Metabolism refers to all the chemical activity within the body.

All chemical reactions either release or require energy. Metabolism has two phases: an energy-generating process called catabolism and an energy-requiring process called anabolism.

Both processes occur simultaneously within the cells, but they are regulated independently. For this reason, the body’s metabolism can be thought of as energy balancing.

a. Catabolism is a degenerative, energy-generating process. Complex molecules of proteins, carbohydrates, and fats are systematically broken down into simpler, smaller molecules by the body’s cells. The bonding energies that hold the atoms of a complex molecule together are released as the molecule is broken down. Much of this energy released by catabolism is captured and stored by the cells in the form of a chemical molecule known as ATP. Digestion is a catabolic process because the breakdown of the food releases energy.

b. Anabolism is a building, energy-requiring process. New, more complex molecules are synthesized from simple molecules. These larger molecules form the body’s structural and functional components. This synthesis requires the expenditure of the cellular energy generated by the cell’s catabolic activities.