Metabolism refers to the dynamic changes of the molecules within a cell, especially those small molecules used as sources of energy and as precursors for the synthesis of proteins, lipids, and nucleic acids. These reactions occur in the steady state rather than all at once. Steady state refers to dynamic equilibrium, or homeostasis, where the individual molecules change but the rate at which they are made equals the rate at which they are destroyed. Concentrations of individual molecules in metabolic reactions are therefore kept relatively constant, while any individual molecules are present only for a brief time. Metabolism therefore is said to be an open chemical system. Metabolic reactions can be catabolic (directed toward the breakdown of larger molecules to produce energy), or anabolic (directed toward the energy‐consuming synthesis of cellular components from smaller molecules).