Metabolism is the combination of biochemical processes by which energy, obtained from an external source, is harnessed to maintain internal order and a state of chemical disequilibrium. It includes all of the chemosynthetic (anabolic, anabolism) and degradative (catabolic, see catabolism) reactions that take place in living organisms. The ability to metabolize is generally considered to be one of the fundamental characteristics of life (see life, nature).
Metabolic reactions are catalyzed by enzymes in a highly integrated and finely controlled manner so that there is no overproduction or underutilization of the energy required to maintain life. All energy required to maintain life is ultimately derived from sunlight by photosynthesis, and most organisms use the products of photosynthesis either directly or indirectly. The energy is stored in most living organisms in a specific chemical compound, adenosine triphosphate (ATP. ATP can transfer its energy to other molecules by a loss of phosphate, later regaining phosphate from catabolic reactions.