electron-transport chain A chain of electron accep-
tors embedded in the inner membrane of the mitochon-
drion. These acceptors separate hydrogen protons from
their electrons. When electrons enter the transport
chain, the electrons lose their energy, and some of it is
used to pump protons across the inner membrane of
the mitochondria, creating an electrochemical gradient
across the inner membrane that provides the energy
needed for ATP (adenosine triphosphate) synthesis. The
function of this chain is to permit the controlled release
of free energy to drive the synthesis of ATP.
See alsoATP.
element Asubstance consisting of atoms that have
the same number of protons in their nuclei. Elements
are defined by the number of protons they possess.
elephantiasis(lymphedema filariasis) Avisibly gro-
tesque enlargement and hardening of the skin and
subcutaneous tissues, usually in the leg or region of the
testis, caused by obstruction of the lymphatic system
when the lymph node is infested by the nematode
worm, Wuchereria bancrofti.
elimination The process achieving the reduction of
the concentration of a XENOBIOTICcompound, includ-
ing its reduction via METABOLISM.
embryo The resulting organism that grows from a fer-
tilized egg following rapid development and eventually
becomes an offspring (in humans, a baby). In plants, it is
the undeveloped plant contained within a seed.
embryo sac A large cell that develops in the ovule of
flowering plants (angiosperms). It contains the egg cell,
the female gametophyte, where pollination occurs, and
when fertilized it becomes an embryo and eventually a
seed. It is formed from the growth and division of the
megaspore into a multicellular structure with eight hap-
loid nuclei.
emigration The process of an individual or group
leaving a population.
EMR(electron magnetic resonance) See ELECTRON
PARAMAGNETIC RESONANCE SPECTROSCOPY.emulsion Droplets of a liquid substance dispersed in
another immiscible liquid. Milk in salad dressing is an
emulsion.enantiomer One of a pair of molecular entities that
are mirror images of each other and nonsuperimposable.endangered species The classification provided to
an animal or plant in danger of extinction within the
foreseeable future throughout all or a significant por-
tion of its range.110 electron-transport chain
Colored scanning electron micrograph (SEM) of a human embryo
at the 16-cell stage, four days after fertilization. Known as a moru-
la, this is a cluster of large rounded cells called blastomeres. The
surface of each cell is covered in microvilli. The smaller spheri-
cal structures seen will degenerate. This embryo is at the early
stage of transformation into a human composed of millions of
cells. Here it is in the process of dividing to form a hollow ball of
cells (the blastocyst). At this 16-cell stage, the morula has not yet
implanted in the uterus (womb). Magnification: ×620 at 6 ×7 cm
size; ×2,300 at 8 ×10 in. size; ×960 at 4 ×5 in. size.(Courtesy ©
Dr. Yorgos Nikas/Photo Researchers, Inc.)