Metabolism (mĕ-tab′ō-lizm; change) is the total of all the chemi-cal reactions that occur in the body. It consists of catabolism (kă-tab′ ō-lizm), the energy-releasing process by which large molecules are broken down into smaller ones, and anabolism (ă-nab′ ō-lizm), the energy-requiring process by which small molecules are joined to form larger ones. Catabolism begins during the process of digestion and is concluded within individual cells. Anabolism occurs in all cells of the body as they divide to form new cells, maintain their own intracellular structure, and produce molecules such as hormones, neurotransmitters, or extracellular matrix molecules for export. The energy derived from catabolism is used to drive anabolic reactions.
Metabolism can be divided into the chemical reactions that occur during digestion and the chemical reactions that occur after the products of digestion are taken up by cells. The chemical reactions that occur within cells are often referred to as cellularmetabolism.The digestive products of carbohydrates, proteins,and lipids can be further broken down inside cells. The energy released during this breakdown can be used to combine ADP and an inorganic phosphate group (Pi) to form ATP (figure 17.3).
ATP is often called the energy currency of the cell. When ATP is broken down to ADP, cells can use the released energy for active transport, muscle contraction, and molecule synthesis. Because the body has high energy demands, it uses ATP rapidly.