metabolism

Chemical processes of living organisms enabling them to grow and to function. It involves a constant alternation of building up complex molecules (anabolism) and breaking them down (catabolism). For example, green plants build up complex organic substances from water, carbon dioxide, and mineral salts (photosynthesis); by digestion animals partially break down complex organic substances, ingested as food, and subsequently resynthesize them for use in their own bodies (see digestive system). Within cells, complex molecules are broken down by the process of respiration. The waste products of metabolism are removed by excretion.