E
(1) see redox potential.
(2) glutamic acid (see amino acids).
E-cadherin
See cadherins.
E. coli
An abbreviation which usually refers to
the bacterium Escherichia coli, but which is also used e.g.
for the protozoan Entamoeba coli..
E∗ precursor cells (immunol.):Cells on whose surface complement fixation has occurred, at 4°c, but
which do not lyse unless the temperature is raised; lysis can be further
inhibited, even at higher temperatures, by edta or zinc or uranyl salts.
E protein
See F plasmid.
E-selectin
Syn. CD62E.
E site (of a ribosome)
See protein synthesis.
E test
A diffusion test for determining the MIC of a given bacterial
strain with respect to particular antibiotic(s). One side of a plastic strip
(placed in contact with the inoculated plate) carries a given antibiotic, the
concentration of which decreases uniformly from one end of the strip; the other
side of the strip is graduated with the concentration of antibiotic. Following incubation,
the MIC is read by noting the lowest concentration of antibiotic (on the scale)
which corresponds to inhibition of growth. Several strips, each with a
different antibiotic, can be used simultaneously on a standard-sized plate.The
E test has been used for various bacteria, including Pseudomonas aeruginosa
(ek-deh-sone)
A steroid hormone that triggers molting in arthropods.
A steroid hormone that triggers molting in arthropods.
ecological
efficiency
The ratio
of net productivity at one trophic level to net productivity at the next lower
level.
ecological
niche
The sum
total of an organism's utilization of the biotic and abiotic resources of its
environment.
ecological
pyramid
A graphic
representation of the quantitative relationships of numbers of organisms,
biomass, or energy flow between the trophic levels of an ecosystem. Because
large amounts of energy and biomass are dissipated at every trophic level,
these diagrams nearly always take the form of pyramids.
ecological
species concept
The idea
that ecological roles (niches) define species.
ecological
succession
Transition
in the species composition of a biological community, often following
ecological disturbance of the community; the establishment of a biological
community in an area virtually barren of life.
ecology
[Gk. oikos, home
+ logos, a discourse]
The study of how organisms interact with their environments.
The study of how organisms interact with their environments.
ecosystem
[Gk. oikos, home
+ systema, that which is put together]
A level of ecological study that includes all the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.
A level of ecological study that includes all the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.
ecotype
Gk. oikos, home
+ L. typus, image]
A locally adapted variant of a species, differing genetically from other ecotypes of the same species.
A locally adapted variant of a species, differing genetically from other ecotypes of the same species.
ectoderm
[Gk. ecto, outside
+ derma, skin]
The outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye.
The outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye.
ectotherm
[Gk. ecto, outside
+ therme, heat]
An animal such as a reptile, fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
An animal such as a reptile, fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
effector
cell
[L. ex, out
of + facere, to make]
definition
definition
efferent
[L. ex, out
of + ferre, to bear]
Carrying away from a center, applied to nerves and blood vessels.
Carrying away from a center, applied to nerves and blood vessels.
egg
A female
gamete, which usually contains abundant cytoplasm and yolk; nonmotile and often
larger than a male gamete.
ejaculatory
duct
In the
male, a duct from each testis that join to form the urethra.
electric
potential
The
difference in the amount of electric charge between a region of positive charge
and a region of negative charge. The establishment of electric potentials
across the plasma membrane and across organelle membranes makes possible a
number of phenomena, including the chemiosmotic synthesis of ATP, the
conduction of nerve impulses, and muscle contraction.
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 generating voltage across the membrane.
electromagnetic
spectrum
The
entire spectrum of radiation; ranges in wavelength from less than a nanometer
to more than a kilometer.
electron
A
particle with a single negative charge; one or more electrons orbit the nucleus
of the atom.
electron
acceptor
Substance
that accepts or receives electrons in an oxidation-reduction reaction, becoming
reduced in the process.
electron
carrier
A
molecule that conveys electrons; one of several membrane proteins in electron
transport chains in cells. Electron carriers shuttle electrons during the redox
reactions that release energy used to make ATP.
electron
donor
Substance
that donates or gives up electrons in an oxidation-reduction reaction, becoming
oxidized in the process.
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 at which an electron orbits 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
tendency for an atom to pull electrons toward itself.
element
Any substance
that cannot be broken down to any other substance.
embryo
(em-bree-oh)
[Gk. en, in + bryein, to swell]
A developing stage of multicellular organisms; in humans, the stage in the development of offspring from the first division of the zygote until body structures begin to appear; about the ninth week of gestation. See Fetus.
