Ecdysone (eck die´ sone) [Gr. ek: out of + dyo: to clothe] • In insects, a hormone that induces molting.

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 assemblage of populations by another in a habitat following some disturbance.

Ecology [Gr. oikos: house + logos: study] • The scientific study of the interaction of organisms with their living and nonliving (abiotic) environment.

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

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; for example, muscle, exocrine glands, chromatophores.

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

Effector phase • Stage of the immune response, when cytotoxic T cells attack virus-infected cells, and helper T cells assist B cells to differentiate into plasma cells.

Efferent [L. ex: out + ferre: to bear] • In physiology, conducting outward or away from an organ or structure. (Contrast with afferent.)

Egg • In all sexually reproducing organisms, the female gamete; in birds, reptiles, and some other vertebrates, a structure witin which early embryonic development occurs. (Compare with amniote egg.)

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 transport • The passage of electrons through a series of proteins with a release of energy which may be captured in a concentration gradient or chemical form such as NADH or ATP.

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

Electrostatic • Pertaining to the attraction and repulsion of negative and positive charges on atoms due to the number and distribution of electrons.

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.

Elongation • Growth of a plant axis or cell primarily in the longitudinal direction.

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

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

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 in 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: native, dwelling in] • Confined to a particular region, thus often having a comparatively restricted distribution.

Endergonic reaction • A chemical reaction that requires the input of energy in order to proceed. (Contrast with exergonic reaction.)

Endocrine gland (en´ doh krin) [Gr. endo: within + 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.

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

Endodermis • In plants, a specialized cell layer marking the inside of the cortex in roots and some stems. Frequently a barrier to free diffusion of solutes.

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

Endoplasmic reticulum (ER) [Gr. endo: within + L. plasma: form + L. reticulum: net] • A system of membranous tubes and flattened sacs found in the cytoplasm of eukaryotes. Exists in two forms: rough ER, studded with ribosomes; and smooth ER, lacking ribosomes.

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

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

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

Endosymbiosis [Gr. endo: within + sym: together + bios: life] • Two species living together, with one living inside the body (or even the cells) of the other.

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

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

End product inhibition • A control capacity of some metabolic pathways in which the final product produced inhibits an early enzyme in the pathway.

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

Energy • The capacity to do work or move matter against an opposing force. The capacity to accomplish change.

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. tropein: to change] • A measure of the degree of disorder in any system. Spontaneous reactions in a closed system are always accompanied by an increase in disorder and entropy.

Environment • Whatever surrounds and interacts with a population, an organism or cell. May be external or internal.

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.

Eosinophils • Phagocytic white blood cells that attack multicellular parasites once they have been coated with antibodies.

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

Epiblast [Gr. epi: upon, over] • The upper or overlying portion of the avian blastula which is joined to the hypoblast at the margins of the blastodisc.

Epicotyl (epp´ i kot´ il) [Gr. epi: over + kotyle: something hollow] • That part of a plant embryo or seedling that is above the cotyledons.

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

Epididymis (epuh did´ uh mus) [Gr. epi: over + 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´ rin) [Gr. epi: over + nephros: kidney] • The "fight or flight" hormone produced by the medulla of the adrenal gland; it also functions as a neurotransmitter. (Also known as adrenaline.)

Epiphyte (ep´ e fyte) [Gr. epi: over + phyton: plant] • A specialized plant that grows on the surface of other plants but does not parasitize them.

Epistasis • 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.

Equatorial plate • In a cell undergoing mitosis, the region in the middle of a cell where the centromeres will align during metaphase.

Equilibrium • Any state of balanced opposing forces and no net change.

ER • See Endoplasmic reticulum.

Erythrocyte (ur rith´ row site) [Gr. erythros: red + kytos: container] • 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) [L. aestivalis: summer] • A state of dormancy and hypometabolism that occurs during the summer; usually a means of surviving drought and/or intense heat. Contrast with hibernation.

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

Estrus (es´ 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=CH₂.

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

Eudicots (yew die´ kots) [Gr. eu: true + di: two + kotyledon: a cup-shaped hollow] • The most diverse and abundant lineage of angiosperms. Eudicot embryos have two cotyledons, and eudicot flowers usually have parts (sepals, petals, etc.) in fours and fives.

Eukaryotes (yew car´ ree oats) [Gr. eu: true + karyon: kernel or nucleus] • Organisms whose cells contain their genetic material inside a nucleus. Includes all life other than the viruses, archaea, and bacteria.

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´ ik ay´ shun) [Gr. eu: truly + trephein: to flourish] • The addition of nutrient materials to a body of water, resulting in changes in ecological processes and 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.

Evolutionarily conserved • Refers to traits that have evolved very slowly and are similar or even identical in individuals of many different phyla.

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

Evolutionary developmental biology • A field that unites embryology, ecology, and genetics.

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.)

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. (Contrast with endocrine gland.)

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

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

Exoskeleton (eks´ oh skel´ e ton) [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 • The basis of the 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 geometric 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.)

Extracellular matrix • In animal tissues, a material of heterogeneous composition surrounding cells and performing many functions including adhesion of cells.

Extraembryonic membranes • The four membranes that support the developing embryo of reptiles, birds, and mammals but are not part of the embryo (amnion, allantois, chorion, and yolk sac)