These cell surface proteins participate in the cell-cell interactions (of helper T cells, macrophages, and B cells) of the humoral immune response.

Class switching • The process whereby a plasma cell changes the class of immunoglobulin that it synthesizes. This results from the deletion of part of the constant region of DNA, bringing in a new C segment. The variable region is the same as before, so that the new irnmunoglobulin has the same antigenic specificity.

Clathrin • A fibrous protein on the inner surfaces of animal cell membranes that strengthens coated vesicles and thus participates in receptor-mediated endocytosis.

Clay • A soil constituent comprising particles smaller than 2 micrometers in diameter.

Clonal anergy • When a naive T cell encounters a self-antigen, the T cell may bind to the antigen but does not receive signals from an antigen-presenting cell. Instead of being activated, the T cell dies (becomes anergic). In this way, we avoid reacting to our own tissue-specific antigens.

Clonal deletion • In immunology, the inactivation or destruction of lymphocyte clones that would produce immune reactions against the animal’s own body.

Clonal selection • The mechanism by which exposure to antigen results in the activation of selected T- or B-cell clones, resulting in an immune response.

Clone [Gr. k/on: twig, shoot] • Genetically identical cells or organisms produced from a common ancestor by asexual means.

Cochlea (kock’ lee uh) [Gr. kokhlos: a land snail] • A spiral tube in the inner ear of vertebrates; it contains the sensory cells involved in hearing.

Codominance • A condition in which two alleles at a locus produce different phenotypic effects and both effects appear in heterozygotes.

Codon • A “triplet” of three nucleotides in messenger RNA that directs the placement of a particular amino acid into a polypeptide chain. (Contrast with anti codon.)

Coelom (see’ lum) [Gr. koiloma: cavity] • The body cavity of certain animals, which is lined with cells of mesodermal origin.

Coelomate • Having a coelom.

Coenzyme • A nonprotein molecule that plays a role in catalysis by an enzyme. The coenzyme may be part of the enzyme molecule or free in solution. Some coenzymes are oxidizing or reducing agents.

Coevolution • Concurrent evolution of two or more species that are mutually affecting each other’s evolution.

Collagen [Gr. kol/a: glue] • A fibrous protein found extensively in bone and connective tissue.

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

Collenchyma (cull eng’ kyma) [Gr. kolla: glue + enchyma: infusion] • A type of plant cell, living at functional maturity, which lends flexible support by virtue of primary cell walls thickened at the corners. (Contrast with parenchyma, sclerenchyma.)

Colon [Gr. kolon: large intestine] • The large intestine.

Commensalism • The form of symbiosis in which one species benefits from the association, while the other is neither harmed nor benefited.

Common bile duct • A single duct that delivers bile from the gallbladder and secretions from the pancreas into the small intestine.

Communication • A signal from one organism (or cell) that alters the pattern of behaviour in another organism (Or cell) in an adaptive fashion.

Community • Any ecologically integrated group of species of microorganisms, plants, and animals inhabiting a given area.

Companion cell • Specialised cell found adjacent to a sieve tube member in flowering plants.

Comparative analysis • An approach to studying evolution in which hypotheses are tested by measuring the distribution of states among a large number of species.

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

Compensation point • The light intensity at which the rates of photosynthesis and of cellular respiration are equal.

Competitive inhibitor • A substance, similar in structure to an enzyme’s substrate, that binds the active site and thus inhibits a reaction.

Competition • In ecology, use of the same resource by two or more species, when the resource is present in insufficient supply for the combined needs of the species.

Competitive exclusion • A result of competition between species for a limiting resource in which one species completely eliminates the other.

Competitive inhibitor • A substance, similar in structure to an enzyme’s substrate, that binds the active site and inhibits a reaction. Complement system • A group of eleven proteins that play a role in some reactions of the immune system. The complement proteins are not immunoglobulins.

Complementary base pairing • The A-T (or A-U), T-A (or U-A), C-G and G-C pairing of bases in double-stranded DNA, in transcription, and between tRNA and mRNA.

Complementary DNA (cDNA) • DNA formed by reverse transcriptase acting with an RNA template; essential intermediate in the reproduction of retroviruses; used as a tool in recombinant DNA technology; lacks introns.

Complete metamorphosis • A change of state during the life cycle of an organism in which the body is almost completely rebuilt to produce an individual with a very different body form. Characteristic of insects such as butterflies, moths, beetles, ants, wasps, and flies.

Compound • (1) A substance made up of atoms of more than one element. (2) Made up of many units, as the compound eyes of arthropods (as opposed to the simple eyes of the same group of organisms).

 Condensation reaction • A reaction in which two molecules become connected by a covalent bond and a molecule of water is released. (AH + BOH Æ AB + H₂0.)

Cones • (1) In the vertebrate retina: photoreceptors responsible for color vision. (2) In gymnosperms: reproductive structures consisting of many sporophylls packed relatively tightly.

Conifer (kahn’ e fer) [Gr. konos: cone + phero: carry] • One of the cone-bearing gymnosperms, mostly trees, such as pines and firs.

Conjugation (kahn’ jew gay’ shun) [L. conjugare: yoke together] • The close approximation of two cells during which they exchange genetic material, as in Paramecium and other ciliates, or during which DNA passes from one to the other through a tube, as in bacteria.

Connective tissue • An animal tissue that connects or surrounds other tissues; its cells are embedded in a collagen-containing matrix.

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

Consensus sequences • Short stretches of DNA that appear, with little variation, in many different genes.

Constant region • The constant region in an immunoglobulin is encoded by a single exon and determines the function, but not the specificity, of the molecule. The constant region of the T cell receptor anchors the protein to the plasma membrane.

Consumer • An organism that eats the tissues of some other organism.

Continental drift • The gradual drifting apart of the world’s continents that has occurred over a period of billions of years.

Convergent evolution • The evolution of similar features independently in unrelated taxa from different ancestral structures.

Cooperative act • Behaviour in which two or more individuals interact to their mutual benefit. No conscious awareness by the actors of the effects of their behaviour is implied.

Copulation • Reproductive behaviour that results in a male depositing sperm in the reproductive tract of a female.

Corepressor • A low molecular weight compound that unites with a protein (the repressor) to prevent transcription in a repressible operon.

Cork • A waterproofing tissue in plants, with suberin-containing cell walls. Produced by a cork cambium.

Corolla (ko role’ lah) [L.: diminutive of corona: wreath, crown] • All of the petals of a flower, collectively.

Coronary (kor’ oh nair ee) • Referring to the blood vessels of the heart.

Corpus luteum (kor’ pus 100’ tee urn) EL. corpus: body + Iuteum: yellow] A structure formed from a follicle after ovulation; it produces hormones important to the maintenance of pregnancy.

Cortex [L.: bark or rind] • (1) In plants, the tissue between the epidermis and the vascular tissue of a stem or root. (2) In animals, the outer tissue of certain organs, such as the adrenal cortex and cerebral cortex.

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

Cotyledon (kot’ ul lee’ dun) [Gr. kotyledon: a hollow space] • A “seed leaf.” An embryonic organ which stores and digests reserve materials; may expand when seed germinates.

Countercurrent exchange • An adaptation that promotes maximum exchange of heat or any diffusible substance between two fluids by the fluids flow in opposite directions through parallel tubes in close approximation to each other. An example is countercurrent heat exchange between arterioles and venules in the extremities of some animals.

Covalent bond • A chemical bond that arises from the sharing of electrons between two atoms. Usually a strong bond.

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

Crista (plural: cristae) • A small, shelflike projection of the inner membrane of a mitochondrion; the site of oxidative phosphorylation.

Critical night length • In the photoperiodic flowering response of short-day plants, the length of night above which flowering occurs and below which the plant remains vegetative. (The reverse applies in the case of long-day plants.)

Critical period • The age during which some particular type of learning must take place or during which it occurs much more easily than at other times. Typical of song learning among birds.

Cross section (also called a transverse section) • A section taken perpendicular to the longest axis of a structure.

Crossing over • The mechanism by which linked markers undergo recombination. In general, the term refers to the reciprocal exchange of corresponding segments between two homologous chromatids.

CRP • The cAMP receptor protein that interacts with the promoter to enhance transcription; a lowered cAMP concentration results in catabolite repression.

Crustacean (crus tay’ see an) • A member of the phylum Crustacea, such as a crab, shrimp, or sowbug.

Cryptic appearance [Gr. kryptos: hidden] • The resemblance of an animal to some part of its environment, which helps it to escape detection by predators.

Cryptochromes [Gr. kryptos: hidden + kroma: color] • Photoreceptors mediating some blue-light effects in plants and animals.

Culture • (1) A laboratory association of organisms under controlled conditions. (2) The collection of knowledge, tools, values, and rules that characterize a human society.

Cuticle • A waxy layer on the outer surface of a plant or an insect, tending to retard water loss.

Cyanobacteria (sigh an’ o bacteria) [Gr. kuanos: the color blue] • A division of photosynthetic bacteria, formerly referred to as blue-green algae; they lack sexual reproduction, and they use chlorophyll a in their photosynthesis.

Cyclic AMP • See cAMP.

Cyclins • Proteins that activate cyclin-dependent kinases, bringing about transitions in the cell cycle.

Cyclin-dependent kinase (cdk) • A kinase is an enzyme that catalzyes the addition of phosphate groups from ATP to target molecules. Cdk s target proteins involved in transitions in the cell cycle and are active only when complexed to additional protein subunits, cyclins.

Cyst (sist) [Gr. kystis: pouch] • (1) A resistant, thick-walled cell formed by some protists and other organisms. (2) An abnormal sac, containing a liquid or semisolid substance, produced in response to injury or illness.

Cytochromes (sy’ toe chromes) [Gr. kytos: container + chroma: color] • Iron-containing red proteins, components of the electron-transfer chains in photophosphorylation and respiration.

Cytokinesis (sy’ toe kine ee’ sis) [Gr. kytos: container + kinein: to move] . The division of the cytoplasm of a dividing cell. (Contrast with mitosis.)

Cytokinin (sy’ toe kine’ in) [Gr. kytos: container + kinein: to move] • A member of a class of plant growth substances playing roles in senescence, cell division, and other phenomena.

Cytoplasm • The contents of the cell, excluding the nucleus.

Cytoplasmic determinants • In animal development, gene products whose spatial distribution may determine such things as embryonic axes.

Cytosine (site’ oh seen) • A nitrogen-containing base found in DNA and RNA.

Cytoskeleton • The network of microtubules and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.

Cytosol • The fluid portion of the cytoplasm, excluding organelles and other solids.

Cytotoxic T cells • Cells of the cellular immune system that recognise and directly eliminate virus-infected cells. (Contrast with helper T cells, suppressor T cells.)

Decomposer • See detritivore.

Degeneracy • The situation in which a single amino acid may be represented by any of two or more different codons in messenger RNA. Most of the amino acids can be represented by more than one codon.

Degradative succession • Ecological succession occuring on the dead remains of the bodies of plants and animals, as when leaves or animal bodies rot.

Deletion (genetic) • A mutation resulting from the loss of a continuous segment of a gene or chromosome. Such mutations never revert to wild type. (Contrast with duplication, point mutation.)

Demographic processes • The events—such as births, deaths, immigration, and emigration—that determine the number of individuals in a population.

Demographic stochasticity • Random variations in the factors influencing the size, density, and distribution of a population.

Demography • The study of dynamical changes in the sizes, densities, and distributions of populations.Denaturation • Loss of activity of an enzyme or nucleic acid molecule as a result of structural changes induced by heat or other means.

Dendrite [Gr. dendron: a tree] • A fibre of a neuron which often cannot carry action potentials. Usually much branched and relatively short compared with the axon, and commonly carries information to the cell body of the neuron.

Denitrification • Metabolic activity by which inorganic nitrogen-containing ions are reduced to form nitrogen gas and other products; carried on by certain soil bacteria.

Density dependence • Change in the severity of action of agents affecting birth and death rates within populations that are directly or inversely related to population density.

Density independence • The state where the severity of action of agents affecting birth and death rates within a population does not change with the density of the population.

Deoxyribonucleic acid • See DNA.

Depolarization • A change in the electric potential across a membrane from a condition in which the inside of the cell is more negative than the outside to a condition in which the inside is less negative, or even positive, with reference to the outside of the cell. (Contrast with hyperpolarization.)

Derived trait • A trait found among members of a lineage that was not present in the ancestors of that lineage.

Dermal tissue system • The outer covering of a plant, consisting of epidermis in the young plant and periderm in a plant with extensive secondary growth. (Contrast with ground tissue system and vascular tissue system.)

Desmosome (dez’ mo sowm) [Gr. desmos: bond + soma: body] • An adhering junction between animal cells.

Determination • Process whereby an embryonic cell or group of cells becomes fixed into a predictable developmental pathway.

Detritivore (di try’ ti yore) IlL, detritus: worn away + vorare: to devour] • An organism that obtains its energy from the dead bodies and/or waste products of other organisms.

Deuterostome • A major evolutionary lineage in animals, characterized by radial cleavage, enterocoelous development, and other traits. (Compare with protostome.)

Development • Progressive change, as in structure or metabolism; in most kinds of organisms, development continues throughout the life of the organism.

Diaphragm (dye’ uh fram) [Gr. diaphrassein, to barricade] • (1) A sheet of muscle that separates the thoracic and abdominal cavities in mammals; responsible for the action of breathing. (2) A method of birth control in which a sheet of rubber is fitted over the woman’s cervix, blocking the entry of sperm.

Diastole (dye ahs’ toll ee) [Gr.: dilation] • The portion of the cardiac cycle when the heart muscle relaxes. (Contrast with systole.)

Dicot (short for dicotyledon) [Gr. di: two + kotyledon: a hollow space] • This term, not used in this book, formerly referred to all angiosperms other than the monocots. (See eudicot, monocot.)

Differentiation • Process whereby originally similar cells follow different developmental pathways. The actual expression of determination.

Diffusion • Random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present.

Digestibility-reducing chemicals • Defensive chemicals produced by plants that make the plant’s tissued difficult to digest.

Digestion • Enzyme-catalysed process by which large, usually insoluble, molecules (foods) are hydrolyzed to form smaller molecules of soluble substances.

Dihybrid cross • A mating in which the parents differ with respect to the alleles of two loci of interest.

Dikaryon (di care’ ee ahn) [Gr. dis: two + karyon: kernel] • A cell or organism carrying two genetically distinguishable nuclei. Common in fungi.

Dioecious (die eesh’ us) [Gr.: two houses] • Organisms in which the two sexes are “housed” in two different individuals, so that eggs and sperm are not produced in the same individuals. Examples: humans, fruit flies, oak trees, date palms. (Contrast with monoecious.)

Diploblastic • Having two cell layers. (Contrast with triploblastic.)

Diploid (dip’ bid) [Gr. diploos: double] • Having a chromosome complement consisting of two copies (homologues) of each chromosome. A diploid individual (or cell) usually arises as a result of the fusion of two gametes, each with just one copy of each chromosome. Thus, the two homologues in each chromosome pair in a diploid cell are of separate origin, one derived from the female parent and one from the male parent.

Directional selection • Selection in which phenotypes at one extreme of the population distribution are favored. (Contrast with disruptive selection; stabilizing selection.)

Disaccharide • A carbohydrate made up of two monosaccharides (simple sugars).

Dispersal stage • Stage in its life history at which an organism moves from its birthplace to where it will live as an adult.

Displacement activity • Apparently irrelevant behaviour performed by an animal under conflict situations, especially when tendencies to attack and escape are closely balanced.

Display • A behaviour that has evolved to influence the actions of other individuals.

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

Distal • Away from the point of attachment or other reference point. (Contrast with proximal.)

Disturbance • A short-term event that disrupts populations, communities, or ecosystems by changing the environment.

Diverticulum (di ver tic’ u lum) [L. divertere: turn away] • A small cavity or tube that connects to a major cavity or tube.

Division • A term used by some microbiologists and formerly by botanists, corresponding to the term phylum.

DNA (deoxyribonucleic acid) • The fundamental hereditary material of all living organisms. In eukaryotes, stored primarily in the cell nucleus. A nucleic acid using deoxyribose rather than ribose.

DNA chip • A small glass or plastic square onto which thousands of single­stranded DNA sequences are fixed. Hybridization of cell-derived RNA or DNA to the target sequences can be performed. (See DNA hybridization.)

DNA hybridization • A process by which DNAs from two species are mixed and heated so that interspecific double helixes are formed.

DNA Iigase • Enzyme that unites Okazaki fragments of the lagging strand during DNA replication; also mends breaks in DNA strands. It connects pieces of a DNA strand and is used in recombinant DNA technology.

DNA methylation • Addition of methyl groups to DNA; plays role in regulation of gene expression; protects a bacterium’s DNA against its restriction endonucleases.

DNA polymerase • Any of a group of enzymes that catalyse the formation of DNA strands from a DNA template.

Domain • The largest unit in the current taxonomic nomenclature. Members of the three domains (Bacteria, Archaea, and Eukarya) are believed to have been evolving independently of each other for at least a billion years.

Dominance • In genetic terminology, the ability of one allelic form of a gene to determine the phenotype of a heterozygous individual, in which the homologous chromosome carries both it and a different allele. For example, if A and a are two allelic forms of a gene, A is said to be dominant to a if AA diploids and Aa diploids are phenotypically identical and are distinguishable from aa diploids. The a allele is said to be recessive.

Dominance hierarchy • In animal behaviour, the set of relationships within a group of animals, usually established and maintained by aggression, in which one individual has precedence over all others in eating, mating, and other activities.

Dormancy • A condition in which normal activity is suspended, as in some seeds and buds.

Dorsal EL. dorsum: back] • Pertaining to the back or upper surface. (Contrast with ventral.)

Double fertilization • Process virtually unique to angiosperms in which one sperm nucleus combines with the egg to produce a zygote, and the other sperm nucleus combines with the two polar nuclei to produce the first cell of the triploid endosperm.

Double helix • Of DNA: molecular structure in which two complementary polynucleotide strands, antiparallel to each other, form a right-handed spiral.

Duodenum (doo’ uh dee’ num) • The beginning portion of the vertebrate small intestine. (Contrast with ileum, jejunum.)

Duplication (genetic) • A mutation resulting from the introduction into the genome of an extra copy of a segment of a gene or chromosome. (Contrast with deletion, point mutation.)

Dynein [Gr. dunamis: power] • A protein that undergoes conformational changes and thus plays a part in the movement of eukaryotic flagella and cilia.

Ecological biogeography • The study of the distributions of organisms from an ecological perspective, usually concentrating on migration, dispersal, and species interactions.

Ecological community • The species living together at a particular site.

Ecological niche (nitch) [L. nidus: nest] • The functioning of a species in relation to other species and its physical environment.

Ecological succession • The sequential replacement of one population assemblage by another in a habitat following some disturbance. Succession sometimes ends in a relatively stable ecosystem.

Ecology [Gr. oikos: house + logos: discourse, study] • The scientific study of the interaction of organisms with their environment, including both the physical environment and the other organisms that live in it.

Ecoregion • A large geographic unit characterized by a typical climate and a widespread assemblage of similar species.

Ecosystem (eek’ oh sis turn) • The organisms of a particular habitat, such as a pond or forest, together with the physical environment in which they live.

Ecto- (eck’ toh) [Gr.: outer, outside] • A prefix used to designate a structure on the outer surface of the body. For example, ectoderm. (Contrast with endo- and meso-.)

Ectoderm [Gr. ektos: outside + derma: skin] • The outermost of the three embryonic tissue layers first delineated during gastrulation. Gives rise to the skin, sense organs, nervous system, etc.

Ectotherm [Gr. ektos: outside + thermos: heat] • An animal unable to control its body temperature. (Contrast with endotherm.)

Edema (i dee’ mah) [Gr. oidema: swelling] • Tissue swelling caused by the accumulation of fluid.

Edge effect • The changes in ecological processes in a community caused by physical and biological factors originating in an adjacent community.

Effector • Any organ, cell, or organelle that moves the organism through the environment or else alters the environment to the organism’s advantage. Examples include muscle, bone, and a wide variety of exocrine glands.

Effector cell • A lymphocyte that performs a role in the immune system without further differentiation.

Effector phase • In this phase of the immune response, effector T cells called cytotoxic T cells attack virus-infected cells, and effector helper T cells assist B cells to differentiate into plasma cells, which release antibodies.

