Metabolism is the sum total of chemical changes that occur in living organisms and which are fundamental to life. All prokaryotic and eukaryotic cells are metabolically active. The sole exception is viruses, but even viruses require a metabolically active host for their replication.

Metabolism involves the use of compounds. Nutrients from the environment are used in two ways by microorganisms. They can be the building blocks of various components of the microorganism (assimilation or anabolism). Or, nutrients can be degraded to yield energy (dissimilation or catabolism). Some so-called amphibolic biochemical pathways can serve both purposes. The continual processes of breakdown and re-synthesis are in a balance that is referred to as turnover. Metabolism is an open system. That is, there are constant inputs and outputs. A chain of metabolic reactions is said to be in a steady state when the concentration of all intermediates remains constant, despite the...