E

E-site: One of a ribosome’s three sites for tRNA during translation. The E -site is the place where discharged tRNAs leave the ribosome (E stands for exit).
Ecdysone: A steroid hormone, secreted by the prothoracic glands, that triggers molting in arthropods.
Ecdysozoan: Member of a major subdivision within the protostomes that includes arthropods, nematodes, and several smaller phyla. Members of this group possess an external covering called a cuticle that molts as the animal grows.
Echinoderm: Member of a major phylum within the deuterostomes that includes sea urchin, starfish, crinoids, and sea cucumbers. Although they possess bilateral symmetry initially, adults usually show pentaradial symmetry.
Ecological capacity: The actual resource base of a country.
Ecological footprint: A method of using multiple constrains to estimate the human carrying capacity of Earth by calculating the aggregate land and water area in various ecosystems categories appropriated by a nation to produce all the resources it consumes and to absorb all the waste it generates.
Ecological niche: The sum total of a species’ use of the biotic and abiotic resources in its environment.
Ecological species concept: Defining species in terms of ecological roles.
Ecological succession: Transition in the species composition of a biological  community, often following ecological disturbance of the community; the establishment of a community in an area virtually barren of life.
Ecology: The scientific study of the distribution and abundance of organisms and the interactions that determine distribution and abundance.
Ecotype: A genotype adapted to a particular environment.
Ecosystem: System formed by the interaction of a community of organisms with their environment.  All the organisms in a given area as well as the biotic factors with which they interact.
Ecosystem ecology: The study of energy flow and the cyclin of chemicals among the various biotic and abiotic factors in an ecosystem.
Ecosystem services: Functions preform by natural ecosystems that directly or indirectly benefit humans.
Ecotone: The transition of one type of habitat or ecosystem to another, such as the transition from a forest to a grassland.
Ectoderm: The outermost of the three cell layers found in bilaterian embryos (the other two being the mesoderm and endoderm). The ectoderm goes on to form structures such as the external skin, mouth, and the nervous system.
Ectomycorrhiza: A type of mycorrhiza in which the mycelium forms a dense sheath, or mantle, over the surface of the root. Hyphae extend from the mantle into the soil, greatly increasing the surface area fro water and mineral absorption.
Ectomycorrhizal fungus: A fungus that forms ectomycorrhiza with plant roots.
Ectoparasite: A parasite that feeds on the external surface of a host.
Ectopic recombination: Recombination between repetitive DNA elements found in different regions of the genome (e.g. between transposable elements at different sites). This may cause chromosomal rearrangements and deletions.
Ectoproct: A sessile, colonial lophophorate commonly called a bryozoan.
Ectotherm: An animal such as a reptile (other than birds), fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
Ectothermic: Referring to organisms that do not produce enough metabolic heat to have much effect on body temperature.
Ediacaran fauna: Earliest generally accepted animal fossils, dating from about 575 million years ago.
Effective population size: An estimate of the size of the population based on the numbers of females and males that successfully breed, generally smaller than the total population.
Effector cell: A muscle cell or a gland cell that performs the body’s response to stimulus, responds to signals from the brain, or other processing center of the nervous system.
Efferent arteriole: The blood vessel draining a nephron.
Egg-polarity gene: Another name for maternal effect gene; a gene that helps control the orientation (polarity) of the egg.
Ejaculation: The propulsion of sperm from the epididymis through the muscular vas deferens, and urethra.
Elastic fiber: A long thread made of the protein elastin. Elastin fibers provide a rubbery quality to the extracellular matrix that complements the nonelastic strength of collagenous fibers.
Electrocardiogram (ECG or EKG): A record of the electrical impulses that travels through cardiac muscle during the heart cycle.
Electrochemical gradient:  The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential.
Electrogenic pump: An ion transport protein that generates voltage across a membrane.
Electromagnetic receptor: A receptor of electromagnetic energy, such as visible light, electricity, and magnetism.
Electromagnetic spectrum: The entire spectrum of radiation raging in wavelength from less of a nanometer to more than a kilometer.
Electron: A subatomic particle with a single negative charge. One or more electrons move around the nucleus of an atom.
Electron microscope (EM): A microscope that focuses an electron beam through a specimen, resulting in resolving power a thousandfold greater than that of a light microscope. A transmission electron microscope (TEM) is used to study the internal structure of thin sections of cells. A scanning electron microscope  (SEM) is used to study the fine details of cell surfaces.
Electron shell: An energy level represented as the distance of an electron from the nucleus of an atom.
Electron transport chain: A sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.
Electronegativity: The attraction of an atom for the electrons of a covalent bond.