Efferent [L. ex: out + ferre: to bear] • Away from, as in neurons that conduct action potentials out from the central nervous system, or arterioles that conduct blood away from a structure. (Contrast with afferent.)

Egg • In all sexually reproducing organisms, the female gamete; in birds, reptiles, and some other vertebrates, a structure within which early embryonic development occurs.

Elasticity • The property of returning quickly to a former state after a disturbance.

Electrocardiogram (EKG) • A graphic recording of electrical potentials from the heart.

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

Electromyogram (EMG) • A graphic recording of electrical potentials from muscle.

Electron (e lek’ tron) [L. electrum: amber (associated with static electricity), from Gr. slektor: bright sun (color of amber)] • One of the three most important fundamental particles of matter, with mass approximately 0.00055 amu and charge -1.

Electronegativity • The tendency of an atom to attract electrons when it occurs as part of a compound.

Electrophoresis (e lek’ tro fo ree’ sis) [L. electrum: amber + Gr. phorein: to bear] • A separation technique in which substances are separated from one another on the basis of their electric charges and molecular weights.

Electrotonic potential • In neurons, a hyperpolarization or small depolarization of the membrane potential induced by the application of a small electric current. (Contrast with action potential, resting potential.)

Elemental substance • A substance composed of only one type of atom.

Embolus (em’ buh lus) [Gr. embolos: inserted object; stopper] • A circulating blood clot. Blockage of a blood vessel by an embolus or by a bubble of gas is referred to as an embolism. (Contrast with thrombus.)

Embryo [Gr. en-: in + bryein: to grow] • A young animal, or young plant sporophyte, while it is still contained within a protective structure such as a seed, egg, or uterus.

Embryo sac • In angiosperms, the female gametophyte. Found within the ovule, it consists of eight or fewer cells, membrane bounded, but without cellulose walls between them.

Emergent property • A property of a complex system that is not exhibited by its individual component parts.

Emigration • The deliberate and usually oriented departure of an organism from the habitat which it has been living.

3’ End (3-prime) • The end of a DNA or RNA strand that has a free hydroxyl group at the 3 ‘-carbon of the sugar (deoxyribose or ribose).

5’ End (5-prime) • The end of a DNA or RNA strand that has a free phosphate group at the 5’-carbon of the sugar (deoxyribose or ribose).

Endemic (en dem’ ik) [Gr. endemos: dwelling in a place] • Confined to a particular region, thus often having a comparatively restricted distribution.

Endergonic reaction • One for which energy must be supplied. (Contrast with exergonic reaction.)

Endo- [Gr.: within, inside] • A prefix used to designate an innermost structure. For example, endoderm, endocrine. (Contrast with edo-, meso-.)

Endocrine gland (en’ doh krin) [Gr. endon: inside + krinein: to separate] • Any gland, such as the adrenal or pituitary gland of vertebrates, that secretes certain substances, especially hormones, into the body through the blood.

Endocrinology • The study of hormones and their actions.

Endocytosis • A process by which liquids or solid particles are taken up by a cell through invagination of the plasma membrane. (Contrast with exocytosis.)

Endoderm [Gr. endon: within + derma: skin] • The innermost of the three embryonic tissue layers first delineated during gastrulation. Gives rise to the digestive and respiratory tracts and structures associated with them.

Endodermis [Gr. endon: within + derma: skin] • In plants, a specialised cell layer marking the inside of the cortex in roots and some stems. Frequently a barrier to free diffusion of solutes.

Endomembrane system • Endoplasmic reticulum plus Golgi apparatus plus, when present, lysosomes; thus, a system of membranes that exchange material with one another.

Endoplasmic reticulum [Gr. endon: within + L. plasma: form; L. reticulum: little net] • A system of membrane-bounded tubes and flattened sacs found in the cytoplasm of eukaryotes. Exists as rough ER, studded with ribosomes; and smooth ER, lacking ribosomes.

Endorphins • Naturally occurring, opiate-like substances in the mammalian brain.

Endoskeleton [Gr. endon: within + skleros: hard] • A skeleton covered by other, soft body tissues. (Contrast with exoskeleton.)

Endosperm [Gr. endon: within + sperma: seed] • A specialised triploid seed tissue found only in angiosperms; contains stored food for the developing embryo.

Endosymbiosis [Gr. endon: within + syn: together + bios: life] • The living together of two species, with one living inside the body (or even the cells) of the other.

Endosymbiotic theory • Theory that the eukaryotic cell evolved from a prokaryote that contained other, endosymbiotic prokaryotes.

Endotherm fGr. endon: within + thermos: hot] • An animal that can control its body temperature by the expenditure of its own metabolic energy. (Contrast with ectotherm.)

Endotoxins [Gr. endon: within + L. toxicum: poison] • Lipopolysaccharides released by the lysis of some Gram-negative bacteria that cause fever and vomiting in a host organism.

Energetic cost • The difference between the energy an animal would have expended had it rested, and that expended in performing a behaviour.

Energy • The capacity to do work.

Enhancer • In eukaryotes, a DNA sequence, lying on either side of the gene it regulates, that stimulates a specific promoter.

Enterocoelous development • A pattern of development in which the coelum is formed by an outpocketing of the embryonic gut (enteron).

Enterokinase (ent uh row kine’ ase) • An enzyme secreted by the mucosa of the duodenum. It activates the zymogen trypsinogen to create the active digestive enzyme trypsin.

Entrainment • With respect to circadian rhythms, the process whereby the period is adjusted to match the 24-hour environmental cycle.

Entropy (en’ tro pee) [Gr. en: in + tropein: to change] • A measure of the degree of disorder in any system. A perfectly ordered system has zero entropy; increasing disorder is measured by positive entropy. Spontaneous reactions in a closed system are always accompanied by an increase in disorder and entropy.

Environment • An organism’s surroundings, both living and nonliving; includes temperature, light intensity, and all other species that influence the focal organism.

Environmental toxicology • The study of the distribution and effects of toxic compounds in the environment.

Enzyme (en’ zime) [Gr. en: in + zyme: yeast] • A protein, on the surface of which are chemical groups so arranged as to make the enzyme a catalyst for a chemical reaction.

Epi- [Gr.: upon, over] • A prefix used to designate a structure located on top of another; for example: epidermis, epiphyte.

Epicotyl (epp’ i kot’ il) [Gr. epi: upon + koty/e: something hollow] • That part of a plant embryo or seedling that is above the cotyledons.

Epidermis [Gr. epi: upon + derma: skin] • In plants and animals, the outermost cell layers. (Only one cell layer thick in plants.)

Epididymis (epuh did’ uh mus) [Gr. epi: upon + didymos: testicle] • Coiled tubules in the testes that store sperm and conduct sperm from the seiminiferous tubules to the vas deferens.

Epinephrine (ep i nef’ nfl) jjGr. epi: upon + nephros: a kidney] • The “fight or flight” hormone. Produced by the medulla of the adrenal gland, it also functions as a neurotransmitter. Also known as adrenaline.

Episome • A plasmid that may exist either free or integrated into a chromosome. (See plasmid.)

Epistasis • An interaction between genes, in which the presence of a particular allele of one gene determines whether another gene will be expressed.

Epithelium • In animals, a layer of cells covering or lining an external surface or a cavity.

Equilibrium • (1) In biochemistry, a state in which forward and reverse reactions are proceeding at counterbalancing rates, so there is no observable change in the concentrations of reactants and products. (2) In evolutionary genetics, a condition in which allele and genotype frequencies in a population are constant from generation to generation.

Erythrocyte ( ur rith’ row sight) [Gr. erythros: red + kytos: hollow vessel] • A red blood cell.

Esophagus (i soff’ i gus) [Gr. oisophagos: gullet] • That part of the gut between the pharynx and the stomach.

Ester linkage • A condensation (water-releasing) reaction in which the carboxyl group of a fatty acid reacts with the hydroxyl group of an alcohol. Lipids are formed in this way.

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

Estrogen • Any of several steroid sex hormones, produced chiefly by the ovaries in mammals.

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

Ethylene • One of the plant hormones, the gas H₂C;h2CH₂.

Euchromatin • Chromatin that is diffuse and non-staining during interphase; may be transcribed. (Contrast with heterochromatin.)

Eudicots (yew di’ kots) [Gr. eu: true + di: two + kotyledon: a cup-shaped hollow] • Members of the angiosperm class Eudicotyledones, flowering plants in which the embryo produces two cotyledons prior to germination. Leaves of most eudicots have major veins arranged in a branched or reticulate pattern.

Eukaryotes (yew car’ ry otes) [Gr. eu: true + karyon: kernel or nucleus] Organisms whose cells contain their genetic material inside a nucleus. Includes all life other than the viruses, Archaebacteria, and Eubacteria.

Eusocial • Term applied to insects, such as termites, ants, and many bees and wasps, in which individuals cooperate in the care of offspring, there are sterile castes, and generations overlap.

Eutrophication (yoo trofe’ 1k ay’ shun) [Gr. eu-: well + trephein: to flourish] The addition of nutrient materials to a body of water, resulting in changes to species composition therein.

Evolution • Any gradual change. Organic evolution, often referred to as evolution, is any genetic and resulting phenotypic change in organisms from generation to generation.

Evolutionary agent • Any factor that influences the direction and rate of evolutionary changes.

Evolutionarily conservative • Traits of organisms that evolve very slowly.

Evolutionary innovations • Major changes in body plans of organisms; these have been very rare during evolutionary history.

Evolutionary radiation • The proliferation of species within a single evolutionary lineage.

Evolutionary reversal • The reappearance of the ancestral state of a trait in a lineage in which that trait had acquired a derived state.

Excision repair • The removal and damaged DNA and its replacement by the appropriate nucleotides.

Excitatory postsynaptic potential (EPSP) • A change in the resting potential of a postsynaptic membrane in a positive (depolarizing) direction. (Contrast with inhibitory postsynaptic potential.)

Excretion • Release of metabolic wastes by an organism.

Exergonic reaction • A reaction in which free energy is released. (Contrast with endergonic reaction.)

Exo- (eks’ oh) • Same as ecto-.

Exocrine gland (eks’ oh krin) [Gr. exo: outside + krinein: to separate] • Any gland, such as a salivary gland, that secretes to the outside of the body or into the gut.

Exocytosis • A process by which a vesicle within a cell fuses with the plasma membrane and releases its contents to the outside. (Contrast with endocytosis.)

Exon • A portion of a DNA molecule, in eukaryotes, that codes for part of a polypeptide. (Contrast with intron.)

Exoskeleton (eks’ oh skel’ e ton) [Gr. exos: outside + skleros: hard] • A hard covering on the outside of the body to which muscles are attached. (Contrast with endoskeleton.)

Exotoxins • Highly toxic proteins released by living, multiplying bacteria.

Experiment • A scientific method in which particular factors are manipulated while other factors are held constant so that the potential influences of the manipulated factors can be determined.

Exponential growth • Growth, especially in the number of organisms in a population, which is a simple function of the size of the growing entity: the larger the entity, the faster it grows. (Contrast with logistic growth.)

Expression vector • A DNA vector, such as a plasmid, that carries a DNA sequence that includes the adjacent sequences for its expression into mRNA and protein in a host cell.

Expressivity • The degree to which a genotype is expressed in the phenotype— may be affected by the environment.

Extensor • A muscle the extends an appendage.

Extinction • The termination of a lineage of organisms.

Extrinsic protein • A membrane protein found only on the surface of the membrane. (Contrast with intrinsic protein.)

F₁ generation • The immediate progeny of a parental (P) mating; the first filial generation.

F₂ generation • The immediate progeny of a mating between members of the F₁ generation.

Facilitated diffusion • Passive movement through a membrane involving a specific carrier protein; does not proceed against a concentration gradient. (Contrast with active transport, free diffusion.)

Family • In taxonomy, the category below the order and above the genus; a group of related, similar genera.

Fat • A triglyceride that is solid at room temperature. (Contrast with oil.)

Fatty acid • A molecule with a long hydrocarbon tail and a carboxyl group at the other end. Found in many lipids.

Fauna (faw’ nah) • All of the animals found in a given area. (Contrast with flora.)

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

Feedback control • Control of a particular step of a multistep process, induced by the presence or absence of a product of one of the later steps. A thermostat regulating the flow of heating oil to a furnace in a home is a negative feedback control device.

Fermentation (fur men tay’ shun) [L. fermentum: yeast] • The degradation of a substance such as glucose to smaller molecules with the extraction of energy, without the use of oxygen (i.e., anaerobically). Involves the glycolytic pathway.

Fertilization • Union of gametes. Also known as syngarny.

Fertilization membrane • A membrane surrounding an animal egg which becomes rapidly raised above the egg surface within seconds after fertilization, serving to prevent entry of a second sperm.

Foetus • The latter stages of an embryo that is still contained in an egg or uterus; in humans, the unborn young from the eighth week of pregnancy to the moment of birth.

Fibre • An elongated and tapering cell of flowering plants, usually with a thick cell wall. Serves a support function.

Fibrin • A protein that polymerises to form long threads that provide structure to a blood clot.

Filter feeder • An organism that feeds upon much smaller organisms, that are suspended in water or air, by means of a straining device.

Filtration • In the excretory physiology of some animals, the process by which the initial urine is formed; water and most solutes are transferred into the excretory tract, while proteins are retained in the blood or hemolyrnph.

First law of thermodynamics • Energy can be neither created nor destroyed.

Fission • Reproduction of a prokaryote by division of a cell into two comparable progeny cells.

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

Fixed action pattern • A behaviour that is genetically programmed.

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

Flexor • A muscle that flexes an appendage.

Flora (flore’ ah) • All of the plants found in a given area. (Contrast with fauna.)

Florigen • A plant hormone (not yet isolated) involved in the conversion of a vegetative shoot apex to a flower.

Flower • The total reproductive structure of an angiosperm; its basic parts include the calyx, corolla, stamens, and carpels.

Fluorescence • The emission of a photon of visible light by an excited atom or molecule.

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

Follicle-stimulating hormone • A gonadotropic hormone produced by the anterior pituitary.

Food chain • A portion of a food web, most commonly a simple sequence of prey species and the predators that consume them.

Food web • The complete set of food links between species in a community; a diagram indicating which ones are the eaters and which are consumed.

Fossil • Any recognizable structure originating from an organism, or any impression from such a structure, that has been preserved over geological time.

Fossil fuel • A fuel (particularly petroleum products) composed of the remains of organisms that lived in the remote past.

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

Fovea [L. fovea; a small pit] • The area, in the vertebrate retina, of most distinct vision.

Frame-shift mutation • A mutation resulting from the addition or deletion of a single base pair in the DNA sequence of a gene. As a result of this, mRNA transcribed from such a gene is translated normally until the ribosome reaches the point at which the mutation has occurred. From that point on, codons are read out of proper register and the amino acid sequence bears no resemblance to the normal sequence. (Contrast with missense mutation, nonsense mutation, synonymous mutation.)

Free energy • That energy which is available for doing useful work, after allowance has been made for the increase or decrease of disorder. Designated by the symbol G (for Gibbs free energy), and defined by: G = H - TS, where H = heat, S = entropy, and T = absolute (Kelvin) temperature.

Frequency-dependent selection • Selection that changes in intensity with the proportion of individuals having the trait.

Fruit • In angiosperms, a ripened and mature ovary (or group of ovaries) containing the seeds. Sometimes applied to reproductive structures of other groups of plants, and includes any adjacent parts which may be fused with the reproductive structures.

Fruiting body • A structure that bears spores.

Fungus (fung’ gus) • A member of the kingdom Fungi, a (usually) multicellular eukaryote with absorptive nutrition.

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

G₂ phase • In the cell cycle, the gap between the S (synthesis) phase and the onset of mitosis.

G protein • A membrane protein involved in signal transduction; characterized by binding guanyl nucleotides. The activation of certain receptors activates the G protein, which in turn activates adenylate cyclase. G protein activation involves binding a GTP molecule in place of a GDP molecule.

Gametangium (gain i tan’ gee urn) [Gr. gamos: marriage + angeion: vessel or reservoir] • Any plant or fungal structure within which a gamete is formed.

Gamete (gain’ eet) [Gr. gamete: wife, gametes: husband] • The mature sexual reproductive cell: the egg or the sperm.

Gametocyte (ga meet’ oh site) [Gr. gamete: wife, gametes: husband + kytos: cell] • The cell that gives rise to sex cells, either the eggs or the sperm. (See oocyte and spermatocyte.)

Gametogenesis (ga meet’ oh jen’ e sis) [Gr. gamete: wife, gametes: husband + genesis: source] • The specialised series of cellular divisions that leads to the production of sex cells (gametes). (Contrast with oogenesis and spermatogenesis.)

Gametophyte (ga meet’ oh fyte) • In plants and photosynthetic protists with alternation of generations, the haploid phase that produces the gametes. (Contrast with sporophyte.)

Ganglion (gang’ glee un) [Gr.: tumour] • A group or concentration of neuron cell bodies.

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

Gas exchange • In animals, the process of taking up oxygen from the environment and releasing carbon dioxide to the environment.

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

Gastrula (gas’ true luh) [Gr. gaster: stomach] • An embryo forming the characteristic three cell layers (ectoderm, endoderm, and mesoderm) which will give rise to all of the major tissue systems of the adult animal.

Gastrulation • Development of a blastula into a gastrula.

Gated channel • A channel (membrane protein) that opens and closes in response to binding of specific molecules or to changes in membrane potential.

Gel electrophoresis (jel ul lec tro for’ eesis) • A semisolid matrix suspended in a salty buffer in which molecules can be separated on the basis of their size and change when current is passed through the gel.

Gene [Gr. gen: to produce] • A unit of heredity. Used here as the unit of genetic function which carries the information for a single polypeptide.

Gene amplification • Creation of multiple copies of a particular gene, allowing the production of large amounts of the RNA transcript (as in rRNA synthesis in oocytes).

Gene cloning • Formation of a clone of bacteria or yeast cells containing a particular foreign gene.

Gene family • A set of identical, or once-identical, genes, derived from a single parent gene; need not be on the same chromosomes; classic example is the globin family in vertebrates.

Gene flow • The exchange of genes between different species (an extreme case referred to as hybridization) or between different populations of the same species caused by migration following breeding.

Gene pool • All of the genes in a population.

Gene therapy • Treatment of a genetic disease by providing patients with cells containing wild type alleles for the genes that are nonfunctional in their bodies.

Generative nucleus • In a pollen tube, a haploid nucleus that undergoes mitosis to produce the two sperm nuclei that participate in double fertilization. (Contrast with tube nucleus.)

Genet • The genetic individual of a plant that is composed of a number of nearly identical but repeated units.

Genetic drift • Changes in gene frequencies from generation to generation in a small population as a result of random processes.

Genetic stochasticity • Variation in the frequencies of alleles and genotypes in a population over time.

Genetics • The study of heredity.

Genetic structure • The frequencies of alleles and genotypes in a population.

Genome (jee’ nome) • The genes in a complete haploid set of chromosomes.

Genotype (jean’ oh type) [Gr. gen: to produce + typos: impression] • An exact description of the genetic constitution of an individual, either with respect to a single trait or with respect to a larger set of traits. (Contrast with phenotype.)

Genus (jean’ us) (plural: genera) [Gr. genos: stock, kind] • A group of related, similar species.

Geotropism • See gravitropism.

Germ cell • A reproductive cell or gamete of a multicellular organism. 

Germination • The sprouting of a seed or spore.

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

Gibberellin (jib er el’ un) IlL. gibberella: hunchback (refers to shape of a reproductive structure of a fungus that produces gibberellins)] • One of a class of plant growth substances playing roles in stem elongation, seed germination, flowering of certain plants, etc. Named for the fungus Gibberella.

Gill • An organ for gas exchange in aquatic organisms.

Gill arch • A skeletal structure that supports gill filaments and the blood vessels that supply them.

Gizzard (giz’ erd) EL. gigeria: cooked chicken parts] • A very muscular port of the stomach of birds that grinds up food, sometimes with the aid of fragments of stone.

Gland • An organ or group of cells that produces and secretes one or more substances.

Glans penis • Sexually sensitive tissue at the tip of the penis.

Glia (glee’ uh) [Gr.: glue] • Cells, found only in the nervous system, which do not conduct action potentials.

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

Glucocorticoids • Steroid hormones produced by the adrenal cortex. Secreted in response to ACTH, they inhibit glucose uptake by many tissues in addition to mediating other stress responses.

