Exotoxin
A toxic protein secreted by a bacterial cell that produces specific symptoms even in the
absence of the bacterium.
Exothermic
Relating to a chemical reaction that releases heat.
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
No longer existing.
Extracellular
Located or occurring outside a cell or cells.
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.
External morphology
The branch of biology that deals with the form and structure of an organism or part, without
regard to function.
Eyelashes
Any of the short curved hairs that grow from the edges of the eyelids.
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