Electrophoresis: A technique in which molecules are pulled through a porous medium by an electric field and so are separated according to their charge and mobility.
Electroporation: A technique to introduce recombinant DNA into cells by applying a brief electrical pulse to a solution containing cells. The electricity creates temporary holes in the cell’s plasma membrane, through which DNA can enter.
Element: Any substance that can not be broken down to any other substance.
Elicitor: A molecule that induces a broad type of host defense response.
Elimination: The passing of undigested material out of the digestive compartment.
Embryo: New developing individual.
Embryo sac: The female gametophyte of angiosperms, form from the growth and division of the megaspore into a multicellular structure with eight haploid nuclei.
Embryonic lethal: A mutation with a phenotype leading to death at the embryo or larval stage.
Embryophyte: Another name for land plants, recognizing that land plants share the common derived trait of multicellular dependent embryos.
Emigration: The movement of individuals out of a population:
Enantiomer: One of two molecules that are mirror image of each other.
Endangered species: A species that is in danger of extinction through out of all or a significance portion of its range.
Endemic: Referring to a species that is confined to a specific, relatively small geographical area.
Endergonic reaction: A non-spontaneous chemical reaction, in which free energy is absorbed from the surroundings.
Endocrine gland: A ductless gland that secretes hormones directly into the intestinal fluid, from which they diffuse into the bloodstream.
Endocrine system: The internal system of chemical communication involving hormones,, the ductless glands that secrete hormones, and the molecular receptors on or in the target cells that respond to hormones; functions in concert with the nervous system to effect internal regulation and maintain homeostasis.
Endocytosis: The engulfment of material found outside a cell by surrounding it with the cell membrane.
Endoderm: One of the three cell layers found in bilaterian embryos (the other two being the mesoderm and ectoderm). The endoderm goes on to form structures such as the lining of the digestive system and portions of organs such as the liver, lung, and pancreas.
Endodermis: The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the vascular cylinder.
Endomembrane system: Series of intracellular membrane compartments found in eukaryotic cells.
Endometrium: The inner lining of the uterus, which is richly supplied with blood vessels.
Endomitosis: Mitosis with no cell division which leads to a doubling of ploidy.
Endomycorrhiza: A type of mycorrhiza that, unlike ectomycorrhiza, does not have a dense mantle ensheathing the root. Instead microscopic fungal hyphae extend from the root int the soil.
Endomycorrhizal fungus: A fungus that forms endomycorrhizae with plant roots.
Endoparasite: A parasite that lives within a host.
Endoplasmic reticulum: An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosomal-studded (rough) and ribosome-free (smooth) regions. It is involved in translation, folding, and transport of proteins.
Endorphin: Any of several hormones produced in the brain and anterior pituitary that inhibits pain perception.
Endoskeleton: A hard skeleton buried within the soft tissues of an animal, such as the spicules of sponges, the plates of echinoderms, and the bony skeleton of vertebrates.
Endosperm: A nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertilization, which provide nourishment to the developing embryo in angiosperm seeds.
Endospore: A thick-coated, resistant cell produced within a bacterial cell exposed to harsh conditions.
Endosymbiosis: A symbiosis in which one organism lives within cells of another.
Endothelium: The innermost, simple squamous layer of cells lining the blood vessels, the only constituent structure of capillaries.
Endotherm: An animal, such as bird or mammal, that uses metabolic heat to regulate body temperature.
Endothermic: Referring to organisms with bodies that are warmed by heat generated by metabolism. This heat is usually used to maintained a relatively stable body temperature higher that that of the external environment.
Endotoxin: A toxic component of the outer membrane of certain gram-negative bacteria that is released only when the bacteria dies.
Energetic hypothesis: The concept that the length of a food chain is limited by the inefficiency of energy transfer along the chain.
Energy: The capacity to do work; to move matter against an opposing force.
Energy coupling: In cellular metabolism, the use of energy released from an exergonic reaction to drive an endergonic reaction.
Energy level: Any of different states of potential energy for electrons in an atom.
Enhancer: A DNA segment containing multiple control elements that may be located far away from the gene it regulates.
Enteric division: Complex network of neurons in the digestive tract, pancreas, gallbladder; normally regulated by the sympathetic and parasympathetic division of the autonomous nervous system.
Enterocoelous: Pattern of formation of the body cavity common in deuterostome development, in which the mesoderm buds from the wall of the archenteron and hollows, forming the body cavity.
Entropy: A quantitative measure of disorder. The Second Law of Thermodynamics states that the entropy of a close system can never decrease.
Environment: The complex of physical, chemical, and biotic factors (such as climate, soil, and living things) that act upon an organism or an ecological community and ultimately determine its form and survival.