Glucagon • A hormone produced and released by cells in the islets of Langerhans of the pancreas. It stimulates the breakdown of glycogen in liver cells.

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

Glucose (glue’ kose) fGr. gleukos: sweet wine mash for fermentation] • The most common sugar, one of several monosaccharides with the formula C₆H₁₂0₆.

Glycerol (gliss’ er ole) • A three-carbon alcohol with three hydroxyl groups, the linking component of phospholipids and triglycerides.

Glycogen (gly’ ko jen) • A branched-chain polymer of glucose, similar to starch (which is less branched and may be of lower molecular weight). Exists mostly in liver and muscle; the principal storage carbohydrate of most animals and fungi.

Glycolysis (gly kol’ Ii sis) [from glucose + Gr. Iysis: loosening] • The enzymatic breakdown of glucose to pyruvic acid. One of the oldest energy-yielding machanisms in living organisms.

Glycosidic linkage • The connection in an oligosaccharide or polysaccharide chain, formed by removal of water during the linking of monosaccharides.by root pressure.

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

Golgi apparatus (goal’ jee) • A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells. Plays a role in the production and release of secretory materials such as the digestive enzymes manufactured in the pancreas. First described by Camillo Golgi (1844-1926).

Gonad (go’ nad) [Gr. gone: seed, that which produces seed] • An organ that produces sex cells in animals: either an ovary (female gonad) or testis (male gonad).

Gonadotropin • A hormone that stimulates the gonads.

Gondwana • The large southern land mass that existed from the Cambrian (540 mya) to the Jurassic (138 mya). Present-day South America, Africa, India, Australia, and Antarctica.

Gram stain • A differential stain useful in characterizing bacteria.

Granum • Within a chloroplast, a stack of thylakoids.

Gravitropism • A directed plant growth response to gravity.

Grazer • An animal that eats the vegetative tissues of herbaceous plants.

Green gland • An excretory organ of crustaceans.

Greenhouse effect • The heating of Earth’s atmosphere by gases that are transparent to sunlight but opaque to radiated heat.

Gross primary production • The total energy captured by plants growing in a particular area.

Ground meristem • That part of an apical meristem that gives rise to the ground tissue system of the primary plant body.

Ground tissue system • Those parts of the plant body not included in the dermal or vascular tissue systems. Ground tissues function in storage, photosynthesis, and support.

Group transfer • The exchange of atoms between molecules

Growth • Irreversible increase in volume (probably the most accurate definition, but at best a dangerous oversimplification).

Growth factors • A group of proteins that circulate in the blood and trigger the normal growth of cells. Each growth factor acts only on certain target cells.

Guanine (gwan ‘een) • A nitrogen-containing base found in DNA, RNA and GTP.

Guard cells • In plants, paired epidermal cells which surround and control the opening of a stoma (pore).

Gut • An animal’s digestive tract

Guttation • The extrusion of liquid water through openings in leaves, caused by root pressure.

Gymnosperm (jim’ no sperm) [Gr. gymnos: naked + sperma: seed] • A plant, such as a pine or other conifer, whose seeds do not develop within an ovary (hence, the seeds are ‘naked”).

Gyrus (plural: gyri) • The raised or ridged portion of the convoluted surface of the brain. (Contrast to sulcus.)

H

Habit • The form or pattern of growth characteristic of an organism. 

Habitat • The environment in which an organism lives.

Habituation (ha bich’ 00 ay shun) • The simplest form of learning, in which an animal presented with a stimulus without reward  or punishment eventually ceases to respond.

Hair cell • A type of mechanoreceptor in animals.

Half-life • The time required for half of a sample of a radioactive isotope to decay to its stable, nonradioactive form.

Halophyte (hal’ oh fyte) [Gr. halos: salt + phyton: plant] • A plant that grows in a saline (salty) environment.

Haploid (hap’ bid) [Gr. haploeides: single] • Having a chromosome complement consisting of just one copy of each chromosome. This is the normal “pboidy” of gametes or of asexual spores produced by meiosis or of organisms (such as the gametophyte generation of plants) that grow from such spores without fertilization.

Hardy-Weinberg equililbrium • The percentages of diploid combinations expected from a knowledge of the proportions of alleles in the population if no agents of evolution are acting on the population.

Haustorium (haw stor’ ee um) [L. haustus: draw up] • A specialised hypha or other structure by which fungi and some parasitic plants draw food from a host plant.

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

Heat-shock proteins • Chaperone proteins expressed in cells exposed to high temperatures or other forms of environmental stress.

Helper T cells • T cells that participate in the activation of B cells and of other I cells; targets of the HIV-I virus, the agent of AIDS. (Contrast with cytotoxic T cells, suppressor T cells.)

Hematocrit (haem at o krit) [Gr. haima: blood + krites: judge] • The proportion of 100 cc of blood that consists of red blood cells.

Hemizygous(hem’ ee zie’ gus) [Gr. hemi: half + zygotos: joined] • In a diploid organism, having only one allele for a given trait, typically the case for X-linked genes in male mammals and Z-linked genes in female birds. (Contrast with homozygous, heterozygous.)

Hemoglobin (hee’ mo glow’ bin) [Gr. haima: blood + L. globus: globe] • The colored protein of vertebrate blood (and blood of some invertebrates) which transports oxygen.

Hepatic (heh pat’ ik) [Gr. hepar: liver] • Pertaining to the liver.

Hepatic duct • The duct that conveys bile from the liver to the gallbladder.

Herbicide (ur’ bis ide) • A chemical substance that kills plants.

Herbivore [L. herba: plant + vorare: to devour] • An animal which eats the tissues of plants. (Contrast with carnivore, detritivore, omnivore.)

Heritable • Able to be inherited; in biology usually refers to genetically determined traits.

Hermaphroditism (her maf’ row dite’ ism) [Gr. hermaphroditos: a person with both male and female traits] • The coexistence of both female and male sex organs in the same organism.

Hertz (abbreviated as Hz) • Cycles per second.

Hetero- [Gr.: other, different] • A prefix used in biology to mean that two or more different conditions are involved; for example, heterotroph, heterozygous. Heterochromatin • Chromatin that retains its coiling during interphase; generally not transcribed. (Contrast with euchromatin.)

Heterocyst • A large, thick-walled cell in the filaments of certain cyanobacteria; performs nitrogen fixation.

Heterogeneous nuclear RNA (hnRNA) • The product of transcription of a eukaryotic gene, including transcripts of introns.

Heteromorphic (het’ er oh more’ fik) [Gr. heteros: different + morphe: form] having a different form or appearance, as two heteromorphic life stages of a plant. (Contrast with isomorphic.)

Heterosporous (het’ er Os’ por us) • Producing two types of spores, one of which gives rise to a female megaspore and the other to a male microspore. Heterosporous plants produce distinct female and male gametophytes. (Contrast with homosporous.)

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

Heterotroph (het’ er oh trof) [Gr. heteros: different + trophe: food] • An organism that requires preformed organic molecules as food. (Contrast with autotroph.)

Heterozygous (het’ er oh zie’ gus) [Gr. heteros: different + zygotos: joined] Of a diploid organism having different alleles of a given gene on the pair of homologues carrying that gene. (Contrast with homozygous.)

Hibernation EL. hibernus: winter] • The state of inactivity of some animals during winter; marked by a drop in body temperature and metabolic rate.

Highly repetitive DNA • Short DNA sequences present in millions of copies in the genome, next to each other (in tandem). In a In a reassociation experiment, denatured highly repetitive DNA reanneals very quickly.

Hippocampus • A part of the forebrain that takes part in long-term memory formation.

Histamine (hiss; tah meen) • A substance released within a damaged tissue by a type of white blood cell. Hista mines are responsible for aspects of allergice reactions, including the increased vascular permeability that leads to edema (swelling).

Histology • The study of tissues.

Histone • Any one of a group of basic proteins forming the core of a nucleosome, the structural unit of a eukaryotic chromosome. (See nucleosome.)

hnRNA • See heterogeneous nuclear RNA.

Homeobox • A 180-base-pair segment of DNA found in a few genes (called Hox genes), perhaps regulating the expression of other genes and thus controlling large-scale developmental processes.

Homeostasis (home’ ee o sta’ sis) [Gr. homos: same + stasis: position] • The maintenance of a steady state, such as a constant temperature or a stable social structure, by means of physiological or behavioural feedback responses.

Homeotherm (home’ ee o therm) [Gr. homos: same + therme: heat] • An animal which maintains a constant body temperature by virtue of its own heating and cooling mechanisms. (Contrast with heterotherm, poikilotherm.)

Homeotic genes (home’ ee Ott’ ic) • Genes that determine what entire segments of an animal become. Drastic mutations in these genes cause the transformation of body segments in Drosophila. Homeotic genes studied in the plant Arabidopsis are called organ identity genes.

Homolog (home’ o log’) [Gr. homos: same + logos: word] • One of a pair, or larger set, of chromosomes having the same overall genetic composition and sequence. In diploid organisms, each chromosome inherited from one parent is matched by an identical (except for mutational changes) chromosome—its homolog—from the other parent.

Homology (ho mol’ o jee) [Gr. homologi(a): agreement] • A similarity between two structures that is due to inheritance from a common ancestor. The structures are said to be homologous. (Contrast with analogy.)

Homoplasy (home’ uh play zee) [Gr. humos: same + plastikos: to mold] • The presence in several species of a trait not present in their most common ancestor. Can result from convergent evolution, reverse evolution, or parallel evolution.

Homosporous • Producing a single type of spore that gives rise to a single type of gametophyte, bearing both female and male reproductive organs. (Contrast with heterosporous.)

Homozygous (home’ o zie’ gus) [Gr. homos: same + zygotos: joined] • Of a diploid organism having identical alleles of a given gene on both homologous chromosomes. An organism may be a “homozygote’ with respect to one gene and, at the same time, a “heterozygote” with respect to another. (Contrast with heterozygous.)

Hormone (hore’ mone) [Gr. hormon: excite, stimulate] • A substance produced in one part of a multicellular organism and transported to another part where it exerts its specific effect on the physiology or biochemistry of the target cells.

Host • An organism that harbours a parasite and provides it with nourishment.

Host-parasite interaction • The dynamic interaction between populations of a host and the parasites that attack it.

Hox genes • See homeobox.

Humoral immune system • The part of the immune system mediated by B cells; it is mediated by circulating antibodies and is active against extracellular bacterial and viral infections.

Humus (hew’ muss) • The partly decomposed remains of plants and animals on the surface of a soil. Its characteristics depend primarily upon climate and the species of plants growing on the site.

Hyaluronidase (hill yew ron uh dase) • An enzyme that digests proteoglycans. Found in sperm cells, it helps digest the coatings surrounding an egg so the sperm can penetrate the egg cell membrane.

Hybrid (high’ brid) [L. hybrida: mongrel] • The offspring of genetically dissimilar parents. In molecular biology, a double helix formed of nucleic acids from different sources.

Hybridoma • A cell produced by the fusion of an antibody-producing cell with a myeloma cell; it produces monoclonal antibodies.

Hybrid zone • A narrow zone where two populations interbreed, producing hybrid individuals.

Hydrocarbon • A compound containing only carbon and hydrogen atoms.

Hydrogen bond • A chemical bond which arises from the attraction between the slight positive charge on a hydrogen atom and a slight negative charge on a nearby fluorine, oxygen, or nitrogen atom. Weak bonds, but found in great quantities in proteins, nucleic acids, and other biological macromolecules.

Hydrological cycle • The sum total of movement of water from the oceans to the atmosphere, to the soil, and back to the oceans. Some water is cycled many times within compartments of the system before completing one full circuit.

Hydrolyze (hi’ dro lize) [Gr. hydro: water + /ysis: cleavage] • To break a chemical bond, as in a peptide linkage, with the insertion of the components of water, -H and -OH, at the cleaved ends of a chain. The digestion of proteins is a hydrolysis.

Hydrophilic [Gr. hydro: water + phi/ia: love] • Having an affinity for water. (Contrast with hydrophobic.)

Hydrophobic [Gr. hydro: water + phobia: fear] • Molecules and amino acid side chains, which are mainly hydrocarbons (compounds of C and H with no charged groups or polar groups), have a lower energy when they are clustered together than when they are distributed through an aqueous solution. Because of their attraction for one another and their reluctance to mix with water they are called “hydrophobic.’ Oil is a hydrophobic substance; phenylalanine is a hydrophobic animo acid in a protein. (Contrast with hydrophilic.)

Hydrostatic skeleton • The incompressible internal liquids of some animals that transfer forces from one part of the body to another when acted upon by the surrounding muscles.

Hydroxyl group • The —OH group, characteristic of alcohols.

Hyperpolarization • A change in the resting potential of a membrane so the inside of a cell becomes more electronegative. (Contrast with depolarization.)

Hypersensitive response • A defensive response of plants to microbial infection; it results in a “dead spot.”

Hypertension • High blood pressure.

Hypotonic [Gk. hypo: beneath, under] • Having a lower solute concentration. Said of one solution in comparing it to another. (Contrast with hypertonic, isotonic.)

Hypha (high’ fuh) (plural: hyphae) [Gr. hyphe: web] • In the fungi, any single filament. May be multinucleate (zygomycetes, ascomycetes) or multicellular (basidiomycetes).

Hypocotyl [Gk. hypo: beneath, under + kotyledon: hollow space] • That part of the embryonic or seedling plant shoot that is below the cotyledons.

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

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

Hypothetico-deductive method • A method of science in which hypotheses are erected, predictions are made from them, and experiments and observations are performed to test the predictions.

Hypotonic [Gk. hypo: beneath, under] • Having a greater solute concentration. Said of one solution in comparing it to another. (Contrast with hypotonic, isotonic.)

 I

Imaginal disc • In insect larvae, groups of cells that develop into specific adult organs.

Immune system [L. immunis: exempt] • A system in mammals that recognises and eliminates or neutralizes either foreign substances or self substances that have been altered to appear foreign.

Immunization • The deliberate introduction of antigen to bring about an immune response.

Immunoglobulins • A class of proteins, with a characteristic structure, active as receptors and effectors in the immune system.

Immunological memory • Certain clones of immune system cells made to respond to an antigen persist. This leads to a more rapid and massive response of the immune system to any subsequenct exposure to that antigen.

Immunological tolerance • A mechanism by which an animal does not mount an immune response to the antigenic determinants of its own macromolecules.

Imprinting • (1) In genetics, the differential modification of a gene depending on whether it is present in a male or a female. (2) In animal behaviour, a rapid form of learning in which an animal comes to make a particular response, which is maintained for life, to some object or other organism.

Inclusive fitness • The sum of an individual’s own fitness (the effect of producing its own offspring: the individual selection component) plus its influence on fitness in relatives other than direct descendants (the kin selection component).

Incomplete dominance • Condition in which the heterozygous phenotype is intermediate between the two homozygous phenotypes.

Incomplete metamorphosis • Insect development in which changes between instars are gradual.

Incus (in’ kus) [L. incus: anvil] • The middle of the three bones that conduct movements of the eardrum to the oval window of the inner ear. (See malleus, stapes.)

Independent assortment • The random separation during meiosis of nonhomologous chromosomes and of genes carried on nonhomologous chromosomes.

Individual fitness • That component of inclusive fitness that results from an organism producing its own offspring. (Contrast with kin selection component.)

Indoleacetic acid • See auxin.

Inducer • (1) In enzyme systems, a small molecule which, when added to a growth medium, causes a large increase in the level of some enzyme. (2) In embryology, a substance that causes a group of target cells to differentiate in a particular way.

Inducible enzyme • An enzyme that is present in much larger amounts when a particular compound (the inducer) has been added to the system. (Contrast with constitutive enzyme.)

Inflammation • A nonspecific defense against pathogens; characterized by redness, swelling, pain, and increased temperature.

Inflorescence • A structure composed of several flowers.

Inhibitor • A substance which binds to the surface of an enzyme and interferes with its action on its substrates.

Inhibitory postsynaptic potential • A change in the resting potential of a postsynaptic membrane in the hyperpolarizing (negative) direction.

Initiation complex • Combination of a ribosomal light subunit, an mRNA molecule, and the tRNA charged with the first amino acid coded for by the mRNA; formed at the onset of translation.

Initiation factors • Proteins that assist in forming the translation initiation complex at the ribosome.

Inositol triphosphate (1P3) • An intracellular second messenger derived from membrane phospholipids.

Instar (in’ star) [L.: image, form] • An immature stage of an insect between molts.

Insulin (in’ su lin) EL. insula: island] • A hormone, synthesized in islet cells of the pancreas, that promotes the conversion of glucose to the storage material, glycogen.

Integrase • An enzyme that integrates retroviral cDNA into the genome of the host cell.

Integrated pest management • A method of control of pests in which natural predators and parasites are used in conjunction with sparing use of chemical methods to achieve control of a pest without causing serious adverse environmental side effects.

Integument [L. integumentum: covering] • A protective surface structure. In gymnosperms and angiosperms, a layer of tissue around the ovule which will become the seed coat. Gymnosperm ovues have one integument, angiosperm ovules two.

Intercatary meristem • A meristematic region in plants which occurs not apically, but between two regions of mature tissue. Intercalary meristems occur in the nodes of grass stems, for example.

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

Interferon • A glycoprotein produced by virus-infected animal cells; increases the resistance of neighboring cells to the virus

Interkinesis • The phase between the first and second meiotic divisions.

Interleukins • Regulatory proteins, produced by macrophages and lymphocytes, that act upon other lymphocytes and direct their development.

Intermediate filaments • Fibrous proteins that stabilize cell structure and resist tension.

Internode • Section between two nodes of a plant stem.

Interphase • The period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. It is during this period that the cell is most active in transcribing and translating genetic information.

Interspecific competition • Competition between members of two or more species.

Intertropical convergence zone • The tropical region where the air rises most strongly; moves north and south with the passage of the sun overhead.

Intraspecific competition • Competition among members of a single species.

Intrinsic protein • A membrane protein that is embedded in the phospholipid bilayer of the membrane. (Contrast with extrinsic protein.)

Intrinsic rate of increase • The rate at which a population can grow when its density is low and environmental conditions are highly favorable.

Intron • A portion of a DNA molecule that, because of RNA splicing, is not involved in coding for part of a polypeptide molecule. (Contrast with exon.)

Invagination • An infolding.

Inversion (genetic) • A rare mutational event that leads to the reversal of the order of genes within a segment of a chromosome, as if that segment had been removed from the chromosome, turned 1800, and then reattached.

Invertebrate • Any animal that is not a vertebrate, that is, whose nerve cord is not enclosed in a backbone of bony segments.

In vitro [L.: in glass] • In a test tube, rather than in a living organism. (Contrast with in vivo.)

In vivo [L.: in the living state] • In a living organism. Many processes that occur in vivo can be reproduced in vitro with the right selection of cellular components. (Contrast with in vitro.)

Ion (eye’ on) [Gr.: wanderer] • An atom or group of atoms with electrons added or removed, giving it a negative or positive electrical charge.

Ion channel • A membrane protein that can let ions pass across the membrane. The channel can be ion-selective, and it can be voltage-gated or ligand-gated.

Ionic bond • A chemical bond which arises from the electrostatic attraction between positively and negatively charged ions. Usually a strong bond.

Iris (eye’ ris) [Gr. iris: rainbow] • The round, pigmented membrane that surrounds the pupil of the eye and adjusts its aperture to regulate the amount of light entering the eye.

Irruption • A rapid increase in the density of a population. Often followed by massive emigration.

Islets of Langerhans • Clusters of hormone-producing cells in the pancreas.

Iso- [Gr.: equal] • Prefix used to denote two separate but similar or identical states of a characteristic. (See isomers, isomorphic, isotope.)

Isolating mechanism • Geographical, physiological, ecological, or behavioural mechanisms that lead to a reduction in the frequency of hybrid matings.

Isomers • Molecules consisting of the same numbers and kinds of atoms, but differing in the way in which the atoms are combined.

Isomorphic (eye’ so more’ fik) [Gr. isos: equal + morphe: form] • having the same form or appearance, as two isomorphic life stages. (Contrast with heteromorphic.)

Isotonic • Having the same solute concentration; said of two solutions. (Contrast with hypertonic, hypotonic.)

Isotope (eye’ so tope) [Gr. isos: equal + topos: place] • Two isotopes of the same chemical element have the same number of protons in their nuclei, but differ in the number of neutrons.

