The term metabolism refers to all the chemical changes that take place in the body's tissues when the cells are producing both energy and essential new organic materials. While these metabolic activities are many and varied, most of them fall into two broad categories: anabolic processes and catabolic ones. These two quite different processes take place constantly and simultaneously.

Anabolism is the cell's synthesizing or building-up phase. Through anabolic reactions, simple substances--usually the molecules of glucose, fatty acids, and amino acids that have been derived from foodstuffs--are combined in various ways to form more complex substances, such as the new cellular material needed for growth and tissue maintenance. By combining amino acids, for instance, the cells can form structural proteins and use them to repair or replace worn-out tissues. The cells can also form functional proteins, such as enzymes, antibodies, and hormones.

Catabolism, the cell's degradative or breaking-down...