excretion The process of separating and removing
waste products of metabolism from the body through
the discharge of urine, feces, or expired air.
exegetic reaction Aspontaneous reaction in which
energy flows out of the system; a decrease in free ener-
gy. A reaction that liberates heat.
exobiology The study of the origin of life other than
on planet Earth.
exocytosis The process in which a cell discharges
large substances to the outside using secretory vesicles,
storage organelles, that are then fused with the plasma
membrane where they open for export.
exogenous Originating externally. In the context
of metalloprotein LIGANDs, exogenous describes
ligands added from an external source, such as CO
or O 2.
exon A section of DNA that carries the coding
SEQUENCEfor a protein or part of it. Exons are separat-
ed by intervening, noncoding sequences (called
INTRONs). In EUKARYOTES, most GENEs consist of a
number of exons.
exoskeleton The hard external skeleton made from
chitin and connective tissue that attaches it to the
underlying parts of a body of animals such as arthro-
pods (insects, spiders, crabs, lobsters). Serves as protec-
tion, antidessicant, and sensory interface with the
environment.
See alsoSKELETON.
exothermic A reaction that produces heat and
absorbs heat from the surroundings.
exotoxin A toxic substance produced by bacteria
and then released outside its cell into its environment.
exponential population growth Rapid population
growth; populations increase at a constant proportion
from one generation to the next. For example, the
human population is doubling every 40 years. The rate
ofincrease is not limited by environmental factors,
only biotic or intrinsic factors. If birth rates exceed
death rates, population size will increase exponentially;
likewise if death rates exceed birth, population size will
decrease exponentially. Also known as J-shaped popu-
lation growth.expression The cellular production of the protein
encoded by a particular GENE. The process includes
TRANSCRIPTIONof DNA, processing of the resulting
mRNA product, and its TRANSLATIONinto an active
protein. A recombinant gene inserted into a host cell by
means of a vector is said to be expressed if the synthe-
sis of the encoded polypeptide can be demonstrated.
For the expression of metalloproteins, usually other
gene products will be required.extended X-ray absorption fine structure (EXAFS)
EXAFS effects arise because of electron scattering by
atoms surrounding a particular atom of interest as that
special atom absorbs X rays and emits electrons. The
atom of interest absorbs photons at a characteristic
wavelength, and the emitted electrons, undergoing con-
structive or destructive interference as they are scat-
tered by the surrounding atoms, modulate the
absorption spectrum. The modulation frequency corre-
sponds directly to the distance of the surrounding
atoms, while the amplitude is related to the type and
number of atoms. EXAFS studies are a probe of the
local structure. EXAFS can be applied to systems that
have local structure, but not necessarily long-range
structure, such as noncrystalline materials. In particu-
lar, bond lengths and local symmetry (COORDINATION
numbers) can be derived. The X-ray absorption spec-
trum can also show detailed structure below the
absorption edge. This X-ray absorption near-edge
structure (XANES) arises from excitation of core elec-
trons to high-level vacant orbitals.extinct species A species no longer in existence.
See alsoENDANGERED SPECIES.extinct species 121