J

Jejunum (jih jew’ num) • The middle division of the small intestine, where most absorption of nutrients occurs. (See duodenum, ileum.)

Joule (jool, or jowl) • A unit of energy, equal to 0.24 calories

K

 Karyotype • The number, forms, and types of chromosomes in a cell. Kelvin temperature scale • See absolute temperature scale.

Keratin (ker’ a tin) [Gr. keras: horn] • A protein which contains sulfur and is part of such hard tissues as horn, nail, and the outermost cells of the skin.

Ketone (key’ tone) • A compound with a C==O group attached to two other  groups, neither of which is an H atom. Many sugars are ketones. (Contrast with aldehyde.)

Keystone species • A species that exerts a major influence on the composition and dynamics of the community in which it lives.

Kidneys • A pair of excretory organs in vertebrates

Kin selection • The component of inclusive fitness resulting from helping the survival of relatives containing the same alleles by descent from a common ancestor.

Kinase (kye’ nase) • An enzyme that transfers a phosphate group from ATP to another molecule. Protein kinases transfer phosphate from ATP to specific proteins, playing important roles in cell regulation.

Kinesis (ki nee’ sis) lGr.: movement] • Orientation behaviour in which the organism does not move in a particular direction with reference to a stimulus but instead simply moves at an increasing or decreasing rate until it ends up farther from the object or closer to it. (Contrast with taxis.)

Kinetochore (kin net’ oh core) [Gr. kinetos: moving + khorein: to move] Specialised structure on a centromere to which microtubules attach.

Koch’s posulates • Four rules for establishing that a particular microorganism causes a particular disease.

Krebs cycle • See citric acid cycle

L

Lactic acid • The end product of fermentation in vertebrate muscle and some microorganisms.

Lagging strand • In DNA replication, the daughter strand that is synthesized discontinuously.

Lamella • Layer.

Larynx (Iar’ inks) • A structure between the pharynx and the trachea that includes the vocal cords.

Larva (plural: larvae) EL.: ghost, early stage] • An immature stage of any invertebrate animal that differs dramatically in appearance from the adult.

Lateral • Pertaining to the side.

Lateral gene transfer • The movement of genes from one prokaryotic species to another.

Lateral meristems • The vascular cambium and cork cambium, which give rise to secondary tissue in plants.

Laterization (lat’ ur iz ay shun) • The formation of a nutrient-poor soil that is rich in insoluble iron and aluminum compounds.

Law of independent assortment • The random separation during meiosis of nonhomologous chromosomes and of genes carried on nonhomologous chromosomes. Mendel’s second law.

Law of segregation • Alleles segregate from one another during gamete formation, Mendel’s first law.

Leader sequence • A sequence of amino acids at the N-terminal end of a newly synthesized protein, determining where the protein will be placed in the cell.

Leading strand • In DNA replication, the daughter strand that is synthesized continuously.

Lenticel • Spongy region in a plant’s periderm, allowing gas exchange.

Leukocyte (100’ ko sight) [Gr. Ieukos: clear + kutos: hollow vessel] • A white blood cell.

Lichen (lie’ kun) [Gr. Ieikhen: licker] • An organism resulting from the symbiotic association of a true fungus and either a cyanobacterium or a unicellular alga.

Life cycle • The entire span of the life of an organism from the moment of fertilization (or asexual generation) to the time it reproduces in turn.

Life history • The stages an individual goes through during its life.

Life table • A table showing, for a group of equal-aged individuals, the proportion still alive at different times in the future and the number of offspring they produce during each time interval.

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

Ligand (hg’ and) • A molecule that binds to a receptor site of another molecule.

Lignin • The principal noncarbohydrate component of wood, a polymer that binds together cellulose fibrils in some plant cell walls.

Limbic system • A group of primitive vertebrate forebrain nuclei that form a network and are involved in emotions, drives, instinctive behaviours, learning, and memory.

Limiting resource • The required resource whose supply most strongly influences the size of a population.

Linkage • Association between genetic markers on the same chromosome such that they do not show random assortment and seldom recombine; the closer the markers, the lower the frequency of recombination.

Lipase (lip’ ase; lye’ pase) • An enzyme that digests fats.

Lipids (lip’ ids) [Gr. Iipos: fat] • Substances in a cell which are easily extracted by organic solvents; fats, oils, waxes, steroids, and other large organic molecules, including those which, with proteins, make up the cell membranes. (See phosphohipids.)

Litter • The partly decomposed remains of plants on the surface and in the upper layers of the soil.

Littoral zone • The coastal zone from the upper limits of tidal action down to the depths where the water is thoroughly stirred by wave action.

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

Lobes • Regions of the human cerebral hemispheres; includes the temporal, frontal, parietal, and occipital lobes.

Locus • In genetics, a specific location on a chromosome. May be considered to be synonymous with ‘‘gene.

Logistic growth • Growth, especially in the size of an organism or in the number of organisms that constitute a population, which slows steadily as the entity approaches its maximum size. (Contrast with exponential growth.)

Loop of Hente (hen’ lee) • Long, hairpin loop of the mammalian renal tubule that runs from the cortex down into the medulla, and back to the cortex. Creates a concentration gradient in the interstitial fluids in the medulla.

Lophophore • A U-shaped fold of the body wall with hollow, ciliated tentacles that encircles the mouth of animals in several different phyla. Used for filtering prey from the surrounding water.

Lordosis (br doe’ sis) [Gk. Iordosis: curving forward] • A posture assumed by females of some mammalian species (especially rodents) to signal sexual receptivity.

Lumen (loo’ men) [L.: light] • The cavity inside any tubular part of an organ, such as a piece of gut or a kidney tubule.

Lungs • A pair of saclike chambers within the bodies of some animals, functioning in gas exchange.

Luteinizing hormone • A gonadotropin produced by the anterior pituitary. It stimulates the gonads to produce sex hormones.

Lymph FL. Iympha: water] • A clear, watery fluid that is formed as a filtrate of blood; it contains white blood cells; it collects in a series of special vessels and is returned to the bloodstream.

Lymph nodes • Specialised tissue regions that act as filters for cells, bacteria and foreign matter.

Lymphocyte • A major class of white blood cells. Includes T cells, B cells, and other cell types important in the immune response.

Lysis (lie’ sis) [Gr.: a loosening] • Bursting of a cell.

Lysogenic • The condition of a bacterium that carries the genome of a virus in a relatively stable form. (Contrast with lytic.)

Lysosome (lie’ so soam) [Gr. Iysis: a loosening + soma: body] • A membrane-bounded inclusion found in eukaryotic cells (other than plants). Lysosomes contain a mixture of enzymes that can digest most of the macromolecules found in the rest of the cell.

Lysozyme (lie’ so zyme) • An enzyme in saliva, tears, and nasal secretions that attacks bacterial cell walls, as one of the body’s nonspecific defense mechanisms.

Lytic • Condition in which a bacterium lyses shortly after infection by a virus; the viral genome does not become stabilized within the bacterial cell. (Contrast with lysogenic.)

M

Macro- (mack’ roh) [Gr. makros: large, long] • A prefix commonly used to denote something large. (Contrast with micro-.)

Macroevolution • Evolutionary changes occurring over long time spans and usually involving changes in many traits. (Contrast with microevolution.)

Macromolecule • A giant polymeric molecule. The macromolecules are proteins, polysaccharides, and nucleic acids.

Macronutrient • A mineral element required by plant tissues in concentrations of at least 1 milligram per gram of their dry matter.

Macrophage (mac’ roh faj) • A type of white blood cell that endocytoses bacteria and other cells.

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

Medulla (meh dull’ luh) [L.: narrow] • (1) The inner, core region of an organ, as in the adrenal medulla (adrenal gland) or the renal medulla (kidneys). (2) The portion of the brain stem that connects to the spinal cord.

Mega- [Gr. megas: large, great] • A prefix often used to denote something large. (Contrast with micro-.)

Megaspore [Gr. megas: large + spora:seed] • In plants, a haploid spore that produces a female gametophyte.

Meiosis (my oh’ sis) [Gr.: diminution] • Division of a diploid nucleus to produce four haploid daughter cells. The process consists of two successive nuclear divisions with only one cycle of chromosome replication.

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

Memory cells • Long-lived lymphocytes produced by exposure to antigen. They persist in the body and are able to mount a rapid response to subsequent exposures to the antigen.

Mendelian population • A local population of individuals belonging to the same species and exchanging genes with one another.

Menopause • The time in a human female’s life when the ovarian and menstrual cycles cease.

Menstrual cycle • The monthly sloughing off of the uterine lining if fertilization does not occur in the female. Occurs between puberty and menopause.

Meristem [Gr. meristos: divided] • Plant tissue made up of actively dividing cells.

Mesenchyme (mez’ en kyme) [Gr. mesos: middle + enchyma: infusion] . Embryonic or unspecialised cells derived from the mesoderm.

Meso- (mez’ oh) [Gr.: middle] • A prefix often used to designate a structure located in the middle, or a stage that appears at some intermediate time. For example, mesoderm, Mesozoic.

Mesoderm [Gr. mesos: middle + derma: skin] • The middle of the three embryonic tissue layers first delineated during gastrulation. Gives rise to skeleton, circulatory system, muscles, excretory system, and most of the reproductive system.

Mesophytl (mez’ a fill) [Gr. mesos: middle + phyllon: leaf] • Chloroplast­containing, photosynthetic cells in the interior of leaves.

Mesosome (mez’ o soam’) [Gr. mesos: middle + soma: body] • A localized infolding of the plasma membrane of a bacterium.

Messenger RNA (mRNA) • A transcript of one of the strands of DNA, it carries information (as a sequence of codons) for the synthesis of one or more proteins.

Meta- [Gr.: between, along with, beyond] • A prefix used in biology to denote a change or a shift to a new form or level; for example, as used in metamorphosis.

Metabolic compensation • Changes in biochemical properties of an organism that render it less sensitive to temperature changes.

Metabolic pathway • A series of enzyme-catalysed reactions so arranged that the product of one reaction is the substrate of the next.

Metabolism (meh tab’ a Iizm) [Gr. metabole: to change] • The sum total of the chemical reactions that occur in an organism, or some subset of that total (as in “respiratory metabolism”).

Metamorphosis (met’ a mor’ fo sis) [Gr. meta: between + morphe: form, shape] • A radical change occurring between one developmental stage and another, as for example from a tadpole to a frog or an insect larva to the adult.

Metaphase (met’ a phase) [Gr. meta: between] • The stage in nuclear division at which the centromeres of the highly supercoiled chromosomes are all lying on a plane (the metaphase plane or plate) perpendicular to a line connecting the division poles.

Metapopulation • A population divided into subpopulations, among which there are occasional exchanges of individuals.

Metastasis (meh tass’ tuh sis) • The spread of cancer cells from their original site to other parts of the body.

Methanogen • Any member of a group of Archaebacteria that release methane as a metabolic product. This group is considered to be an extremely ancient one.

MHC • See major histocompatibility complex.

Micro- (mike’ roh) [Gr. mikros: small] • A prefix often used to denote something small. (Contrast with macro-, mega-.)

Microbiology [Gr. mikros: small + bios: life + logos: discourse] • The scientific study of microscopic organisms, particularly bacteria, unicellular algae, protists, and viruses.

Microevolution • The small evolutionary changes typically occurring over short time spans; generally involving a small number of traits and minor genetic changes. (Contrast with macroevolution.)

Microfilament • Minute fibrous structure generally composed of actin found in the cytoplasm of eukaryotic cells. They play a role in the motion of cells.

Micronutrient • A mineral element required by plant tissues in concentrations of less than 100 micrograms per gram of their dry matter.

Micropyle (mike’ roh pile) [Gr. mikros: small + pyle: gate] • Opening in the integument(s) of a seed plant ovule through which pollen grows to reach the female gametophyte within.

Microspores [Gr. mikros: small + spora: seed] • In plants, a haploid spore that produces a male gametophyte.

Microtubules • Minute tubular structures found in centrioles, spindle apparatus, cilia, flagella, and other places in the cytoplasm of eukaryotic cells. These tubules play roles in the motion and maintenance of shape of eukaryotic cells.

Microvilli (singular: microvillus) • The projections of epithelial cells, such as the cells lining the small intestine, that increase their surface area.

Middle lamella • A layer of derivative polysaccharides that separates plant cells; a common middle lamella lies outside the primary walls of the two cells.

Migration • The regular, seasonal movements of animals between breeding and nonbreeding ranges.

Mimicry (mim’ ik ree) • The resemblance of one kind of organism to another, or to some inanimate object; serves the function of making the organism difficult to find, of discouraging potential enemies or of attracting potential prey.

Mineral • An inorganic substance other than water.

Mineralocorticoid • A hormone produced by the adrenal cortex that influences mineral ion balance; aldosterone.

Mismatch repair • When a single base in DNA is changed into a different base, or the wrong base inserted during DNA replication, there is a mismatch in base pairing with the base on the opposite strand. A repair system removes the incorrect base and inserts the proper one for pairing with the opposite strand.

Missense mutation • A nonsynonymous mutation, or one that changes a codon for one amino acid to a codon for a different amino acid. (Contrast with frame-shift mutation, nonsense mutation, synonymous mutation.)

Mitochondrial matrix • The fluid interior of the mitochondrion, enclosed by the inner mitochondrial membrane.

Mitochondrion (my’ toe kon’ dree un) (plural: mitochondria) [Gr. mitos: thread + chondros: cartilage, or grain] • An organelle that occurs in eukaryotic cells and contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation. A mitochondrion is bounded by a double membrane.

Mitosis (my toe’ sis) [Gr. mitos: thread] • Nuclear division in eukaryotes leading to the formation of two daughter nuclei each with a chromosome complement identical to that of the original nucleus.

Mitotic centre • Cellular region that organizes the microtubules for mitosis. In animals a centrosome serves as the mitotic centre.

Moderately repetitive DNA • DNA sequences that appear hundreds to thousands of times in the genome. They include the DNA sequences coding for rRNA5 and tRNAs, as well as the DNA at telomeres.

Modular organism • An organism which grows by producing additional units of body construction (modules) that are very similar to the units of which it is already composed.

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

Molecular clock • The theory that macromolecules diverge from one another over evolutionary time at a constant rate, and that discovering this rate gives insight into the phylogenetic relationships of organisms.

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

Molecule • A particle made up of two or more atoms joined by covalent bonds or ionic attractions.

Molting • The process of shedding part or all of an outer covering, as the shedding of feathers by birds or of the entire exoskeleton by arthropods.

Mono- [Gr. monos: one] • Prefix denoting a single entity. (Contrast with poly.)

Monoclonal antibody • Antibody produced in the laboratory from a clone of hybridoma cells, each of which produces the same specific antibody.

Monocot (short for monocotyledon) [Gr. monos: one + kotyledon: a cup-shaped hollow] • Any member of the angiosperm class Monocotyledones, plants in which the embryo produces but a single cotyledon (seed leaf). Leaves of most monocots have their major veins arranged parallel to each other.

Monocytes • White blood cells that produce macrophages.

Monoecious (mo nee’ shus) [Gr.: one house] • Organisms in which both sexes are “housed in a single individual, which produces both eggs and sperm. (In some plants, these are found in different flowers within the same plant.) Examples: corn, peas, earthworms, hydras. (Contrast with dioecious, perfect flower.)

Monohybrid cross • A mating in which the parents differ with respect to the alleles of only one locus of interest.

Monomer [Gr.: one unit] • A small molecule, two or more of which can be combined to form oligomers (consisting of a few monomers) or polymers (consisting of many monomers).

Monophyletic (mon’ oh fih leht’ ik) [Gk. monos: single + phylon: tribe] • Being descended from a single ancestral stock.

Monosaccharide • A simple sugar. Oligosaccharides and polysaccharides are made up of monosaccharides.

Monosynaptic reflex • A neural reflex that begins in a sensory neuron and makes a single synapse before activating a motor neuron.

Morphogens • Diffusible substances whose concentration gradients determine patterns of development in animals and plants.

Morphogenesis (more’ fo jen’ e sis) IIGr. morphe: form + genesis: origin] The development of form. Morphogenesis is the overall consequence of determination, differentiation, and growth.

Morphology (more fol’ o jee) [Gr. morphe: form + logos: discourse] • The scientific study of organic form, including both its development and function.

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

Motor end plate • The modified area on a muscle cell membrane where a synapse is formed with a motor neuron.

Motor neuron • A neuron carrying information from the central nervous system to an effector such as a muscle fibre.

Motor unit • A motor neuron and the set of muscle fibres it controls. mRNA • (See messenger RNA.)

Mucosa (mew koh’ sah) • An epithelial membrane containing cells that secrete mucus. The inner cell layers of the digestive and respiratory tracts.

Müllerian mimicry • The resemblance of two or more unpleasant or dangerous kinds of organisms to each other.

Multicellular EL. multus: much + cella: chamber] • Consisting of more than one cell, as for example a multicellular organism. (Contrast with unicellular.)

Muscle • Contractile tissue containing actin and myosin organized into polymeric chains called microfilaments. In vertebrates, the tissues are either cardiac muscle, smooth muscle, or striated (skeletal) muscle.

Muscle fibre • A single muscle cell. In the case of striated muscle, a syncitial, multinucleate cell.

Muscle spindle • Modified muscle fibres encased in a connective sheet and functioning as stretch receptors.

Mutagen (mute’ ah jen) [L. mutare: change + Gr. genesis: source] • Any agent (e.g., chemicals, radiation) that increases the mutation rate.

Mutation • An inherited change along a very narrow portion of the nucleic acid sequence.

Mutation pressure • Evolution (change in gene proportions) by different mutation rates alone.

Mutualism • The type of symbiosis, such as that exhibited by fungi and algae or cyanobacteria in forming lichens, in which both species profit from the association.

Mycelium (my seel’ ee yum) [Gr. mykes: fungus] • In the fungi, a mass of hyphae.

Mycorrhiza (my’ ka rye’ za) [Gr. mykes: fungus + rhiza: root] • An association of the root of a plant with the mycelium of a fungus.

Myelin (my’ a un) • A material forming a sheath around some axons. It is formed by Schwann cells that wrap themselves about the axon. It serves to insulate the axon electrically and to increase the rate of transmission of a nervous impulse.

Myofibril (my’ oh fy’ bril) [Gr. mys: muscle + L. fibrilla: small fibre] • A polymeric unit of actin or myosin in a muscle.

Myogenic (my oh jen’ ik) [Gr. mys: muscle + genesis: source] • Originating in muscle.

Myoglobin (my’ oh globe’ in) [Gr. mys: muscle + L. globus: sphere] • An oxygen-binding molecule found in muscle. Consists of a haem unit and a single globin chain, and carries less oxygen than haemoglobin

Myosin [Gr. mys: muscle] • One of the two major proteins of muscle, it makes up the thick filaments. (See actin.)

N

NAD (nicotinamide adenine dinucleotide) • A compound found in all living cells, existing in two interconvertible forms: the oxidizing agent NAD+ and the reducing agent NADH.

NADP (nicotinamide adenine dinucleotide phosphate) • Like NAD, but possessing another phosphate group; plays similar roles but is used by different enzymes.

Natural selection • The differential contribution of offspring to the next generation by various genetic types belonging to the same population. The mechanism of evolution proposed by Charles Darwin.

Necrosis (nec roh’ sis) • Tissue damage resulting from cell death.

Negative control • The situation in which a regulatory macromolecule (generally a repressor) functions to turn off transcription. In the absence of a regulatory macromolecule, the structural genes are turned on.

Nekton [Gr. nekhein: to swim] • Animals, such as fish, that can swim against currents of water. (Contrast with plankton.)

Nematocyst (ne mat’ o sist) [Gr. nema: thread + kystis: cell] • An elaborate, threadlike structure produced by cells of jellyfish and other cnidarians, used chiefly to paralyze and capture prey.

Nephridium (nef rid’ ee urn) [Gr. nephros: kidney] • An organ which is involved in excretion, and often in water balance, involving a tube that opens to the exterior at one end.

Nephron (nef’ ron) [Gr. nephros: kidney] • The basic component of the kidney, which is made up of numerous nephrons. Its form varies in detail, but it always has at one end a device for receiving a fii₁rate of blood, and then a tubule that absorbs selected parts of the filtrate back into the bloodstream.

