Metabolism

Metabolism, sum total of all chemical reactions that occur in a living organism. It can be subdivided into anabolism, which describes reactions that build up more complex substances from smaller ones, and catabolism, which describes reactions that break down complex substances into simpler ones. Anabolic reactions require energy, while catabolic reactions liberate energy.

Metabolic reactions are set off 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 this energy is ultimately derived from sunlight by the photosynthesis in plants, and most organisms use the products of photosynthesis either directly or indirectly.