The sum total of the enzymatic reactions occurring in a cell, organ, or organism. Metabolism serves four major functions: it obtains chemical energy from fuel molecules or from light; it converts exogenous nutrients into the precursors of macromolecular cell components; it assembles these precursors into macromolecules; and it synthesizes molecules to carry out specialized functions in a particular cell. Metabolism can be broadly divided into catabolic and anabolic processes. Catabolism, the breakdown of complex bio-molecules to release energy, occurs in three stages. Macromolecules are broken down in the first stage into their constituent subunits. These subunits are then converted into a few simple molecules ready for stage three, the complete oxidation of the simple molecules to carbon dioxide and water. Stage three, of which the TCA cycle is an important component, is also the first stage of anabolism (see biosynthetic pathway
); hence stage three is both anabolic and catabolic and is termed amphibolic. Particular metabolic processes are located in specific areas of the cell. Thus protein synthesis takes place on the ribosomes and the enzymes of the TCA cycle are found in the mitochondria. There is a continuous turnover of the end products of cell metabolism. This metabolic turnover is a characteristic of all living systems.