Environmental deviation: The difference E between the expected trait value of a given genotype and its actual value.
Environmental genomics: Large-scale sequencing of DNA isolated directly from environmental samples (e.g. soil, air, water).
Environmental variation: Variation between genetically identical individuals.
Enzymatic hydrolysis: The process in digestion that splits macromolecules from food by the enzymatic addition of water.
Enzyme: A protein serving as a catalyst, A chemical agent that changes the rate of a reaction without being consumed by the reaction.
Enzyme-substrate complex:  A temporary complex formed when an enzyme binds to its substrate molecule(s).
Eosinophil: A type of white blood cell with low phagocytic activity That is thought to play a role in the defense against parasitic worms by releasing enzymes toxic t the invaders.
Epicotyl: In an angiosperm embryo, the embryonic axis above the point  of attachment of the cotyledon(s).
Epidermis: (1) The dermal tissue system of non-woody plants, usually consisting of a single layer of tightly packed cells. (2) The outer covering of animals.
Epididymis: A coiled tube located adjacent to the testes where sperm are stored.
Epigenesis: The development of an organism from a zygote through cell differentiation and formation of morphology.
Epigenetics: Changes in gene expression due to modifications on proteins associated with DNA or altering the chemical structure of some DNA bases, such as attaching a methyl group to cytosine. Epigenetic changes called epimutations,  do not change the sequence of DNA.
Epiglottis: A cartilaginous flap that blocks the top of the windpipe, the glottis,  during swallowing, which prevents the entry of food or fluid into the respiratory system.
Epinephrine: A catecholamine hormone secreted by the adrenal medulla that mediates “flight-or-fight” response to short-term stress ; also functions as a neurotransmitter.
Epiphyte: A plant that nourish itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
Episome: A genetic element that can exist either as a plasmid or as a part of the bacterial chromosome.
Epistasis: Interactions between alleles in their effect on a trait. If a quantitative trait is given by adding up the contributions from different loci, then we say there is no epistasis.
Epithalamus: A brain region, derived from the diencephalon, that contain several clusters of capillaries that produce cerebrospinal fluid.
Epithelial tissue: Sheets of tightly packed cells that line organs and body cavities.
Epitope: A small, Accessible region of an antigen to which an antigen receptor or antibody binds; also called antigen determinant.
Equilibrium potential: The magnitude of a cell’s membrane voltage at equilibrium, calculated using the Nernst equation.
Erythrocyte: A red blood cell, contains hemoglobin, which functions in transporting oxygen in the circulatory system.
Erythropoetin (EPO): A hormone produced in the kidney when tissues in the body do not receive enough oxygen. This hormone stimulates the production of erythrocytes.
Esophagus: A channel that conducts food, by peristalsis, from the pharynx to the stomach.
Essential amino acid: An amino acid that an animal can not synthesize itself and must by obtained from food. Eight amino acids are essential in the human adult.
Essential element: In plants, a chemical element that is required for the plant to grow from a seed and complete the life cycle, producing another generation of seeds.
Essential fatty acid: Certain unsaturated fatty acids that animals can not make.
Essential nutrient: A substance that an organism must absorb in preassembled form because it can not be synthesized from any other material. In humans, there are essential vitamin, minerals, amino acids, and fatty acids.
Estivation: Summer torpor; a physiological state that is characterized by slow metabolism and inactivity and that permits survival during long periods of elevated temperature and diminish water supplies.
Estrogen: Any steroid hormone, such as estradiol, that stimulates the development and maintenance of the female reproductive system and secondary sex characteristics.
Estrous cycle: A type of reproductive cycle in all female mammals except higher primates, in which the non-pregnant endometrium is reabsorbed rather than shed, and sexual response occurs only during mid-cycle at estrus.
Estrus: A period os sexual activity associated with ovulation.
Estuary: The area where a freshwater stream or river merges with the ocean.
Ethology: The study of animal behavior in natural conditions
Ethylene: The only gaseous plant hormone. Among its many effects are response to mechanical stress, programed cell death, leaf abscission, and fruit ripening.
Etiolation: Plant morphological adaptions for growing in darkness.
Euchromatin: The part of the eukaryotic genome that is not condense and that contains most active genes. It contrasts with heterochromatin.
Eudicots: A clade consisting of the vast majority of flowering plants that have two embryonic seed leaves or cotyledons.
Eugenics: The believe that the human gene pool can be improved by selective breeding.
Euglenoid: Member of a class of eukaryotic microorganisms bearing flagella. Most members live in fresh water and many possess chloroplasts.
Eukaryote: One of the three domains of life. Its species are characterized by the presence of a nucleus.