A developing stage of multicellular organisms; in humans, the stage in the development of offspring from the first division of the zygote until body structures begin to appear; about the ninth week of gestation. See Fetus.
embryo
sac
The
female gametophyte of angiosperms, formed from the growth and division of the
megaspore into a multicellular structure with eight haploid nuclei.
enantiomer
(eh-nan-she-uh-mer)
One of a pair of molecules that are mirror-image isomers of each other.
One of a pair of molecules that are mirror-image isomers of each other.
endangered
species
A species
that is in danger of extinction throughout all or a significant portion of its
range.
endemic
species
Species
that are confined to a specific, relatively small geographic area.
endemic
An
organism found only in one particular location.
endergonic
reaction
(en-dur-gon-ik)
[Gk. endon, within + ergon, work]
A nonspontaneous chemical reaction in which free energy is absorbed from the surroundings.
A nonspontaneous chemical reaction in which free energy is absorbed from the surroundings.
endocrine
gland
(en-doh-krin)
[Gk. endon, within + krinein, to separate]
A ductless gland that secretes hormones directly into the bloodstream.
A ductless gland that secretes hormones directly 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 target cells that
respond to hormones; functions in concert with the nervous system to effect
internal regulation and maintain homeostasis.
endocytosis
(en-doh-sy-toh-sis)
[Gk. endon, within + kytos, vessel]
The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.
The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.
endoderm
(en-doh-durm)
[Gk. endon, within + derma, skin]
The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract.
The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract.
endodermis
(en-doh-dur-mis)
[Gk. endon, within + derma, skin]
The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the stele.
The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the stele.
endogenous
Arising
from internal structures or functional causes.
endomembrane
system
The
collection of membranes inside and around a eukaryotic cell, related either
through direct physical contact or by the transfer of membranous vesicles.
endometrium
(en-doh-mee-tree-um)
[Gk. endon, within + metrios, of the womb]
The inner lining of the uterus, which is richly supplied with blood vessels.
The inner lining of the uterus, which is richly supplied with blood vessels.
endoplasmic
reticulum
(en-doh-plaz-mik
reh-tik-yoo-lum) [Gk. endon, within + plasma, from
cytoplasm; L. reticulum,network]
An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
endorphin
A hormone
produced in the brain and anterior pituitary that inhibits pain perception.
endoskeleton
(en-doh-skel-eh-ton)
A hard skeleton buried within the soft tissues of an animal, such as the spicules of sponges, the plates of echinoderms, and the bony skeletons of vertebrates.
A hard skeleton buried within the soft tissues of an animal, such as the spicules of sponges, the plates of echinoderms, and the bony skeletons of vertebrates.
endosperm
(en-doh-spurm)
[Gk. endon, within + sperma, seed]
A nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertilization, which provides nourishment to the developing embryo in angiosperm seeds.
A nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertilization, which provides nourishment to the developing embryo in angiosperm seeds.
endospore
A
thick-coated, resistant cell produced within a bacterial cell exposed to harsh
conditions.
endosymbiotic
theory
(en-doh-sim-by-ot-ic)
A hypothesis about the origin of the eukaryotic cell, maintaining that the forerunners of eukaryotic cells were symbiotic associations of prokaryotic cells living inside larger prokaryotes.
A hypothesis about the origin of the eukaryotic cell, maintaining that the forerunners of eukaryotic cells were symbiotic associations of prokaryotic cells living inside larger prokaryotes.
endothelium
(en-doh-theel-ee-um)
[Gk. endon, + thele, nipple]
The innermost, simple squamous layer of cells lining the blood vessels; the only constituent structure of capillaries.
The innermost, simple squamous layer of cells lining the blood vessels; the only constituent structure of capillaries.
endotherm
(en-doh-thurm)
[Gk. endon, within + therme, heat]
An animal that uses metabolic energy to maintain a constant body temperature, such as a bird or mammal. See also Homeotherm.
An animal that uses metabolic energy to maintain a constant body temperature, such as a bird or mammal. See also Homeotherm.
endotoxin
(en-doh-toks-in)
A component of the outer membranes of certain gram-negative bacteria responsible for generalized symptoms of fever and ache.
A component of the outer membranes of certain gram-negative bacteria responsible for generalized symptoms of fever and ache.
energy
The
capacity to do work by moving matter against an opposing force.
energy of
activation (EA)
The
amount of energy that reactants must absorb before a chemical reaction will
start.
enhancer
A DNA
sequence that recognizes certain transcription factors that can stimulate
transcription of nearby genes.
entropy
(en-truh-pee)
[Gk. en, in + trope, turning]
A quantitative measure of disorder or randomness, symbolized by S.