Nephrostome (nef’ ro stome) [Gr. nephros: kidney + stoma: opening] An opening in a nephridium through which body fluids can enter.

Nerve • A structure consisting of many neuronal axons and connective tissue.

Net primary production • Total photosynthesis  minus respiration by plants.

Neural plate • A thickened strip of ectoderm along the dorsal side of the early vertebrate embryo; gives rise to the central nervous system.

Neural tube • An early stage in the development of the vertebrate nervous system consisting of a hollow tube created by two opposing folds of the dorsal ectoderm along the anterior-posterior body axis.

Neuromuscular junction • The region where a motor neuron contacts a muscle fibre, creating a synapse.

Neuron (floor’ on) [Gr. neuron: nerve, sinew] • A cell derived from embryonic ectoderm and characterized by a membrane potential that can change in response to stimuli, generating action potentials. Action potentials are generated along an extension of the cell (the axon), which makes junctions (synapses) with other neurons, muscle cells, or gland cells.

Neurotransmitter • A substance, produced in and released by one neuron, that diffuses across a synapse and excites or inhibits the postsynaptic neuron.

Neurula (nure’ you Ia) fGr. neuron: nerve] • Embryonic stage during formation of the dorsal nerve cord by two ectodermal ridges.

Neutral allele • An allele that does not alter the functioning of the proteins for which it codes.

Neutral theory • A view of molecular evolution that postulates that most mutations do not affect the amino acid being coded for, and that such mutations accumulate in a population at rates driven by genetic drift and mutation rates.

Neutron (new’ tron) [E.: neutral] • One of the three most fundamental particles of matter, with mass approximately 1 amu and no electrical charge.

Nicotinamide adenine dinucleotide • (See NAD.)

Nicotinamide adenine dinucleotide phosphate • (See NADP.)

Nitrification • The oxidation of ammonia to nitrite and nitrate ions, performed by certain soil bacteria.

Nitrogenase • In nitrogen-fixing organisms, an enzyme complex that mediates the stepwise reduction of atmospheric N₂ to ammonia.

Nitrogen fixation • Conversion of nitrogen gas to ammonia, which makes nitrogen available to living things. Carried out by certain prokaryotes, some of them free-living and others living within plant roots.

Node [L. nodus: knob, knot] • In plants, a (sometimes enlarged) point on a stem where a leaf is or was attached.

Node of Ranvier • A gap in the myelin sheath covering an axons, where the axonal membrane can fire action potentials.

Noncompetitive inhibitor • An inhibitor that binds the enzyme at a site other than the active site. (Contrast with competitive inhibitor.)

Nondisjunction • Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.

Nonpolar molecule • A molecule whose electric charge is evenly balanced from one end of the molecule to the other.

Nonsense (chain-terminating) mutation • Mutations that change a codon for an amino acid to one of the codons (UAG, UAA, or UGA) that signal termination of translation. The resulting gene product is a shortened polypeptide that begins normally at the amino-terminal end and ends at the position of the altered codon. (Contrast with frame-shift mutation, missense mutation, synonymous mutation.)

Nonspecific defenses • Immunologic responses directed against most or all pathogens, generally without reference to the pathogens’ antigens. These defenses include the skin, normal flora, lysozyme, the acidic stomach, interferon, and the inflammatory response.

Nonsynonymous mutation • A nucleotide substitution that that changes the amino acid specified (i.e., AGC /E AGA, or serine /E arginine). (Compare with frame-shift mutation, missense mutation, nonsense mutation.)

Nonsynonymous substitution • The situation when a nonsynonymous mutation becomes widespread in a population. Typically influenced by natural selection. (Contrast with synonymous substitution.)

Nontracheophytes • Those plants lacking well-developed vascular tissue; the liverworts, hornworts, and mosses. (Contrast with tracheophytes.)

Normal flora • The bacteria and fungi that live on animal body surfaces without causing disease.

Norepinephrine • A neurotransmitter found in the central nervous system and also at the postganglionic nerve endings of the sympathetic nervous system. Also called noradrenaline.

Notochord (no’ tow kord) [Gr. notos: back + chorde: string] • A flexible rod of gelatinous material serving as a support in the embryos of all chordates and in the adults of tunicates and lancelets.

Nuclear envelope • The surface, consisting of two layers of membrane, that encloses the nucleus of eukaryotic cells.

Nucleic acid (new klay’ ik) [E.: nucleus of a cell] • A long-chain alternating polymer of deoxyribose or ribose and phosphate groups, with nitrogenous bases— adenine, thymine, uracil, guanine, or cytosine (A, T, U, G, or C)—as side chains. DNA and RNA are nucleic acids.

Nucleoid (new’ klee oid) • The region that harbours the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not bounded by a membrane.

Nucleolar organizer (new klee’ o lar) • A region on a chromosome that is associated with the formation of a new nucleolus following nuclear division. The site of the genes that code for ribosomal RNA.

Nucleolus (new klee’ oh lus) [from L. diminutive of flux: little kernel or little nut] • A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.

Nucleoplasm (new’ klee o plazm) • The fluid material within the nuclear envelope of a cell, as opposed to the chromosomes, nucleoli, and other particulate constituents.

Nucleosome • A portion of a eukaryotic chromosome, consisting of part of the DNA molecule wrapped around a group of histone molecules, and held together by another type of histone molecule. The chromosome is made up of many nucleosomes.

Nucleotide • The basic chemical unit (monomer) in a nucleic acid. A nucleotide in RNA consists of one of four nitrogenous bases linked to ribose, which in turn is linked to phosphate. In DNA, deoxyribose is present instead of ribose.

Nucleus (new’ klee us) [from L. diminutive of flux: kernel or nut] • (1) In chemistry, the dense central portion of an atom, made up of protons and neutrons, with a positive charge. Surrounded by a cloud of negatively charged electrons. (2) In cells, the centrally located chamber of eukaryotic cells that is bounded by a double membrane and contains the chromosomes. The information centre of the cell.

Null hypothesis • The assertion that an effect proposed by its companion hypothesis does not in fact exist.

Nutrient • A food substance; or, in the case of mineral nutrients, an inorganic element required for completion of the life cycle of an organism.

O

Oil • A triglyceride that is liquid at room temperature. (Contrast with fat.)

Okazaki fragments • Newly formed DNA strands making up the lagging strand in DNA replication. DNA Iigase links the Okazaki fragments to give a continuous strand.

Olfactory • Having to do with the sense of smell.

Oligomer [Cr.: a few units] • A compound molecule of intermediate size, made up of two to a few monomers. (Contrast with monomer, polymer.)

Oligosaccharins • Plant hormones, derived from the plant cell wall, that trigger defenses against pathogens.

Ommatidium [Gr. omma: an eye] • One of the units which, collected into groups of up to 20,000, make up the compound eye of arthropods.

Omnivore EL. omnis: all, everything + vorare: to devour] • An organism that eats both animal and plant material. (Contrast with carnivore, detritivore, herbivore.)

Oncogenic (ong’ cc jen’ ik) [Cr. onkos: mass, tumour + genes: born] •Causing cancer.

Oocyte (oh’ eh site) [Cr. oon: egg 4- kytos: cell] • The cell that gives rise to eggs in animals.

Oogenesis (oh’ eh jen e sis) [Cr. oon: egg + genesis: source] • Female gametogenesis, leading to production of the egg.

Oogonium (oh’ eh go’ nee um) • In some algae and fungi, a cell in which an egg is produced.

Operator • The region of an operon that acts as the binding site for the repressor.

Operon • A genetic unit of transcription, typically consisting of several structural genes that are transcribed together; the operon contains at least two control regions: the promoter and the operator.

Opportunity cost • The sum of the benefits an animal forfeits by not being able to perform some other behaviour during the time when it is performing a given behaviour.

Opsin (op’ sin) [Cr. opsis: sight] • The protein protion of the visual pigment rhodopsin. (See rhodopsin.)

Optic chiasm • Stucture on the lower surface of the vertebrate brain where the two optic nerves come together.

Optical isomers • Isomers that differ in the configuration of the four different groups attached to a single carbon atom; so named because solutions of the two isomers rotate the plane of polarized light in opposite directions. The two isomers are mirror images of one another.

Optimality models • Models developed to determine the structures or behaviours that best solve particular problems faced by organisms.

Order • In taxonomy, the category below the class and above the family; a group of related, similar families.

Organ • A body part, such as the heart, liver, brain, root, or leaf, composed of different tissues integrated to perform a distinct function for the body as a whole.

Organ identity genes • Plant genes that specify the various parts of the flower. See homeotic genes.

Organ of Corti • Structure in the inner ear that transforms mechanical forces produced from pressure waves (“sound waves”) into action potentials that are sensed as sound.

Organelles (or’ gan els’) EL.: little organ] • Organized structures that are found in or on cells. Examples: ribosomes, nuclei, mitochrondria, chloroplasts, cilia, and contractile vacuoles.

Organic • Pertaining to any aspect of living matter, e.g., to its evolution, structure, or chemistry. The term is also applied to any chemical compound that contains carbon.

Organism • Any living creature.

Organizer, embryonic • A region of an embryo which directs the development of nearby regions. In amphibian early gastrulas, the dorsal lip of the blastopore.

Origin of replication • A DNA sequence at which helicase unwinds the DNA double helix and DNA polymerase binds to initiate DNA replication.

Osmoregulation • Regulation of the chemical composition of the body fluids of an organism.

Osmoreceptor • A neuron that converts changes in the osmotic potential of interstial fluids into action potentials.

Osmosis (oz mo’ sis) [Gr. osmos: to push] • The movement of water through a differentially permeable membrane from one region to another where the water potential is more negative. This is often a region in which the concentration of dissolved molecules or ions is higher, although the effect of dissolved substances may be offset by hydrostatic pressure in cells with semi-rigid walls.

Ossicle (ah’ sick ul) EL. os: bone] • The calcified construction unit of echinoderm skeletons.

Osteoblasts • Cells that lay down the protein matrix of bone.

Osteoclasts • Cells that dissolve bone.

Otolith (oh’ tuh lith) [Gk.otikos: ear + Iithos: stone[ • Structures in the vertebrate vestibular apparatus that mechanically stimulate hair cells when the head moves or changes position.

Outgroup • A taxon that separated from another taxon, whose lineage is to be inferred, before the latter underwent evolutionary radiation.

Oval window • The flexible membrane which, when moved by the bones of the middle ear, produces pressure waves in the inner ear

Ovary (oh’ var ee) • Any female organ, in plants or animals, that produces an egg.

Oviduct EL. ovum: egg + ducere: to lead] • In mammals, the tube serving to transport eggs to the uterus or to outside of the body.

Oviparous (oh vip’ uh rus) • Reproduction in which eggs are released by the female and development is external to the mother’s body. (Contrast with viviparous.)

Ovulation • The release of an egg from an ovary.

Ovule (oh’ yule) [L. ovulum: little egg] • In plants, an organ that contains a gametophyte and, within the gametophyte, an egg; when it matures, an ovule becomes a seed.

Ovum (oh’ vum) EL.: egg] • The egg, the female sex cell.

Oxidation (ox i day’ shun) • Relative loss of electrons in a chemical reaction; either outright removal to form an ion, or the sharing of electrons with substances having a greater affinity for them, such as oxygen. Most oxidation, including biological ones, are associated with the liberation of energy. (Contrast with reduction.)

Oxidative phosphorylation • ATP formation in the mitochondrion, associated with flow of electrons through the respiratory chain.

Oxidizing agent • A substance that can accept electrons from another. The oxidizing agent becomes reduced; its partner becomes oxidised.

P

P generation • Also called the parental generation. The individuals that mate in a genetic cross. Their immediate offspring are the F₁ generation.

Pacemaker • That part of the heart which undergoes most rapid spontaneous contraction, thus setting the pace for the beat of the entire heart. In mammals, the sinoatrial (SA) node. Also, an artificial device, implanted in the heart, that initiates rhythmic contraction of the organ.

Pacinian corpuscle • A sensory neuron surrounded by sheaths of connective tissue. Found in the deep layers of the skin, where it senses touch and vibration.

Pair rule genes • Segmentation genes that divide the Drosophila larva into two segments each.

Paleomagnetism • The record of the changing direction of Earth’s magnetic field as stored in lava flows. Used to accurately date extremely ancient events

Paleontology (pale’ ee on tol’ oh jee) [Gr. palaios: ancient, old + logos: discourse] • The scientific study of fossils and all aspects of extinct life.

Pancreas (pan’ cree us) • A gland, located near the stomach of vertebrates, that secretes digestive enzymes into the small intestine and releases insulin into the bloodstream.

Pangaea (pan jee’ uh) [Gk. pan: all, every) • The single land mass formed when all the continents came together in the Permian period. (Contrast with Gondwana.)

Parabronchi • Passages in the lungs of birds through which air flows.

Paradigm • A general framework within which a scientific or philosophical discipline is viewed and within which questions are asked and hypotheses are developed. Scientific revolutions usually involve major paradigm changes. (Contrast with hypothesis, theory.)

Parallel evolution • Evolutionary patterns that exist in more than one lineage. Often the result of underlying developmental processes.

Parapatric speciation [Gr. pare: beside + patria: fatherland] • Development of reproductive isolation when the barrier is not geographic but is a difference in some other physical condition (such as soil nutrient content) that prevents gene flow between the subpopulations. (Contrast with allopatric speciation, sympatric speciation.)

Paraphyletic taxon • A taxon that includes some, but not all, of the descendants of a single ancestor.

Parasite • An organism that attacks and consumes parts of an organism much larger than itself. Parasites sometimes, but not always, kill the host.

Parasitoid • A parasite that is so large relative to its host that only one individual or at most a few individuals can live within a single host.

Parasympathetic nervous system • A portion of the autonomic (involuntary) nervous system. Activity in the parasympathetic nervous system produces effects such as decreased blood pressure and decelerated heart beat. (Contrast with sympathetic nervous system.)

Parathormone • Hormone secreted by the parathyroid glands. Stimulates osteoclast activity and raises blood calcium levels.

Parathyroids • Four glands on the posterior surface of the thyroid that produce and release parathormone.

Parenchyma (pair eng’ kyma) [Gr. para: beside + enchyma: infusion] • A plant tissue composed of relatively unspecialised cells without secondary walls

Parental investment • Investment in one offspring or group of offspring that reduces the ability of the parent to assist other offspring.

Parsimony • The principle of preferring the simplest among a set of plausible explanations of a phenomenon. Commonly employed in evolutionary and biogeographic studies.

Parthenocarpy • Formation of fruit from a flower without fertilization.

Parthenogenesis (par’ then oh jen’ e sis) [Cr. parthenos: virgin ± genesis: source] • The production of an organism from an unfertilized egg.

Partial pressure • The portion of the barometric pressure of a mixture of gases that is due to one component of that mixture. For example, the partial pressure of oxygen at sea level is 20.9% of barometric pressure.

Patch clamping • A technique for isolating a tiny patch of membrane to allow the study of ion movement through a particular channel.

Pathogen (path’ o jen) [Cr. pathos: suffering + gignon’iai: causing] • An organism that causes disease.

Pattern formation • In animal embryonic development, the organization of differentiated tissues into specific structures such as wings.

Pedigree • The pattern of transmission of a genetic trait in a family.

Pelagic zone (puh ladj’ ik) [Cr. pelagos: the sea] • The open waters of the ocean.

Penetrance • Of a genotype, the proportion of individuals with that genotype who show the expected phenotype.

PEP carboxylase • The enzyme that combines carbon dioxide with PEP to form a 4-carbon dicarboxylic acid at the start of C₄ photosynthesis or of Crassulacean acid metabolism ( CAM ).

Pepsin [Cr. pepsis: digestion] • An enzyme, in gastric juice, that digests protein.

Peptide linkage • The connecting group in a protein chain, -CO-NH-, formed by removal of water during the linking of amino acids, -COOH to -NH₂.

Peptidoglycan • The cell wall material of many prokaryotes, consisting of a single enormous molecule that surrounds the entire cell.

Perennial (per ren’ ee al) [L. per: through + annus: a year] • Referring to a plant that lives from year to year. (Contrast with annual, biennial.)

Perfect flower • A flower with both stamens and carpels, therefore hermaphroditic.

Pericycle [Cr. pen: around + kyklos: ring or circle] • In plant roots, tissue just within the endodermis, but outside of the root vascular tissue. Meristematic activity of pericycle cells produces lateral root primordia.

Periderm • The outer tissue of the secondary plant body, consisting primarily of cork..

Period • (1) A minor category in the geological time scale. (2) The duration of a cyclical event, such as a circadian rhythm.

Peripheral nervous system • Neurons that transmit information to and from the central nervous system and whose cell bodies reside outside the brain or spinal cord.

Peristalsis (pair’ i stall’ sis) [Gr. pen: around + ste//em: place] • Wavelike muscular contractions proceeding along a tubular organ, propelling the contents along the tube.

Peritoneum • The mesodermal lining of the coelom among coelomate animals.

Permease • A membrane protein that specifically transports a compound or family of compounds across the membrane.

Peroxisome • An organelle that houses reactions in which toxic peroxides are formed. The peroxisome isolates these peroxides from the rest of the cell.

Petal • In an angiosperm flower, a sterile modified leaf, nonphotosynthetic, frequently brightly colored, and often serving to attract pollinating insects.

Petiole (pet’ ee ole) EL. petio/us: small foot] • The stalk of a leaf.

pH • The negative logarithm of the hydrogen ion concentration; a measure of the acidity of a solution. A solution with pH = 7 is said to be neutral; pH values higher than 7 characterize basic solutions, while acidic solutions have pH values less than 7.

Phage (fayj) • Short for bacteriophage.

Phagocyte • A white blood cell that ingests microorganisms by endocytosis.

Phagocytosis [Gr.: phagein to eat; cell-eating] • A form of endocytosis, the uptake of a solid particle by forming a pocket of plasma membrane around the particle and pinching off the pocket to form an intracellular particle bounded by membrane. (Contrast with pinocytosis.)

Pharynx [Gr.: throat] • The part of the gut between the mouth and the esophagus.

Phenotype (fee’ no type) [Gr. phanein: to show] • The observable properties of an individual as they have developed under the combined influences of the genetic constitution of the individual and the effects of environmental factors. (Contrast with genotype.)

Phenotypic plasticity • The fact that the phenotype of an organism is determined by a complex series of developmental processes that are affected by both its genotype and its environment.

Pheromone (feer’ o mone) [Gr. phero: carry + hormon: excite, arouse] • A chemical substance used in communication between organisms of the same species.

Phloem (flo’ urn) [Gr. phloos: bark] • In vascular plants, the food-conducting tissue. It consists of sieve cells or sieve tubes, fibres, and other specialised cells.

Phosphate group • The functional group -0P0₃H₂; the transfer of energy from one compound to another is often accomplished by the transfer of a phosphate group.

Phosphodiester linkage • The connection in a nucleic acid strand, formed by linking two nucleotides.

Phospholipids • Cellular materials that contain phosphorus and are soluble in organic solvents. An example is lecithin (phosphatidyl choline). Phospholipids are important constituents of cellular membranes. (See lipids.)

Phosphorylation • The addition of a phosphate group.

Photoautotroph • An organism that obtains energy from light and carbon from carbon dioxide. (Contrast with chernoautotroph, chemoheterotroph, photoheterotroph)

Photoheterotroph • An organism that obtains energy from light but must obtain its carbon from organic compounds. (Contrast with chemoautotroph, chemoheterotroph, photoautotroph.)

Photon (foe’ tohn) [Gr. photos: light] • A quantum of visible radiation; a packet’ of light energy.

Photoperiod (foe’ tow peer’ ee ud) . The duration of a period of light, such as the length of time in a 24-hour cycle in which daylight is present. The regulation of processes such as flowering by the changing length of day (or of night) is known as photoperiodism.

Photoreceptor • (1) A protein (pigment) that triggers a physiological response when it absorbs a photon. (2) A cell that senses and responds to light energy.

Photorespiration • Light-driven uptake of oxygen and release of carbon dioxide, the carbon being derived from the early reactions of photosynthesis.

Photosynthesis (foe tow sin’ the sis) [literally, “synthesis out of light”] Metabolic processes, carried out by green plants, by which visible light is trapped and the energy used to synthesize compounds such as ATP and glucose.

Phototropin • A yellow protein that is the photoreceptor responsible for phototropism.

Phototropism [Gr. photos: light + trope: a turning] • A directed plant growth response to light.

