Metabolism is the coupled balance of two opposing classes of chemical reaction in living organisms: catabolism, the degradation of macromolecules into their monomeric subunits with release of chemical energy, and anabolism, the energy-requiring resynthesis of organism-specific macromolecules from those subunits. The two are linked by the ATP/ADP cycle, in which catabolic oxidation of fuel monomers regenerates ATP from ADP and inorganic phosphate, and hydrolysis of ATP back to ADP supplies the free energy that drives anabolic construction — so that catabolism fuels anabolism. This concept sits in biochemistry and physiology as the organizing framework for the four classes of biomolecule (proteins, lipids, carbohydrates, nucleic acids) and their subunits, and it is regulated at the whole-organism level by hormones that determine whether tissues occupy a net catabolic or net anabolic state.
Metabolism is the coupled balance of two opposing classes of chemical reaction in living organisms: catabolism, the degradation of macromolecules into their monomeric subunits with release of chemica…