A set of physical and chemical reactions that occur in living organisms in order to maintain life, such as digesting food and nutrients, eliminating waste through urine and feces, circulating blood, and breathing. The chemical reactions can be catabolic (catabolism) ─ breaking down organic molecules (e.g., nutrients) to generate energy ─ or anabolic (anabolism) ─ using energy to construct components of cells such as proteins and nucleic acids.