Phylogenetic tree • Graphic representation of lines of descent among organisms.

Phylogeny (fy loj’ e nee) [Gr. phylon: tribe, race + genesis: source] • The evolutionary history of a particular group of organisms; also, the diagram of the “family tree” that shows genetic linkages between ancestors and descendants.

Phylum (plural: phyla) [Cr. phy/on: tribe, stock] • In taxonomy, a high-level category just beneath kingdom and above the class; a group of related, similar classes.

Physiology (fiz’ ee ol’ o jee) [Cr. physis: natural form ± logos: discourse, study] • The scientific study of the functions of living organisms and the individual organs, tissues, and cells of which they are composed.

Phytoalexins • Substances toxic to fungi, produced by plants in response to fungal infection.

Phytochrome (fy’ tow krome) [Cr. phyton: plant ± chroma: color] • A plant pigment regulating a large number of developmental and other phenomena in plants; can exist in two different forms, one of which is active and the other is not. Different wavelengths of light can drive it from one form to the other.

Phytoplankton (fy’ tow plangk’ ton) [Cr. p/iyton: plant + p/ar,ktos: wandering] The autotrophic portion of the plankton, consisting mostly of algae.

Pigment • A substance that absorbs visible light.

Pilus (pill’ us) [Lat. p//us: hair] • A surface appendage by which some bacteria adhere to one another during conjugation.

Pinocytosis [Cr.: drinking cell] • A form of endocytosis; the uptake of liquids by engulfing a sample of the external medium into a pocket of the plasma membrane followed by pinching off the pocket to form an intracellular vesicle. (Contrast with phagocytosis and endocytosis.)

Pistil EL. pistil/urn: pestle] • The female structure of an angiosperm flower, within which the ovules are borne. May consist of a single carpel, or of several carpels fused into a single structure. Usually differentiated into ovary, style, and stigma.

Pith • In plants, relatively unspecialised tissue found within a cylinder of vascular tissue.

Pituitary • A small gland attached to the base of the brain invertebrates. Its hormones control the activities of other glands. Also known as the hypophysis.

Placenta (pla sen’ ta) [Cr. p/ax: flat surface] • The organ found in most mammals that provides for the nourishment of the foetus and elimination of the fetal waste products.

Placental (pla sen’ tal) • Pertaining to mammals of the subclass Eutheria, a group characterized by the presence of a placenta; contains the majority of living species of mammals.

Plankton [Cr. p/anktos: wandering] • The free-floating organisms of the sea and fresh water that for the most part move passively with the water currents. Consisting mostly of microorganisms and small plants and animals. (Contrast with nekton.)

Plant • A member of the kingdom Plantae. Multicellular, gaining its nutrition by photosynthesis.

Planula (plan’ yew Ia) [L. planum: something flat] • The free-swimming, ciliated larva of the cnidarians.

Plaque (plack) [Fr.: a metal plate or coin] • (1) A circular clearing in a turbid layer (lawn) of bacteria growing on the surface of a nutrient agar gel. Produced by successive rounds of infection initiated by a single bacteriophage. (2) An accumulation of prokaryotic organisms on tooth enamel. Acids produced by the metabolism of these microorganisms can cause tooth decay.

Plasma (plaz’ muh) [Gr. plassein: to mold] • The liquid portion of blood, in which blood cells and other particulates are suspended.

Plasma cell • An antibody-secreting cell that developed from a B cell. The effector cell of the humoral immune system

Plasma membrane • The membrane that surrounds the cell, regulating the entry and exit of molecules and ions. Every cell has a plasma membrane

Plasmid • A DNA molecule distinct from the chromosome(s); that is, an extra chromosomal element. May replicate independently of the chromosome.

Plasmodesma (plural: plasmodesmata) [Gr. plasma: formed or molded + desmos: band] • A cytoplasmic strand connecting two adjacent plant cells.

Plasmolysis (plaz mol’ I sis) • Shrinking of the cytoplasm and plasma membrane away from the cell wall, resulting from the osmotic outflow of water. Occurs only in cells with rigid cell walls.

Plastid • Organelle in plants that serves for food manufacture (by photosynthesis) or food storage; bounded by a double membrane.

Platelet • A membrane-bounded body without a nucleus, arising as a fragment of a cell in the bone marrow of mammals. Important to blood-clotting action.

Pleiotropy (plee’ a tro pee) [Gr. pleioc: more] • The determination of more than one character by a single gene.

Pleural membrane [Gk. pleuras: rib, side] • The membrane lining the outside of the lungs and the walls of the thoracic cavity. Inflammation of these membranes is a condition known as pleurisy.

Podocytes • Cells of Bowman’s capsule of the nephron that cover the capillaries of the glomerulus, forming filtration slits.

Poikilotherm (poy’ kill o therm) [Gr. poikilos: varied + therme: heat] • An animal whose body temperature tends to vary with the surrounding environment. (Contrast with homeotherm, heterotherm.)

Point mutation • A mutation that results from a small, localized alteration in the chemical structure of a gene. Such mutations can give rise to wild-type revertants as a result of reverse mutation. In genetic crosses, a point mutation behaves as if it resided at a single point on the genetic map. (Contrast with deletion.)

Planula (plan’ yew Ia) [L. planum: something flat] • The free-swimming, ciliated larva of the cnidarians.

Plaque (plack) [Fr.: a metal plate or coin] • (1) A circular clearing in a turbid layer (lawn) of bacteria growing on the surface of a nutrient agar gel. Produced by successive rounds of infection initiated by a single bacteriophage. (2) An accumulation of prokaryotic organisms on tooth enamel. Acids produced by the metabolism of these microorganisms can cause tooth decay.

Plasma (plaz’ muh) [Gr. plassein: to mold] • The liquid portion of blood, in which blood cells and other particulates are suspended.

Plasma cell • An antibody-secreting cell that developed from a B cell. The effector cell of the humoral immune system

Plasma membrane • The membrane that surrounds the cell, regulating the entry and exit of molecules and ions, Every cell has a plasma membrane

Plasmid • A DNA molecule distinct from the chromosome(s); that is, an extrachromosomal element. May replicate independently of the chromosome.

Plasmodesma (plural: plasmodesmata) [Gr. plasma: formed or molded + desmos: band] • A cytoplasmic strand connecting two adjacent plant cells.

Plasmolysis (plaz moE’ i sis) • Shrinking of the cytoplasm and plasma membrane away from the cell wall, resulting from the osmotic outflow of water. Occurs only in cells with rigid cell walls.

Plastid • Organelle in plants that serves for food manufacture (by photosynthesis) or food storage; bounded by a double membrane.

Platelet • A membrane-bounded body without a nucleus, arising as a fragment of a cell in the bone marrow of mammals. Important to blood-clotting action.

Pleiotropy (piee’ a tro pee) [Gr. pie/on: more] • The determination of more than one character by a single gene.

Pleural membrane [Gk. pieuras: rib, side] • The membrane lining the outside of the lungs and the walls of the thoracic cavity. Inflammation of these membranes is a condition known as pleurisy.

Podocytes • Cells of Bowman’s capsule of the nephron that cover the capillaries of the glomerulus, forming filtration slits.

Poikilotherm (poy’ kill o therm) [Gr. polk/los: varied + therme: heat] • An animal whose body temperature tends to vary with the surrounding environment. (Contrast with homeotherm, heterotherm.)

Point mutation • A mutation that results from a small, localized alteration in the chemical structure of a gene. Such mutations can give rise to wild-type revertants as a result of reverse mutation. In genetic crosses, a point mutation behaves as if it resided at a single point on the genetic map. (Contrast with deletion.)

Polar body • A nonfunctional nucleus produced by meiosis, accompanied by very little cytoplasm. The meiosis which produces the mammalian egg produces in addition three polar bodies.

Polar molecule • A molecule in which the electric charge is not distributed evenly in the covalent bonds

Polarity • In development, the difference between one end and the other. In chemistry, the property that makes a polar molecule.

Pollen [L.: fine powder, dust] • The fertilizing element of seed plants, containing the male gametophyte and the gamete, at the stage in which it is shed.

Pollination • Process of transferring pollen from the anther to the receptive surface (stigma) of the ovary in plants.

Poly- [Gr. poly: many] • A prefix denoting multiple entities.

Polygamy [Gr. pci>’: many + games: marriage] • A breeding system in which an individual acquires more than one mate. In polyandry, a female mates with more than one male, in polygyny, a male mates with more than one female.

Polygenes • Multiple loci whose alleles increase or decrease a continuously variable phenotypic trait.

Polymer • A large molecule made up of similar or identical subunits called monomers. (Contrast with monomer, oligomer.)

Polymerase chain reaction (PCR) • A technique for the rapid production of millions of copies of a particular stretch of DNA.

Polymerization reactions • Chemical reactions that generate polymers by means of condensation reactions.

Polymorphism (p01’ lee mor’ fiz um) [Gr. poly: many + mcrphe: form, shape]

(1)  In genetics, the coexistence in the same population of two distinct hereditary types based on different alleles. (2) In social organisms such as colonial cnidarians and social insects, the coexistence of two or more functionally different castes within the same colony.

Polyp • The sessile, asexual stage in the life cycle of most cnidarians.

Polypeptide • A large molecule made up of many amino acids joined by peptide linkages. Large polypeptides are called proteins.

Polyphyletic group • A group containing taxa, not all of which share the most recent common ancestor.

Polyploid (p01’ lee ploid) • A cell or an organism in which the number of complete sets of chromosomes is greater than two.

Polysaccharide • A macromolecule composed of many monosaccharides (simple sugars). Common examples are cellulose and starch.

Polysome • A complex consisting of a threadlike molecule of messenger RNA and several (or many) ribosomes. The ribosomes move along the mRNA, synthesizing polypeptide chains as they proceed.

Polytene (pol’ lee teen) [Gr. poly: many ± taenia: ribbon] • An adjective describing giant interphase chromosomes, such as those found in the salivary glands of fly larvae. The characteristic, reproducible pattern of bands and bulges seen on these chromosomes has provided a method for preparing detailed chromosome maps of several organisms.

Pans EL. pans: bridge] • Region of the b am stem anterior to the medulla.

Population • Any group of organisms coexisting at the same time and in the same place and capable of interbreeding with one another.

Population density • The number of individuals (or modules) of a population in a unit of area or volume.

Population genetics • The study of genetic variation and its causes within populations.

Population structure • The proportions of individuals in a population belonging to different age classes (age structure). Also, the distribution of the population in space.

Portal vein • A vein connecting two capillary beds, as in the hepatic portal system.

Positive control • The situation in which a regulatory macromolecule is needed to turn transcription of structural genes on. In its absence, transcription will not occur.

Positive cooperativity • Occurs when a molecule can bind several ligands and each one that binds alters the conformation of the molecule so that it can bind the next ligand more easily. The binding of four molecules of 02 by hemoglobin is an example of positive cooperativity.

Postabsorptive period • When there is no food in the gut and no nutrients are being absorbed.

Postsynaptic cell • The cell whose membranes receive the neurotransmitter released at a synapse.

Predator • An organism that kills and eats other organisms. Predation is usually thought of as involving the consumption of animals by animals, but it can also mean the eating of plants.

Presynaptic excitation/inhibition • Occurs when a neuron modifies activity at a synapse by releasing a neurotransmitter onto the presynaptic nerve terminal.

Prey [L. praeda: booty] • An organism consumed as an energy source.

Primary active transport • Form of active transport in which ATP is hydrolyzed, yielding the energy required to transport ions against their concentration gradients. (Contrast with secondary active transport.)

Primary growth • In plants, growth produced by the apical meristems. (Contrast with secondary growth.)

Primary producer • A photosynthetic or chemosynthetic organism that synthesizes complex organic molecules from simple inorganic ones.

Primary succession • Succession that begins in an areas initially devoid of life, such as on recently exposed glacial till or lava flows.

Primary structure • The specific sequence of amino acids in a protein.

Primary wall •Cellulose-rich cell wall layers laid down by a growing plant cell.

Primate (pry’ mate) • A member of the order Primates, such as a lemur, monkey, ape, or human.

Primer • A short, single-stranded segment of DNA serving as the necessary starting material for the synthesis of a new DNA strand, which is synthesized from the 3’ end of the primer.

Primitive streak • A line running axially along the blastodisc, the site of inward cell migration during formation of the three-layered embryo. Formed in the embryos of birds and fish.

Primordium [L. primordium: origin] • The most rudimentary stage of an organ or other part.

Principle of continuity • States that because life probably evolved from nonlife by a continuous, gradual process, all postulated stages in the evolution of life should be derivable from preexisting states. (Compare with signature principle.)

Pro- [L.: first, before, favoring] • A prefix often used in biology to denote a developmental stage that comes first or an evolutionary form that appeared earlier than another. For example, prokaryote, prophase.

Probe • A segment of single stranded nucleic acid used to identify DNA molecules containing the complementary sequence.

Procambium • Primary meristem that produces the vascular tissue.

Progesterone [L. pro: favoring + gestate: to bear] • A vertebrate female sex hormone that maintains pregnancy.

Prokaryotes (pro kar’ ry otes) EL. pro: before ± Gk. karyon: kernel, nucleus] Organisms whose genetic material is not contained within a nucleus. The bacteria. Considered an earlier stage in the evolution of life than the eukaryotes.

Prometaphase • The phase of nuclear division that begins with the disintegration of the nuclear envelope.

Promoter • The region of an operon that acts as the initial binding site for RNA polymerase.

Proofreading • The correction of an error in DNA replication just after an incorrectly paired base is added to the growing polynucleotide chain.

Prophage (pro’ fayj) • The noninfectious units that are linked with the chromosomes of the host bacteria and multiply with them but do not cause dissolution of the cell. Prophage can later enter into the lytic phase to complete the virus life cycle.

Prophase (pro’ phase) • The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies.

Prostaglandin • Any one of a group of specialised lipids with hormone-like functions. It is not clear that they act at any considerable distance from the site of their production.

Prosthetic group • Any nonprotein portion of an enzyme.

Protease (pro’ tee ase) • See proteolytic enzyme.

Protein (pro’ teen) [Gr. protos: first] • One of the most fundamental building substances of living organisms. A long-chain polymer of amino acids with twenty different common side chains. Occurs with its polymer chain extended in fibrous proteins, or coiled into a compact macromotecule in enzymes and other globular proteins.

Proteolytic enzyme • An enzyme whose main catalytic function is the digestion of a protein or polypeptide chain. The digestive enzymes trypsin, pepsin, and carboxypeptidase are all proteolytic enzymes (proteases).

Protist • Those eukaryotes not included in the kingdoms Animalia, Fungi, or Plantae.

Protobiont • Aggregates of abiotically produced molecules that cannot reproduce but do maintain internal chemical environments that differ from their surroundings.

Protoderm • Primary meristem that gives rise to epidermis.

Proton (pro’ ton) [Gr. protos: first] • One of the three most fundamental particles of matter, with mass approximately 1 amu and an electrical charge of +1.

Proto-oncogenes . The normal alleles of genes possessing oncogenes (cancer-causing genes) as mutant alleles. Proto-oncogenes encode growth factors and receptor proteins.

Protostome • One of the major lineages of animal evolution. Characterized by spiral, determinate cleavage of the egg, and by schizocoelous development. (Compare with deuterostome.)

Prototroph (pro’ tow trofe’) [Gr. protos: first + trophein: to nourish] • The nutritional wild type, or reference form, of an organism. Any deviant form that requires growth nutrients not required by the prototrophic form is said to be a nutritional mutant, or auxotroph.

Protozoa • A group of single-celled organisms classified by some biologists as a single phylum; includes the flagellates, amoebas, and ciliates. This textbook follows most modern classifications in elevating the protozoans to a distinct kingdom (Protista) and each of their major subgroups to the rank of phylum.

Proximal • Near the point of attachment or other reference point. (Contrast with distal.)

Pseudocoelom • A body cavity not surrounded by a peritoneum. Characteristic of nematodes and rotifers.

Pseudogene • A DNA segment that is homologous to a functional gene but contains a nucleotide change that prevents its expression

Pseudoplasmodium [Gr. pseudes: false + plasma: mold or form] • In the cellular slime molds such as Dictyostelium, an aggregation of single amoeboid cells. Occurs prior to formation of a fruiting structure.

Pseudopod (S00’ do pod) [Gr. pseudes: false + podos: foot] • A temporary, soft extension of the cell body that is used in location, attachment to surfaces, or engulfing particles.

Pulmonary • Pertaining to the lungs.

Punctuated equiilibrium • An evolutionary pattern in which periods of rapid change are separated by longer periods of little or no change.

Pupa (pew’ pa) [L.: doll, puppet] • In certain insects (the Holometabola), the encased developmental stage that intervenes between the larva and the adult.

Pupil • The opening in the vertebrate eye through which light passes.

Purine (pure’ een) • A type of nitrogenous base. The purines adenine and guanine are found in nucleic acids.

Purkinje fibres • Specialised heart muscle cells that conduct excitation throughout the ventricular muscle.

Pyramid of biomass • Graphical representation of the total body masses at different trophic levels in an ecosystem.

Pyramid of energy • Graphical representation of the total energy contents at different trophic levels in an ecosystem.

Pyrimidine (peer im’ a deen) • A type of nitrogenous base. The pyrimidines cytosine, thymine, and uracil are found in nucleic acids.

Pyruvate • A three-carbon acid; the end product of glycolysis and the raw material for the citric acid cycle.

Q

Quaternary structure • Of aggregating proteins, the arrangement of pofypeptide subunits.

R

R factor (resistance factor) • A plasmid that contains one or more genes that encode resistance to antibiotics.

Radial symmetry • The condition in which two halves of a body are mirror images of each other regardless of the angle of the cut, providing the cut is made along the centre line. Thus, a cylinder cut lengthwise down its centre displays this form of symmetry. (Contrast with biradial symmetry.)

Radioisotope • A radioactive isotope of an element. Examples are carbon-14 (‘t) and hydrogen-3, or tritium (³H).

Radiometry • The use of the regular, known rates of decay of radioisotopes of elements to determine dates of events in the distant past.

Rain shadow • A region of low precipitation on the leeward side of a mountain range.

Random genetic drift • Evolution (change in gene proportions) by chance processes alone.

Rate constant • Of a particular chemical reaction, a constant which, when multiplied by the concentration(s) of reactant(s), gives the rate of the reaction.

Reactant • A chemical substance that enters into a chemical reaction with another substance.

Reaction, chemical • A process in which atoms combine or change bonding partners.

Realised niche • The actual niche occupied by an organism; it differs from the fundamental niche because of the presence of other species.

Receptive field • Of a neuron, the area on the retina from which the activity of that neuron can be influenced.

Receptor potential • The change in the resting potential of a sensory cell when it is stimulated.

Recessive • See dominance.

Reciprocal altruism • The exchange of altruistic acts between two or more individuals. The acts may be separated considerably in time.

Reciprocal crosses • A pair of crosses, in one of which a female of genotype A mates with a male of genotype B and in the other of which a female of genotype B mates with a male of genotype A.

Recognition site (also called a restriction site) • A sequence of nucleotides in DNA to which a restriction enzyme binds and then cuts the DNA.

Recombinant • An individual, meiotic product, or single chromosome in which genetic materials originally present in two individuals end up in the same haploid complement of genes. The reshuffling of genes can be either by independent segragation, or by crossing over between homologous chromosomes. For example, a human may pass on genes from both parents in a single haploid gamete.

Recombinant DNA technology . The application of genetic tools (restriction endonucleases, plasmids, and transformation) to the production of specific proteins by biological “factories^t’ such as bacteria.

Rectum • The terminal portion of the gut, ending at the anus.

Redox reaction • A chemical reaction in which one reactant becomes oxidised

and the other becomes reduced.

Reducing agent • A substance that can donate electrons to another substance.

The reducing agent becomes oxidised, and its partner becomes reduced.

Reduction (re duk’ shun) • Gain of electrons; the reverse of oxidation. Most reductions lead to the storage of chemical energy, which can be released later by an oxidation reaction. Energy storage compounds such as sugars and fats are highly reduced compounds. (Contrast with oxidation.)

Reflex • An automatic action, involving only a few neurons (in vertebrates, often in the spinal cord), in which a motor response swiftly follows a sensory stimulus.

Refractory period • Of a neuron, the time interval after an action potential, during which another action potential cannot be elicited.

