Metabolism is the total set of chemical reactions occurring in a living organism. It is what keeps organisms alive — the continuous chemical activity that extracts energy from the environment, builds and maintains cellular structures, and disposes of waste products. An organism that stops metabolizing is dead.
Metabolism divides into two broad categories. Catabolism breaks down complex molecules into simpler ones, releasing energy. Digestion is catabolic: food molecules are broken into smaller units. Cellular respiration is catabolic: glucose is oxidized to carbon dioxide and water, releasing energy captured in ATP (adenosine triphosphate). Anabolism uses energy to build complex molecules from simpler ones. Protein synthesis is anabolic: amino acids are assembled into proteins using energy from ATP. Growth is anabolic: new cellular material is constructed from nutrients.
The energy currency of metabolism is ATP. Nearly all cellular work — muscle contraction, nerve impulse transmission, active transport across membranes, biosynthesis — is powered by ATP hydrolysis. ATP is produced primarily through cellular respiration (in mitochondria, in eukaryotes) and, in photosynthetic organisms, through photosynthesis (in chloroplasts). The continuous cycle of ATP production and consumption is what makes metabolism a dynamic process rather than a static state.
Metabolic pathways are sequences of enzyme-catalyzed reactions. Each enzyme catalyzes one specific reaction, and the product of one reaction becomes the substrate of the next. These pathways are regulated by feedback mechanisms: when the end product of a pathway accumulates, it inhibits an earlier enzyme, slowing the pathway down. This is one mechanism of homeostasis at the molecular level — the cell adjusts its metabolic activity to match its current needs.
Related terms
- Cell — the site where metabolism occurs
- Organism — the living system sustained by metabolism
- Homeostasis — the regulatory processes that keep metabolic conditions stable
- Phenotype — the observable outcome of metabolic and developmental processes