Eukaryotic cell: A type of cells with a membrane-enclosed nucleus and membrane-enclosed organelles. Include protists, plants, fungi, and animals.
Eumetazoan: Member of the clade Eumetazoan, animals with true tissues: all animals except sponges.
Euryhaline: Referring to organisms that can tolerate substantial changes in external osmolarity.
Eurypterid: An extinct carnivorous cheliceri-form also called a water scorpion.
Eusocial: Fully social organisms in which only one or a few individuals in a colony reproduce.
Eustachian tube: The tube that connects the middle ear to the pharynx.
Eutherian mammal: A mammal having a placenta. This includes all mammals except monotremes and marsupials.
Eutrophic lake: A nutrient rich and oxygen poor lake, having a high rate of biological productivity.
Eutrophication: A process by which nutrients particularly phosphorus and nitrogen, became highly concentrated in a body of water, leading to the increased growth of organisms such as algae.
Euxinic: Environments that are anoxic and sulfidic.
Evaporation: The removal of heat energy form the surface of a liquid that is losing some of its molecules.
Evaporative cooling: The property of a liquid whereby the surface becomes cooler during evaporation, owing to a loss of highly kinetic molecules to the gaseous state.
Evapotranspiration: The evaporation of water from soil plus the transpiration of water from plants.
Evolution: All the changes that have transformed life on Earth from its earliest beginning to the diversity that characterized it today. Change through time.
Evolutionary adaptation: An accumulation of inherited characteristics that enhance organisms’ ability to survive and reproduce in specific environments.
Evolutionary ecology: Study of the evolutionary consequences of interaction between species and between a species and its environment.
Evolutionary game: An interaction between individuals in which the payoff depends on the strategy played by each of them.
Evolutionary rescue: A mechanism of evolution that prevents extinction after environmental change.
Evolutionary synthesis: The synthesis during the 1930 and 1940 of population genetics with other fields of biology (e.g. paleontology, systematics, botany).
Evolvability: Ability to generate heritable variation that can be exploited by selection.
Exaptation: Structures that evolved by natural selection in one context but become co-opted for another function.
Excavate: Member of a kingdom of eukaryotes. All are single-celled species and none are known to have mitochondria.
Excitatory postsynaptic potential (EPSP):  An electrical change (depolarization) in the membrane of a postsynaptic neuron caused by the binding of an excitatory neurotransmitter from a presynaptic cells to a postsynaptic receptor.
Excretion: The disposal of nitrogen-containing waste product of metabolism.
Exergonic reaction: A spontaneous chemical reaction in which there is a net release of free energy.
Exocytosis: Cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
Exoenzyme: A powerful hydrolytic enzyme secreted by a fungus outside its body to digest food.
Exon: A protein-coding region of a protein-coding gene. Exons are separated from each other by introns.
Exon shuffling: Recombination events that mix exons from two different genes.
Exoskeleton: A hard encasement on the surface of an animal, such as the shell of a mollusc or the cuticle of an arthropod, that provides protection and points of attachment for the muscles.
Exotoxin: A toxic protein that is secreted by a prokaryote and that produces specific symptoms even in the absence or the prokaryote.
Expansins: Plant enzymes that break the crosslinks (hydrogen bonds) Between cellulose microfibrils and the other cell constituents, loosening the wall’s fabric.
Exponential population growth: The geometric increase of a population as it grows in an ideal, unlimited environment.
Expression vector: A cloning vector that contains the requisite prokaryotic promoter just upstream of a restriction site where a eukaryotic gene can be inserted
Extant: Currently existing, alive. The opposite of extinct.
Extended phenotype: The phenotype of all the individuals affected by a gene.
External fertilization: The fusion of gametes that parents have discharged into the environment.
Exteroreceptor: A sensory receptor that detects stimuli outside the body, such as heat, light pressure, and chemicals.
Extinction vortex: A downward population spiral in which positive-feedback loops of inbreeding and genetic drift cause a small population to shrink and, unless reversed, become extinct.
Extracellular digestion: The breakdown of  food outside cells.
Extracellular matrix (ECM): The substance in which animal tissue cells are embedded, consisting of protein and polysaccharides.
Extraembryonic membranes: Four membranes ( yolk sac, amnion, chorion, allantois) that support the developing embryo in mammals and birds and other reptiles.
Extreme halophile: A prokaryote that lives in a highly saline environment, such as the Great Salt Lake or the Dead Sea.
Extreme thermophile: A prokaryote that thrives in hot environments; often between 60-80 ºC or hotter.
Extremophile: An organism that thrives in environments that are at the extremes of conditions where life is normally found. Include methanogens, extreme halophiles, and extreme thermophiles.