The conditions in which an organism lives. The external environment includes the nonliving physical and chemical factors such as light, nutrients, etc., together with the effects of other living organisms. Genetically similar plants growing in different environments may appear different (see
plasticity). Such intraspecific variation is termed environmental variation and is not inherited. An organism also has an internal environment, which is the result of its own metabolism. enzyme A protein molecule specialized to catalyse biological reactions. The extremely high specificity and activity of enzymes enables the living cell to function at physiological temperatures and pH values. Without enzymes, nearly all metabolic processes would require high temperatures and extreme pH values, or would produce excessive amounts of heat.
The catalytic activity of an enzyme is due to its possession of an *
active site. The three-dimensional conformation and the charge distribution of the active site are critical; in some enzymes these are entirely maintained by the tertiary structure of the protein, but in other enzymes *
cofactors or *
coenzymes are required for catalytic activity. Enzymes are classified according to their function by an internationally recognized system. There are six classes of enzyme, the *
oxidoreductases, the *
transferases , the *
hydrolases , the *
lyases , the *
isomerases , and the *
ligases. Each class has a code number, and each is subdivided into subclasses and sub-subclasses. Thus any enzyme has a common or recommended name, a systematic name indicating the reaction that it catalyses, and a four-digit code number. See also
substrate.