Regulative development • A pattern of animal embryonic development in which the fates of the first blastomeres are not absolutely fixed. (Contrast with mosaic development.)

Regulatory gene • A gene that contains the information for making a regulatory macromolecule, often a repressor protein.

Releaser • A sensory stimulus that triggers a fixed action pattern.

Releasing hormone • One of several hypothalamic hormones that stimulates the secretion of anterior pituitary hormone.

REM sleep • A sleep state characterized by dreaming, skeletal muscle relaxation, and rapid eye movements.

Renal [L. renes: kidneys] • Relating to the kidneys.

Replication fork • A point at which a DNA molecule is replicating. The fork forms by the unwinding of the parent molecule.

Repressible enzyme • An enzyme whose synthesis can be decreased or prevented by the presence of a particular compound. A repressible opren often controls the syhthesis of such an enzyme.

Repressor • A protein coded by the regulatory gene. The repressor can bind to a specific operator and prevent transcription of the operon.

Reproductive isolating mechanism • Any trait that prevents individuals from two different populations from producing fertile hybrids.

Reproductive isolation • The condition in which a population is not exchanging genes with other populations of the same species.

Resolving power • Of an optical device such as a microscope, the smallest distance between two lines that allows the lines to be seen as separate from one another.

Resource • Something in the environment required by an organism for its maintenance and growth that is consumed in the process of being used.

Resource defense polygamy • A breeding system in which individuals of one sex (usually males) defend resources that are attractive to individuals of the other sex (usually females); individuals holding better resources attract more mates.

Respiration (res pi ra’ shun) EL. spirare: to breathe] • (1) Cellular respiration; the oxidation of the end products of glycolysis with the storage of much energy in ATP. The oxidant in the respiration of eukaryotes is oxygen gas. Some bacteria can use nitrate or sulfate instead of 02. (2) Breathing.

Respiratory chain • The terminal reactions of cellular respiration, in which electrons are passed from NAD or FAD, through a series of intermediate carriers, to molecular oxygen, with the concomitant production of ATP.

Resting potential • The membrane potential of a living cell at rest. In cells at rest, the interior is negative to the exterior. (Contrast with action potential, electrotonic potential.)

Restoration ecology • The science and practice of restoring damaged or degraded ecosystems.

Restriction endonuclease • Any one of several enzymes, produced by bacteria, that break foreign DNA molecules at very specific sites. Some produce “sticky ends.” Extensively used in recombinant DNA technology.

Restriction map • A partial genetic map of a DNA molecule, showing the points at which particular restriction endonuclease recognition sites reside.

Reticular system • A central region of the vertebrate brain stem that includes complex fibre tracts conveying neural signals between the forebrain and the spinal cord, with collateral fibres to a variety of nuclei that are involved in autonomic functions, including arousal from sleep.

Retina (rett’ in uh) [L. rete: net] • The light-sensitive layer of cells in the vertebrate or cephalopod eye.

Retinal • The light-absorbing portion of visual pigment molecules. Derived from b-carotene.

Retrovirus • An RNA virus that contains reverse transcriptase. Its RNA serves as a template for cDNA production, and the cDNA is integrated into a chromosome of the mammalian host cell.

Reverse transcriptase • An enzyme that catalyses the production of DNA (cDNA), using RNA as a template; essential to the reproduction of retroviruses.

RFLP (Restriction fragment length polymorphism) • Coexistence of two or more patterns of restriction fragments (patterns produced by restriction enzymes), as revealed by a probe. The polymorphism reflects a difference in DNA sequence on homologous chromosomes.

Rhizoids (rye’ zoids) [Gr. rhiza: root] • Hairlike extensions of cells in mosses, liverworts, and a few vascular plants that serve the same function as roots and root hairs in vascular plants. The term is also applied to branched, rootlike extensions of some fungi and algae.

Rhizome (rye’ zome) [Gr. rhizome: mass of roots] • A special underground stem (as opposed to root) that runs horizontally beneath the ground.

Rhodopsin • A photopigment used in the visual process of transducing photons of light into changes in the membrane potential of photoreceptor cells.

Ribonucleic acid • See RNA.

Ribosomal RNA (rRNA) • Several species of RNA that are incorporated into the ribosome. Involved in peptide bond formation.

Ribosome • A small organelle that is the site of protein synthesis.

Ribozyme • An RNA molecule with catalytic activity.

Ribulose 1,5-bisphosphate (RuBP) • The compound in chloroplasts which reacts with carbon dioxide in the first reaction of the Calvin-Benson cycle.

Risk cost • The increased chance of being injured or killed as a result of performing a behaviour, compared to resting.

RNA (ribonucleic acid) • A nucleic acid using ribose. Various classes of RNA are involved in the transcription and translation of genetic information. RNA serves as the genetic storage material in some viruses.

RNA polymerase • An enzyme that catalyses the formation of RNA from a DNA template.

RNA splicing • The last stage of RNA processing in eukaryotes, in which the transcripts of introns are excised through the action of small nuclear ribonucleoprotein particles (5nRNP).

Rods • Light-sensitive cells (photoreceptors) in the retina. (Contrast with cones.)

Root cap • A thimble-shaped mass of cells, produced by the root apical meristem, that protects the meristem and that is the organ that perceives the gravitational stimulus in root gravitropism.

Root hair • A specialised epidermal cell with a long, thin process that absorbs water and minerals from the soil solution.

rRNA • See ribosomal RNA.

Rubisco (RuBP carboxylase) • Enzyme that combines carbon dioxide with ribulose bisphosphate to produce 3-phosphoglycerate, the first product of C₃ photosynthesis. The most abundant protein on Earth.

Rumen (rew’ mun) • The first division of the ruminant stomach. It stores and initiates bacterial fermentation of food. Food is regurgitated from the rumen for further chewing.

S

S phase • In the cell cycle, the stage of interphase during which DNA is replicated. (Contrast with G₁ phase, G₂ phase.)

Saprobe [Gr. sapros: rotten + b/os: life] • An organism (usually a bacterium or fungus) that obtains its carbon and energy directly from dead organic matter.

Sarcomere (sark’ o meer) [Gr. sark: flesh + meros: a part] • The contractile unit of a skeletal muscle.

Saturated hydrocarbon • A compound consisting only of carbon and hydrogen, with the hydrogen atoms connected by single bonds.

Schizocoelous development • Formation of a coelom during embryological development by a splitting of mesodermal masses.

Schwann cell • A glial cell that wraps around part of the axon of a peripheral neuron, creating a myelin sheath.

Sclereid [Gr. sk/eros: hard] • A type of sclerenchyma cell, commonly found in nutshells, that is not elongated.

Sclerenchyma (skier eng’ kyma) [Gr. skleros: hard + kym us, juice] • A plant tissue composed of cells with heavily thickened cell walls, dead at functional maturity. The principal types of sclerenchyma cells are fibres and sclereids.

Secondary active transport • Form of active transport in which ions or molecules are transported against their concentration gradient using energy obtained by relaxation of a gradient of sodium ion concentration rather than directly from AlP. (Contrast with primary active transport.)

Secondary compound • A compound synthesized by a plant that is not needed for basic cellular metabolism. Typically has an antiherbivore or antiparasite function.

Secondary growth • In plants, growth produced by vascular and cork cambia, contributing to an increase in girth. (Contrast with primary growth.)

Secondary structure • Of a protein, localized regularities of structure, such as the

a helix and the b pleated sheet.

Secondary succession • Ecological succession after a disturbance that does not eliminate all the organisms that originally lived on the site

Secondary wall • Wall layers laid down by a plant cell that has ceased growing; often impregnated with lignin or suberin.

Second law of thermodynamics • States that in any real (irreversible) process, there is a decrease in free energy and an increase in entropy.

Second messenger • A compound, such as cyclic AMP, that is released within a target cell after a hormone or other “first messenger” has bound to a surface receptor on a cell; the second messenger triggers further reactions within the cell.

Secretin (si kreet’ in) • A peptide hormone secreted by the upper region of the small intestine when acidic chyme is present. Stimulates the pancreatic dud to secrete bicarbonate ions.

Section • A thin slice, usually for microscopy, as a tangential section or a transverse section.

Seed • A fertilized, ripened ovule of a gymnosperm or angiosperm. Consists of the embryo, nutritive tissue, and a seed coat.

Seed crop • The number of seeds produced by a plant during a particular bout of reproduction.

Seedling • A young plant that has grown from a seed (rather than by grafting or by other means.)

Segmentation genes • In insect larvae, genes that determine the number and polarity of larval segments.

Segment polarity genes • Genes that determine the boundaries and front-to-back organization of the segments in the Drosophila larva.

Segregation (genetic) • The separation of alleles, or of homologous chromosomes, from one another during meiosis so that each of the haploid daughter nuclei produced by meiosis contains one or the other member of the pair found in the diploid mother cell, but never both.

Selective permeability • A characteristic of a membrane, allowing certain substances to pass through while other substances are excluded.

Selfish act • A behavioural act that benefits its performer but harms the recipients.

Semelparous organism • An organism that reproduces only once in its lifetime. (Contrast with iteroparous.)

Semen (see’ men) EL.: seed] • The thick, whitish liquid produced by the male reproductive organ in mammals, containing the sperm.

Semicircular canals • Part of the vestibular system of mammals.

Semiconservative replication • The common way in which DNA is synthesized. Each of the two partner strands in a double helix ads as a template for a new partner strand. Hence, after replication, each double helix consists of one old and one new strand.

Seminiferous tubules • The tubules within the testes within which sperm production occurs.

Senescence [L. senescere: to grow old] • Aging; deteriorative changes with aging; the increased probability of dying with increasing age.

Sensory neuron • A neuron leading from a sensory cell to the central nervous system. (Contrast with motor neuron.)

Sepal (see’ pul) • One of the outermost structures of the flower, usually protective in function and enclosing the rest of the flower in the bud stage.

Septum [L.: partition] • A membrane or wall between two cavities.

Sertoli cells • Cells in the seminiferous tubules that nuture the developing sperm.

Serum • That part of the blood plasma that remains after clots have formed and been removed.

Sessile (sess’ ul) [L. sedere: to sit] • Permanently attached; not moving.

Set point • In a regulatory system, the threshold sensitivity to the feedback stimulus.

Sex chromosome • In organisms with a chromosomal mechanism of sex determination, one of the chromosomes involved in sex determination.

Sex linkage • The pattern of inheritance characteristic of genes located on the sex chromosomes of organisms having a chromosomal mechanism for sex determination.

Sexual selection • Selection by one sex of characteristics in individuals of the opposite sex. Also, the favoring of characteristics in one sex as a result of competition among individuals of that sex for mates.

Shoot • The aerial part of a vascular plant, consisting of the leaves, stem(s), and flowers.

Sieve tube • A column of specialised cells found in the phloem, specialised to conduct organic matter from sources (such as photosynthesizing leaves) to sinks (such as roots). Found principally in flowering plants.

Sieve tube member • A single cell of a sieve tube, containing cytoplasm but relatively few organelles, with highly specialised perforated end walls leading to elements above and below.

Sign stimulus • The single stimulus, or one out of a very few stimuli, by which an animal distinguishes key objects, such as an enemy, or a mate, or a place to nest, etc.

Signal sequence • The sequence of a protein that directs the protein through a particular cellular membrane

Signal transduction pathway • The series of biochemical steps whereby a stimulus to a cell (such as a hormone or neurotransmitter binding to a receptor) is translated into a response of the cell.

Signature principle • States that because of continuity, prebiotic processes should leave some trace in contemporary biochemistry. (Compare with principle of continuity.)

Silencer • A sequence of eukaryotic DNA that binds proteins that inhibit the transcription of an associated gene.

Silent mutations • Genetic changes that do not lead to a phenotypic change. At the molecular level, these are DNA sequence changes that, because of the redundancy of the genetic code, result in the same amino acids in the resulting protein. See synonymous mutation.

Similarity matrix • A matrix to compare the structures of two molecules constructed by adding the number of their amino acids that are identical or different

Sinoatrial node (sigh’ no ay’ tree al) • The pacemaker of the mammalian heart.

Sinus (sigh’ nus) [L. sinus: a bend, hollow] • A cavity in a bone, a tissue space, or an enlargement in a blood vessel.

Skeletal muscle • See striated muscle.

Sliding filament theory • A proposed mechanism of muscle contraction based on formation and breaking of crossbridges between actin and myosin filaments, causing them to slide together.

Small intestine • The portion of the gut between the stomach and the colon, consisting of the duodenum, the jejunum, and the ileum.

Small nuclear ribonucleoprotein particle (snRNP) • A complex of an enzyme and a small nuclear RNA molecule, functioning in RNA splicing.

Smooth muscle • One of three types of muscle tissue. Usually consists of sheets of mononucleated cells innervated by the autonomic nervous system.

Society • A group of individuals belonging to the same species and organized in a cooperative manner; in the broadest sense, includes parents and their offspring.

Sodium-potassium pump • The complex protein in plasma membranes that is responsible for primary active transport; it pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.

Solute • A substance that is dissolved in a liquid (solvent).

Solute potential • A property of any solution, resulting from its solute contents; it may be zero or have a negative value.

Solution • A liquid (solvent) and its dissolved solutes.

Solvent • A liquid that has dissolved or can dissolve one or more solutes.

Somatic [Gr. soma: body] • Pertaining to the body, or body cells (rather than to germ cells).

Somite (so’ might) • One of the segments into which an embryo becomes divided longitudinally, leading to the eventual segmentation of the animal as illustrated by the spinal column, ribs, and associated muscles.

Spatial summation • In the production or inhibition of action potentials in a postsynaptic neuron, the interaction of depolarizations and hyperpolarizations produced by several terminal boutons.

Spawning • The direct release of sex cells into the water.

Speciation (spee’ shee ay’ shun) • The process of splitting one population into two populations  that are reproductively isolated from one another.

Species (spee’ shees) [L.: kind] • The basic lower unit of classification, consisting of a population or series of populations of closely related and similar organisms. The more narrowly defined biological species” consists of individuals capable of interbreeding freely with each other but not with members of other species.

Species diversity • A weighted representation of the species of organisms living in a region; large and common species are given greater weight than are small and rare ones. (Contrast with species richness.)

Species richness • The number of species of organisms living in a region. (Contrast with species diversity.)

Specific heat • The amount of energy that must be absorbed by a gram of a substance to raise its temperature by one degree centigrade. By convention, water is assigned a specific heat of one.

Sperm (Gr. sperma: seed] • A male reproductive cell.

Spermatocyte (spur mat’ oh site) [Gr. sperma: seed ± kytos: cell] • The cell that gives rise to the sperm in animals.

Spermatogenesis (spur mat oh jen’ e sis) [Gr. sperma: seed + genesis: source] • Male gametogenesis, leading to the production of sperm.

Spermatogonia . Undifferentiated germ cells that give rise to primary spermatocytes and hence to sperm.

Sphincter (sfingk’ ter) [Cr. sphinkter: that which binds tight] • A ring of muscle that can close an orifice, for example at the anus.

Spindle apparatus • An array of microtubules stretching from pole to pole of a dividing nucleus and playing a role in the movement of chromosomes at nuclear division. Named for its shape.

Spiracle (spy’ nh kel) [L. spirare: to breathe] • An opening of the treacheal respiratory system of terrestrial arthorpods

Spiteful act • A behavioural act that harms both the actor and the recipient of the act.

Spliceosome • An RNA-protein complex that splices out introns from eukaryotic pre-mRNA5.

Splicing • The removal of introns and connecting of exons in eukaryotic pre­mRNAs.

Spontaneous generation • The idea that life is generated continually from nonliving matter. Usually distinguished from the current idea that life evolved from nonliving matter under primordial conditions at an early stage in the history of earth.

Spontaneous reaction • A chemical reaction which will proceed on its own, without any outside influence. A spontaneous reaction need not be rapid

Sporangium (spor an’ gee um) [Gr. spora: seed + angeion: vessel or reservoir] • In plants and fungi, any specialised stucture within which one or more spores are formed.

Spore [Gr. spora: seed] • Any asexual reproductive cell capable of developing into an adult plant without gametic fusion. Haploid spores develop into gametophytes, diploid spores into sporophytes. In prokaryotes, a resistant cell capable of surviving unfavorable periods.

Sporophyte (spor’ o fyte) [Gr. spora: seed + phyton: plant] • In plants with alternation of generations, the diploid phase that produces the spores. (Contrast with gametophyte.)

Stabilizing selection • Selection against the extreme phenotypes in a population, so that the intermediate types are favored. (Contrast with disruptive selection.)

Stamen (stay’ men) [L.: thread] • A male (pollen-producing) unit of a flower, usually composed of an anther, which bears the pollen, and a filament, which is a stalk supporting the anther.

Starch [0±. stern-c: stiff] • An a-linked polymer of glucose; used by plants as a means of storing energy and carbon atoms.

Start codon • The mRNA triplet (AUG) that acts as signals for the beginning of translation at the ribosome. (Compare with stop codons. There are a few mnior exceptions to these codons.)

Stasis • Period during which little or no evolutionary change takes place within a lineage or groups of lineages.

Statocyst (stat’ oh sist) [Gk. statos: stationary + kystos: pouch] • An organ of equilibrium in some invertebrates.

Statolith (stat’ oh lith) [Gk. statos: stationary + Iithos: stone] • A solid object that responds to gravity or movement and stimulates the mechanoreceptors of a statocyst.

Stele (steel) [Gr. ste/c: pillar] • The central cylinder of vascular tissue in a plant stem.

Stem cell • A cell capable of extensive proliferation, generating more stem cells and a large clone of differentiated progeny cells, as in the formation of red blood cells.

Step dine • A sudden change in one or more traits of a species along a geographical gradient.

Steroid • Any of numerous lipids based on a 17-carbon atom ring system.

Sticky ends • On a piece of two-stranded DNA, short, complementary, one-stranded regions produced by the action of a restriction endonuclease. Sticky ends allow the joining of segments of DNA from different sources.

Stigma [L.: mark, brand] • The part of the pistil at the apex of the style, which is receptive to pollen, and on which pollen germinates.

Stimulus . Something causing a response; something in the environment detected by a receptor.

Stolon • A horizontal stem that forms roots at intervals.

Stoma (plural: stomata) [Gr. stoma: mouth, opening] • Small opening in the plant epidermis that permits gas exchange; bounded by a pair of guard cells whose osmotic status regulates the size of the opening.

Stop codons • Triplets (UAG, UGA, IJAA) in mRNA that act as signals for the end of translation

at the ribosome. (See also start codon. There are a few mnior exceptions to these codons.)

Stratosphere • The part of the atmosphere above the troposphere; extends upward to approximately 50 kilometers above the surface of the earth; contains very little water.

Stratum (plural strata) • A layer or sedimentary rock laid down at a particular time in a past.

Striated muscle • Contractile tissue characterized by multinucleated cells containing highly ordered arrangements of actin and myosin microfilaments. Also known as skeletal muscle.

Stroma • The fluid contents of an organelle, such as a chloroplast.

Stromatolite • A composite, flat-to-domed structure composed of successive mineral layers. Some are known to be produced by the action of bacteria in salt or fresh water, and some ancient ones are considered to be evidence for early life on the earth.

Structural formula • A representation of the positions of atoms and bonds in a molecule.

Structural gene • A gene that encodes the primary structure of a protein.

Style [Gr. stylos: pillar or column] • In flowering plants, a column of tissue extending from the tip of the ovary, and bearing the stigma or receptive surface for pollen at its apex.

Sub- [L.: under] • A prefix often used to designate a structure that lies beneath another or is less than another. For example, subcutaneous, subspecies.

Submucosa • (sub mew koe’ sah) • The tissue layer just under the epithelial lining of the lumen of the digestive tract. (Contrast with mucosa.)

Substrate (sub’ strayte) • (1) The molecule or molecules on which an enzyme exerts catalytic action. (2) The base material on which an organism lives

Substrate level phosphorylation • ATP formation resulting from direct transfer of a phosphate group to ADP from an intermediate in glycolysis. (Contrast with oxidative phosphorylation.)

Succession • In ecology, the gradual, sequential series of changes in species composition of a community following a disturbance.

Sulcus (plural: sulci) [L. sulcare: to plow] • The valleys or creases between the raised portions of the convoluted surface of the brain, (Contrast to gyrus.)

Sulfhydryl group • The —SH group.

