The sum of the chemical reactions that occur within living organisms. The various compounds that take part in or are formed by these reactions are called metabolites. In animals many metabolites are obtained by the digestion of food, whereas in plants only the basic starting materials (carbon dioxide, water, and minerals) are externally derived. The synthesis (anabolism) and breakdown (catabolism) of most compounds occurs by a number of reaction steps, the reaction sequence being termed a metabolic pathway. Some pathways (e.g. glycolysis) are linear; others (e.g. the Krebs cycle) are cyclic. The changes at each step in a pathway are usually small and are promoted by efficient biological catalysts—the enzymes. In this way the amounts of energy required or released at any given stage are minimal, which helps in maintaining a constant internal environment. Various feedback mechanisms exist to govern metabolic rates.