A quantitative measure of disorder or randomness, symbolized by S.
environmental
grain
An ecological
term for the effect of spatial variation, or patchiness, relative to the size
and behavior of an organism.
enzyme
[Gk. en, in
+ zyme, leaven]
A class of proteins serving as catalysts, chemical agents that change the rate of a reaction without being consumed by the reaction.
A class of proteins serving as catalysts, chemical agents that change the rate of a reaction without being consumed by the reaction.
epidermis
(ep-eh-der-mis)
[Gk. epi, on or over + derma, skin]
(1) The dermal tissue system in plants. (2) The outer covering of animals.
(1) The dermal tissue system in plants. (2) The outer covering of animals.
epigenesis
(ep-eh-jen-eh-sis)
The progressive development of form in an embryo.
The progressive development of form in an embryo.
epididymis
(ep-ih-did-eh-mis)
A long coiled tube into which sperm pass from the testis and are stored until mature and ejaculated.
A long coiled tube into which sperm pass from the testis and are stored until mature and ejaculated.
epigenesis
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 hormone
produced as a response to stress; also called adrenaline. See Adrenaline.
epiphyte
(ep-eh-fite)
A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
episome
A plasmid
capable of integrating into the bacterial chromosome.
epistasis
[Gk. a
stopping]
A phenomenon in which one gene alters the expression of another gene that is independently inherited.
A phenomenon in which one gene alters the expression of another gene that is independently inherited.
epithelial
tissue
(ep-eh-theel-ee-ul)
[Gk. epi, on or over + thele, nipple]
Sheets of tightly packed cells that line organs and body cavities.
Sheets of tightly packed cells that line organs and body cavities.
epitope
A
localized region on the surface of an antigen that is chemically recognized by
antibodies; also called antigenic determinant.
equilibrium
[L. aequus, equal
+ libra, balance]
The state of a system in which no further net change is occurring; result of counterbalancing forward and backward processes.
The state of a system in which no further net change is occurring; result of counterbalancing forward and backward processes.
equilibrium
species
Species
characterized by low reproduction rates, long development times, large body
size, and long adult life with repeated reproductions.
erythrocyte
(er-rith-ro-site)
[Gk. erythros, red + kytos, vessel]
A red blood cell; contains hemoglobin, which functions in transporting oxygen in the circulatory system.
A red blood cell; contains hemoglobin, which functions in transporting oxygen in the circulatory system.
esophagus
(eh-sof-eh-gus)
A channel that conducts food, by peristalsis, from the pharynx to the stomach.
A channel that conducts food, by peristalsis, from the pharynx to the stomach.
essential
amino acids
The amino
acids that an animal cannot synthesize itself and must obtain from food. Eight
amino acids are essential in the human adult.
estivation
(es-teh-vay-shun)
A physiological state characterized by slow metabolism and inactivity, which permits survival during long periods of elevated temperature and diminished water supplies.
A physiological state characterized by slow metabolism and inactivity, which permits survival during long periods of elevated temperature and diminished water supplies.
estrogens
[Gk. oistros, frenzy
+ genos, origin, descent]
The primary female steroid sex hormones, which are produced in the ovary by the developing follicle during the first half of the cycle and in smaller quantities by the corpus luteum during the second half. Estrogens stimulate the development and maintenance of the female reproductive system and secondary sex characteristics.
The primary female steroid sex hormones, which are produced in the ovary by the developing follicle during the first half of the cycle and in smaller quantities by the corpus luteum during the second half. Estrogens stimulate the development and maintenance of the female reproductive system and secondary sex characteristics.
estrous
cycle
(es-trus)
A type of reproductive cycle in all female mammals except higher primates, in which the nonpregnant endometrium is reabsorbed rather than shed, and sexual response occurs only during midcycle at estrus.
A type of reproductive cycle in all female mammals except higher primates, in which the nonpregnant endometrium is reabsorbed rather than shed, and sexual response occurs only during midcycle at estrus.
ethology
[Gk. ethos, habit,
custom + logos, discourse]
The comparative study of patterns of animal behavior, with emphasis on their adaptive significance and evolutionary origin.
The comparative study of patterns of animal behavior, with emphasis on their adaptive significance and evolutionary origin.
ethylene
(eth-ul-een)
The only gaseous plant hormone, responsible for fruit ripening, growth inhibition, leaf abscission, and aging.
The only gaseous plant hormone, responsible for fruit ripening, growth inhibition, leaf abscission, and aging.
etiolation
[Fr. etioler, to
blanch]
In plants, a condition characterized by stem elongation, poor leaf development, and lack of chlorophyll; occurs in plants growing in the dark or with greatly reduced light.
