Ecdysone (eck die´ sone) [Gr. ek: out of + dyo: to clothe] • In insects, a hormone that induces molting.
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 tum) • 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 witin 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 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: 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 ecto-, 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 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 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 specialized 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 [Gr. 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 behavior.
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 + kotyle: 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´ rin) [Gr. 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.
Epiphyte (ep´ e fyte) [Gr. epi: upon + phyton: plant] • A specialized plant that grows on the surface of other plants but does not parasitize them.
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) [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;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´ ik 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.)