Metabolism refers to the highly integrated network of chemical reactions by which living cells grow and sustain themselves. This network is composed of two major types of pathways: anabolism and catabolism. Anabolism uses energy stored in the form of adenosine triphosphate (ATP) to build larger molecules from smaller molecules. Catabolic reactions degrade larger molecules in order to produce ATP and raw materials for anabolic reactions.

Together, these two general metabolic networks have three major functions: (1) to extract energy from nutrients or solar energy; (2) to synthesize the building blocks that make up the large molecules of life: proteins, fats, carbohydrates, nucleic acids, and combinations of these substances; and (3) to synthesize and degrade molecules required for special functions in the cell.

These reactions are controlled by enzymes, protein catalysts that increase the speed of chemical reactions in the cell without themselves being changed. Each enzyme catalyzes a specific chemical reaction...