In plants, a condition characterized by stem elongation, poor leaf development, and lack of chlorophyll; occurs in plants growing in the dark or with greatly reduced light.
euchromatin
(yoo-kroh-muh-tin)
The more open, unraveled form of eukaryotic chromatin, which is available for transcription.
The more open, unraveled form of eukaryotic chromatin, which is available for transcription.
eukaryotic
cell
A type of
cell with a membrane-enclosed nucleus and membrane-enclosed organelles, present
in protists, plants, fungi, and animals; also called eukaryote.
eukaryote
[Gk. eu, good
+ karyon, nut, kernel]
An organism whose cells contain membraine-bound organelles and whose DNA is enclosed in a cell nucleus and is associated with proteins.
An organism whose cells contain membraine-bound organelles and whose DNA is enclosed in a cell nucleus and is associated with proteins.
eumetazoa
(yoo-met-uh-zoh-uh)
Members of the subkingdom that includes all animals except sponges.
Members of the subkingdom that includes all animals except sponges.
eusocial
[Gk. eu, good
+ L. socius, companion]
Applied to animal societies, such as those of certain insects, in which sterile individuals work on behalf of reproductive individuals.
Applied to animal societies, such as those of certain insects, in which sterile individuals work on behalf of reproductive individuals.
eutherian
mammals
Placental
mammals; those whose young complete their embryonic development within the
uterus, joined to the mother by the placenta.
eutrophic
lake
A highly
productive lake, having a high rate of biological productivity supported by a
high rate of nutrient cycling.
eutrophication
A process
in which an aquatic environment accumulates high nutrient levels due to factors
such as industrial or urban pollution or run-off of fertilizers from nearby
agricultural lands. The nutrients lead to dense blooms of algae and aquatic
plants that cloud lake water, deplete specific minerals and dissolved gases,
and can cause natural plant and animal populations to decline.
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.
evolution
[L. e-, out
+ volvere, to roll]
All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.
All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.
evolutionary
species concept
The idea
that evolutionary lineages and ecological roles can form the basis of species
identification.
exaptation
A
structure that evolves and functions in one environmental context but that can
perform additional functions when placed in some new environment.
excitatory
postsynaptic potential (EPSP)
(post-sin-ap-tik)
An electrical change (depolarization) in the membrane of a postsynaptic neuron caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neuron to generate an action potential.
An electrical change (depolarization) in the membrane of a postsynaptic neuron caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neuron to generate an action potential.
excretion
The
disposal of nitrogen-containing waste products of metabolism.
excretory
system
The organ
system that disposes of nitrogen-containing metabolic wastes.
exergonic
reaction
(eks-ur-gon-ik)
[Gk. ex, out of + ergon, work]
A spontaneous chemical reaction in which there is a net release of free energy.
A spontaneous chemical reaction in which there is a net release of free energy.
exocrine
glands
[Gk. ex, out
of + krinein, to separate]
Glands, such as sweat glands and digestive glands, that secrete their products into ducts that empty onto surfaces, such as the skin, or into cavities, such as the interior of the stomach.
Glands, such as sweat glands and digestive glands, that secrete their products into ducts that empty onto surfaces, such as the skin, or into cavities, such as the interior of the stomach.
exocytosis
(eks-oh-sy-toh-sis)
[Gk. ex, out of + kytos, vessel]
The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
exon
The
coding region of a eukaryotic gene that is expressed. Exons are separated from
each other by introns.
exoskeleton
A hard
encasement on the surface of an animal, such as the shells of mollusks or the
cuticles of arthropods, that provides protection and points of attachment for
muscles.
exotoxin
(eks-oh-toks-in)
A toxic protein secreted by a bacterial cell that produces specific symptoms even in the absence of the bacterium.
A toxic protein secreted by a bacterial cell that produces specific symptoms even in the absence of the bacterium.
exponential
growth
In
populations, the increasingly accelerated rate of growth due to the increasing
number of individuals being added to the reproductive base. Exponential growth
is very seldom approached or sustained in natural populations.
expression
vector
A vector
that allows a DNA sequence cloned into it to be transcribed when the vector is
introduced into a cell.
expressivity
In
genetics, the degree to which a particular genotype is expressed in the
phenotype of individuals with that genotype.
extinct
[L. exstinctus, to
be extinguished]
No longer existing.
No longer existing.
extracellular
matrix (ECM)
The
substance in which animal tissue cells are embedded; consists of protein and
polysaccharides.
extraembryonic
membranes
Four
membranes (yolk sac, amnion, chorion, allantois) that support the developing
embryo in reptiles, birds, and mammals.