3. Metabolism Overview: Catabolic vs. Anabolic Pathways

Metabolism is the sum of all chemical reactions happening inside an organism. Biologists sort these reactions into two broad categories based on the direction energy flows.

Catabolic pathways break large molecules into smaller ones and release energy in the process. Digesting a meal and running cellular respiration are both catabolic: complex molecules like starch or glucose are broken down, and usable energy comes out.

Anabolic pathways do the opposite. They build larger, more complex molecules from smaller building blocks, which requires an input of energy. Building proteins from amino acids, and photosynthesis building glucose from carbon dioxide and water, are both anabolic.

Catabolic and anabolic pathways are linked: the energy released by catabolic reactions (often captured as ATP) is what powers anabolic reactions elsewhere in the cell. A useful way to picture metabolism is a constant back-and-forth between breaking things down for energy and building things up using that energy.