Metabolism is the complete set of chemical reactions that occurs in living cells. These metabolic processes are the basis of life, as they allow cells to maintain their structures, respond to their environments, grow and reproduce. Metabolism is usually divided into two categories: catabolic reactions that yield energy, such as the breakdown of food, and anabolic reactions that use this energy to construct components of the cell.

The chemical reactions of metabolism are organized into metabolic pathways, where one chemical is transformed to another by a sequence of enzymes. Enzymes are crucial to metabolism because they allow cells to drive unfavorable reactions by coupling them to favorable ones. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell’s environment or signals from other cells.

The metabolism of an organism determines what substances it will find nutritious as well as what substances it will find poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, while this gas is poisonous to animals. The speed of metabolism, the metabolic rate, also influences how much food an organism will require.

A striking feature of metabolism is how similar the basic metabolic pathways are between even vastly different species. For example, the series of chemical steps in a pathway such as the citric acid cycle are universal between living cells as diverse as the unicellular bacteria Escherichia coli and multicellular elephants. This shared metabolic structure is most likely the result of both the high efficiency of these pathways, and their early appearance in evolutionary history

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