All cells have the ability to sense whether nutrients are scarce or abundant so that appropriate anabolic or catabolic programs can be initiated. ~ Norwegian cytologists Hilde Abrahamsen & Harald Stenmark
Metabolism comprises the cellular chemical reactions that provide energy to sustain life. The mitochondria in eukaryotic cells are the site of respiratory metabolism. Via metabolism, organisms grow, maintain themselves, ecologically interact, and reproduce.
Metabolism is sometimes defined more expansively, for an organism rather than at the cellular level, thus including digestion and the transport of substances between cells. By this broader definition, metabolic cellular processes are more specifically called catabolism.
Catabolism is the controlled cellular process of breaking down organic matter to harvest energy via cellular respiration. During catabolism, polymers are reduced to monomers. Polysaccharides are broken down into monosaccharides; lipids into fatty acids; nucleic acids into nucleotides; and proteins into amino acids. Cells use resultant monomers either for energy, by further breakdown, or to construct new polymers, via anabolism.