Summation • The ability of a neuron to fire action potentials in response to numerous subthreshold postsynaptic potentials arriving simultaneously at differentiated places on the cell, or arriving at the same site in rapid succession.

Surface area-to-volume ratio • For any cell, organism, or geometrical solid, the ratio of surface area to volume; this is an important factor in setting an upper limit on the size a cell or organism can attain.

Surfactant • A substance that decreases the surface tension of a liquid. Lung surfactant, secreted by cells of the alveoli, is mostly phospholipid and decreases the amount of work necessary to inflate the lungs.

Symbiosis (sim’ bee oh’ sis) [Sr.: to live together] • The living together of two or more species in a prolonged and intimate ecological relationship. (See parasitism, commensalism, mutualism.)

Symmetry • In biology, the property that two halves of an object are mirror images of each other. (See bilateral symmetry and biradial symmetry.)

Sympathetic nervous system • A division of the autonomic (involuntary) nervous system. Its activities include increasing blood pressure and acceleration of the heartbeat. The neurotransmitter at the sympathetic terminals is epinephrine or norepinephrine. (Contrast with parasympathetic nervous system.)

Sympatric speciation (sim pat’ rik) [Sr. sym: same + patria: homeland] • The occurrence of genetic reproduction isolation and the subsequent formation of new species without any physical separation of the subpopulation. (Contrast with allopatric speciation, parapatric speciation.)

Symplast • The continuous meshwork of the interiors of living cells in the plant body, resulting from the presence of plasmodesmata. (Contrast with apoplast.)

Symport • A membrane transport process that carries two substances in the same direction across the membrane. (Contrast with antiport.)

Synapse (sin’ aps) [Gr. syn: together + haptein: to fasten] • The narrow gap between the terminal bouton of one neutron and the dendrite or cell body of another.

Synapsis (sin ap’ sis) • The highly specific parallel alignment (pairing) of homologous chromosomes during the first division of meiosis.

Synaptic vesicle • A membrane-bounded vesicle, containing neurotransmitter, which is produced in and discharged by the presynaptic neuron.

Syngamy (sing’ guh mee) [Gr. sun-: together + gamos: marriage] • Union of gametes. Also known as fertilization.

Synonymous mutation • A mutation that substitutes one nucleotide for another but does not change the amino acid specified (i.e., UUA Æ UUG , both specifying leucine). (Compare with frame-shift mutation, missense mutation, nonsense mutation.)

Synonymous substitution • The situation when a synonymous mutation becomes widespread in a population. Typically not influenced by natural selection, these substitutions can accumulate in a population. (Contrast with nonsynonymous substitution.)

Systematics • The scientific study of the diversity of organisms.

Systemic circulation • The part of the circulatory system serving those parts of the body other  than the lungs or gills.

Systemin • The only polypeptide plant hormone; participates in response to tissue damage.

Systole (sis’ tuh lee) [Gr.: contraction] • Contraction of a chamber of the heart, driving blood forward in the circulatory system.

T

T cell • A type of lymphocyte, involved in the cellular immune response. The final stages of its development occur in the thymus gland. (Contrast with B cell; see also cytotoxic T cell, helper T cell, suppressor T cell.)

T cell receptor • A protein on the surface of a T cell that recognises the antigenic determinant for which the cell is specific.

T tubules • A system of tubules that runs throughout the cytoplasm of muscle fibres, through which action potentials spread.

Target cell • A cell with the appropriate receptors to bind and respond to a particular hormone or other chemical mediator.

Taste bud • A structure in the epithelium of the tongue that includes a cluster of chemoreceptors innervated by sensory neurons.

TATA box • An eight-base-pair sequence, found about 25 base pairs before the starting point for transcription in many eukaryotic promoters, that binds a transcription factor and thus helps initiate transcription.

Taxis (tak’ sis) [Cr. taxis: arrange, put in order] • The movement of an organism in a particular direction with reference to a stimulus. A taxis usually involves the employment of one sense and a movement directly toward or away from the stimulus, or else the maintenance of a constant angle to it. Thus a positive phototaxis is movement toward a light source, negative geotaxis is movement upward (away from gravity), and so on.

Taxon • A unit in a taxonomic system.

Taxonomy (taks on’ oh me) [Gr. taxis: arrange, classify] • The science of classification of organisms.

Telomeres (tee’ 10 merz) [Gr. telos: end] • Repeated DNA sequences at the ends of eukaryotic chromosomes.

Telophase (tee’ lo phase) [Gr. telos: end] • The final phase of mitosis or meiosis during which chromosomes became diffuse, nuclear envelopes reform, and nucleoli begin to reappear in the daughter nuclei.

Template • In biochemistry, a molecule or surface upon which another molecule is synthesized in complementary fashion, as in the replication of DNA. In the brain, a pattern that responds to a normal input but not to incorrect inputs.

Template strand • In a stretch of double-stranded DNA, the strand that is transcribed.

Temporal summation • In the production or inhibition of action potentials in a postsynaptic neuron, the interaction of depolarizations or hyperpolarizations produced by rapidly repeated stimulation of a single point.

Tendon • A collagen-containing band of tissue that connects a muscle with a bone.

Terrestrial (ter res’ tree al) [L. terra: earth] • Pertaining to the land. (Contrast with aquatic, marine.)

Territory • A fixed area from which an animal or group of animals excludes other members of the same species by aggressive behaviour or display.

Tertiary structure • In reference to a protein, the relative locations in three-dimensional space of all the atoms in the molecule. The overall shape of a protein. (Contrast with primary, secondary, and quaternary structures.)

Test cross • A cross of a dominant-phenotype individual (which may be either heterozygous or homozygous) with a homozygous-recessive individual.

Testis (tes’ tis) (plural: testes) EL.: witness] • The male gonad; that is, the organ that produces the male sex cells.

Testosterone (tes toss’ tuhr own) • A male sex steroid hormone.

Tetanus [Gr. tetanos: stretched] • (1) In physiology, a state of sustained, maxima? muscular contraction caused by rapidly repeated stimulation. (2) In medicine, an often-fatal disease (“lockjaw”) caused by the bacterium Clostridium tetani.

Thalamus • A region of the vertebrate forebrain; involved in integration of sensory input.

Thallus (thai’ us) [Gr.: sprout] • Any algal body which is not differentiated into root, stem, and leaf.

Theory • An explanation or hypothesis that is supported by a wide body of evidence. (Contrast with hypothesis, paradigm.)

Thermoneutral zone • The range of temperatures over which an endotherm does not have to expend extra energy to thermoregulate.

Thermoreceptor • A cell or structure that responds to changes in temperature.

Thoracic cavity • The portion of the mammalian body cavity bounded by the ribs, shoulders, and diaphragm. Contains the heart and the lungs.

Thorax • In an insect, the middle region of the body, between the head and abdomen. In mammals, the part of the body between the neck and the diaphragm.

Thrombin • An enzyme that converts fibrinogen to fibrin, thus triggering the formation of blood clots.

Thrombus (throm’ bus) [Gk. thrombos: clot] • A blood clot that forms within a blood vessel and remains attached to the wall of the vessel. (Contrast with embolus.)

Thylakold • A flattened sac within a chloroplast. The membranes of the numerous thylakoids contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.

Thymine • A nitrogen-containing base found in DNA.

Thymus • A ductless, glandular portion of the lymphoid system, involved in development of the immune system of vertebrates.

Thyroid [Gr. thyreos: door-shaped] • A two-lobed gland in vertebrates. Produces the hormone thyroxin.

Thyrotropic hormone • A hormone that is produced in the pituitary gland of amphibia such as frogs and transported in the bloodstream to the thyroid gland, inducing the thyroid gland to produce the thyroid hormone that regulates metamorphosis from tadpole to adult frog.

Tight junction • A junction between epithelial cells, in which there is no gap whatever between the adjacent cells. Materials may get through a tight junction only by entering the epithelial cells themselves.

Tissue • A group of similar cells organized into a functional unit and usually integrated with other tissues to form part of an organ such as a heart or leaf.

Tonus • A low level of muscular tension that is maintained even when the body is at rest.

Totipotency • In a cell, the condition of possessing all the genetic information and other capacities necessary to form an entire individual.

Toxigenicity [L. toxicum: poison] • The ability of a bacterium to produce chemical substances injurious to the tissues of the host organism.

Trachea (tray’ kee ah) [Gr. trakhoia: a small tube] • A tube that carries air to the bronchi of the lungs of vertebrates, or to the cells of arthropods.

Tracheid (tray’ kee id) • A distinctive conducting and supporting cell found in the xylem of nearly all vascular plants, characterized by tapering ends and walls that are pitted but not perforated.

Tracheophytes [Gr. trakhoia: a small tube + phyton: plant] • Those plants with xylem and phloem, including psilophytes, club mosses, horsetails, ferns, gymnosperms, and angiosperms. (Contrast with nontrachoephytes.)

Trait • One form of a character: Eye color is a character; brown eyes and blue eyes are traits

Transcription • The synthesis of RNA, using one strand of DNA as the template.

Transcription factors • Proteins that assemble on a eukaryotic chromosome, allowing RNA polymerase II to perform transcription.

Transduction • (1) Transfer of genes from one bacterium to another, with a bacterial virus acting as the carrier of the genes. (2) In sensory cells, the transformation of a stimulus (e.g., light energy, sound pressure waves, chemical or electrical stimulants) into action potentials.

Transfection • Uptake, incorporation, and expression of recombinant DNA.

Transfer cell • A modified parenchyma cell that transports solutes from its cytoplasm into its cell wall, thus moving the solutes from the symplast into the apoplast.

Transfer RNA (tRNA) • A category of relatively small RNA molecules (about 75 nucleotides). Each kind of transfer RNA is able to accept a particular activated amino acid from its specific activating enzyme, after which the amino acid is added to a growing polypeptide chain.

Transformation • Mechanism for transfer of genetic information in bacteria in which pure DNA extracted from bacteria of one genotype is taken in through the cell surface of bacteria of a different genotype and incorporated into the chromosome of the recipient cell.

Transgenic • Containing recombinant DNA incorporated into its genetic material.

Translation • The synthesis of a protein (polypeptide). This occurs on ribosomes, using the information encoded in messenger RNA.

Translocation • (1) In genetics, a rare mutational event that moves a portion of a chromosome to a new location, generally on a nonhomologous chromosome.(2)  In vascular plants, movement of solutes in the phloem.

Transpiration [L. spit-are: to breathe] • The evapouration of water from plant leaves and stem, driven by heat from the sun, and providing the motive force to raise water (plus ions) from the roots.

Transposable element • A segment of DNA that can move to, or give rise to copies at, another locus on the same or a different chromosome.

Triglyceride • A simple lipid in which three fatty acids are combined with one molecule of glycerol.

Triplet • See codon.

Triplet repeat • Occurrence of repeated triplet of bases in a gene, often leading to genetic disease, as does excessive repetition of CGG in the gene responsible for fragile-X syndrome.

Triploblastic • Having three cell layers. (Contrast with diploblastic.)

Trisomic • Containing three,rather than two members of a chromosome pair. tRNA • See transfer RNA.

Trochophore (troke’ 0 fore) [Gr. trochos: wheel + phoreus: bearer] • The free­swimming larva of some annelids and molluscs, distinguished by a wheel-like band of cilia around the middle, and indicating an evolutionary relationship between these two groups.

Trophic level • A group of organisms united by obtaining their energy from the same part of the food web of a biological community.

Tropic hormones • Hormones of the anterior pituitary that control the secretion of hormones by other endocrine glands.

Tropism [Gr. tropos: to turn] • In plants, growth toward or away from a stimulus such as light (phototropism) or gravity (gravitropism).

Tropomyosin (troe poe my’ oh sin) • A protein that, along with actin, constitutes the thin filaments of myofibrils. It controls the interactions of actin and myosin necessary for muscle contraction.

Troposphere • The atmospheric zone reaching upward approximately 17 km in the tropics and subtropics but only to about 10 km at higher latitudes. The zone in which virtually all the water vapour in the atmosphere is located.

Trypsin • A protein-digesting enzyme. Secreted by the pancreas in its inactive form (trypsinogen), it becomes active in the duodenum of the small intestine.

T-tubules • A set of transverse tubes that penetrates skeletal muscle fibres and terminates in the sarcoplasmic reticulum. The T-system transmits impulses to the sacs, which then release Ca²⁴ to initiate muscle contraction.

Tube nucleus • In a pollen tube, the haploid nucleus that does not participate in double fertilization. (Contrast with generative nucleus.)

Tubulin • A protein that polymerises to form microtubules.

Tumour • A disorganized mass of cells, often growing out of control. Malignant tumours spread to other parts of the body.

Tumour suppressor genes • Genes which, when homozygous mutant, result in cancer. Such genes code for protein products that inhibit cell proliferation.

Twitch • A single unit of muscle contraction.

Tympanic membrane [Gr. tympanum: drum] • The eardrum.

U

Umbilical cord • Tissue made up of embryonic membranes and blood vessels that connects the embryo to the placenta in eutherian mammals.

Understory • The aggregate of smaller plants growing beneath the canopy of dominant plants in a forest.

Unicellular (yoon’ e sell’ yer ler) [L. unus: one + ce/Ia: chamber] • Consisting of a single cell; as for example a unicellular organism. (Contrast with multicellular.)

Uniport • A membrane transport process that carries a single substance (Contrast with antiport, symport.)

Unsaturated hydrocarbon • A compound containing only carbon and hydrogen atoms. One or more pairs of carbon atoms are connected by double bonds.

Upwelling • The upward movement of nutrient-rich, cooler water from deeper layers of the ocean.

Urea • A compound serving as the main excreted form of nitrogen by many animals, including mammals

Ureotelic • Describes an organism in which the final product of the breakdown of nitrogen-containing compounds (primarily proteins) is urea. (Contrast with ammonotelic, uricotelic.)

Ureter (your’ uh tur) [Gr. ouron: urine] • A long duct leading from the vertebrate kidney to the urinary bladder or the cloaca.

Urethra (you ree’ thra) [Gr. ouron: urine] • In most mammals, the canal through which urine is discharged from the bladder and which serves as the genital dud in males.

Uric acid • A compound that serves as the main excreted form of nitrogen in some animals, particularly those which must conserve water, such as birds, insects, and reptiles.

Uricotelic • Describes an organism in which the final product of the breakdown of nitrogen-containing compounds (primarily proteins) is uric acid. (Contrast with ammonotelic, ureotelic.)

Urinary bladder • A structure that receives urine from the kidneys via the ureter, stores it, and expels it periodically through the urethra.

Urine (you’ rin) [Gk. ouron: urine] • In vertebrates, the fluid waste product containing the toxic nitrogenous by-products of protein and amino acid metabolism.

Uterus (yoo’ ter us) EL.: womb] • The uterus or womb is a specialised portion of the female reproductive tract in certain mammals. It receives the fertilized egg and nurtures the embryo in its early development.

V

  Vaccination • Injection of virus or bacteria or their proteins into the body, to induce immunization. The injected material is usually attenuated (weakened) before injection.

Vacuole (vac’ yew ole) [Fr.: small vacuum] • A liquid-filled cavity in a cell, enclosed within a single membrane. Vacuoles play a wide variety of roles in cellular metaboii~tii, some being digestive chambers, some storage chambers, some waste bins, and so forth.

Vagina (vuh jine’ uh) [L.: sheath] • In female mammals, the passage leading from the external genital orifice to the uterus; receives the copulatory organ of the male in mating.

van der Waals Interaction • A weak attraction between atoms resulting from the interaction of the electrons of one atom with the nucleus of the other atom. This attraction is about one-fourth as strong as a hydrogen bond.

Variable regions • The part of an immunoglobulin molecule or T-celI receptor that includes the antigen-binding site.

Vascular (vas’ kew lar) • Pertaining to organs and tissues that conduct fluid, such as blood vessels in animals and phloem and xylem in plants.

Vascular bundle • In vascular plants, a ~strand of vascular tissue, including conducting cells of xylem and phloem as well as thick-walled fibres.

Vascular ray • In vascular plants, radially oriented sheets of cells produced by the vascular cambium, carrying materials laterally between the wood and the phloem.

Vascular tissue system • The conductive system of the plant, consisting primarily of xylem and phloem. (Contrast with dermal tissue system, ground tissue system.)

Vasopressln • See antidiuretic hormone.

Vector • (1) An agent, such as an insect, that carries a pathogen affecting another species. (2) A plasmid or virus that carries an inserted piece of DNA into a bacterium for cloning purposes in recombinant DNA technology.

Vegetal hemIsphere • The lower portion of some animal eggs, zygotes, and embryos, in which the dense nutrient yolk settles. The vegetal pole refers to the very bottom of the egg or embyro. (Contrast with animal hemisphere.)

Vegetative • Nonreproductive, or nonflowering, or asexual.

Vein [L. vena: channel] • A blood vessel that returns blood to the heart. (Contrast with artery.)

Ventral [L. venter: belly, womb] • Toward or pertaining to the belly or lower side. (Contrast with dorsal.)

Ventricle • A muscular heart chamber that pumps blood through the body.

Vernalization [L. vernalis: belonging to spring] • Events occurring during a required chilling period, leading eventually to flowering.

Vertebral column • The jointed, dorsal column that is the primary support structure of vertebrates.

Vertebrate • An animal whose nerve cord is enclosed in a backbone of bony segments, called vertebrae. The principal groups of vertebrate animals are the fishes, amphibians, reptiles, birds, and mammals.

Vessel [L. vasculum: a small vessel] • In botany, a tube-shaped portion of the xylem consisting of hollow cells (vessel elements) placed end to end and connected by perforations. Together with tracheids, vessel elements conduct water and minerals in the plant.

Vestibular apparatus (yes tib’ yew lar) [L. vestibulum: an enclosed passage] Structures associated with the vertebrate ear; these structures sense changes in position or momentum of the head, affecting balance and motor skills.

Vestigial (yes tij’ ee al) [L. vestigium: footprint, track] • The remains of body structures that are no longer of adaptive value to the organism and therefore are not maintained by selection.

Vicariance (vye care’ ee unce) [L. vicus: change] • The splitting of the range of a taxon by the imposition of some barrier to dispersal of its members.

Vicariant distribution • A distribution resulting from the disruption of a formerly continuous range by a vicariant event.

Villus (vii’ lus) (plural: villi) [L.: shaggy hair] • A hairlike projection from a membrane; for example, from many gut walls.

Virlon (veer’ e on) • The virus particle, the minimum unit capable of infecting a cell.

Viroid (we’ roid) • An infectious agent consisting of a single-stranded RNA molecule with no protein coat; produces diseases in plants.

Virus [L.: poison, slimy liquid] • Any of a group of ultramicroscopic infectious particles constructed of nucleic acid and protein (and, sometimes, lipid) that can reproduce only in living cells.

Visceral mass • The major internal organs of a mollusc.

Vitamin [L. vita: life] • Any one of several structurally unrelated organic compounds that an organism cannot synthesize itself, but nevertheless requires in small quantity for normal growth and metabolism.

Viviparous (we vip' uh rus)[L. vivus: alive]-Reproduction in which fertilization of the egg and development of the embryo occur inside the mother's body (Contrast with ovparous)

W

Water potential • In osmosis, the tendency for a system (a cell or solution) to take up water from pure water, through a differentially permeable membrane. Water flows toward the system with a more negative water potential. (Contrast with osmotic potential, turgor pressure.)

Water vascular system • The array of canals and tubelike appendages that serves as the circulatory system, locomotory system, and food-capturing system of many echinoderms; is in direct connection with the surrounding sea water.

Wavelength • The distance between successive peaks of a wave train, such as electromagnetic radiation.

Wild type • Geneticists’ term for standard or reference type. Deviants from this standard, even if the deviants are found in the wild, are said to be mutant.

X

X-llnked (also called sex-linked) • A character that is coded for by a gene on the X chromosome.

Xerophyte's (zee’ row fyte) [Gr. xerox: dry + phyton: plant] • A plant adapted to an environment with a limited water supply.

Xylem (zy’ lum) [Gr. xy/an: wood] • In vascular plants, the woody tissue that conducts water and minerals; xylem consists, in various plants, of tracheids, vessel elements, fibres, and other highly specialised cells

Y

 Yolk •  The stored food material in animal eggs, usually rich in protein and lipid.

Z

Zygote (zye’ gote)  [Gr. zygotos: yoked]• The cell created by the union of two gametes, in which the gamete nuclei are also fused. The earliest stage of the diploid generation