exocrine gland A gland that secretes its materials through ducts (tubes). For example, sweat glands conduct perspiration through ducts to the skin.

exocytosis The process in which a transport vesicle fuses with the plasma membrane of a cell and the contents of the vesicle are ejected outside the cell. Exocytosis can be used to expel waste products in single-celled organisms or to export protein products in multi-celled organisms.

exon A segment DNA, or the messenger RNA transcript complementary to it, that encodes information for the sequencing of amino acids in a protein. Exon is short for an "expressed sequence" of mRNA or DNA. Compare to intron.

exoskeleton An external material covering the animal body, providing support and protection.

exponential growth Growth in numbers that occurs from exponential increase, in which the number produced is proportional to the number already in existence. This type of growth in the size of a population can produce huge numbers of individuals over time.

exponential increase An increase in numbers that is proportional to the number already in existence. This type of increase occurs in populations of living things, and it carries the potential for enormous growth of populations.

extremophile Type of organism that flourishes in extreme conditions such as high heat, high pressure, high salt, or extreme pH. Many archaea and some bacteria are extremophiles, and the enzymes that allow them to thrive in their environments can be used by biotechnologists to make products useful to humans.

extrinsic isolating mechanism A barrier to interbreeding of populations that is not an inherent characteristic of the organisms. Geographic separation of populations is an extrinsic isolating mechanism.