Metabolism refers to the sum of all chemical reactions within a living organism. Chemical reactions either release or require energy. Metabolism can be viewed as an energy-balancing act .

Catabolism – enzyme-regulated chemical reactions that release energy. Complex organic compounds are broken down into simpler ones. These reactions are called catabolic or degradative reactions. They are generally hydrolytic reactions (reactions that use water and in which chemical bonds are broken), and they are exergonic (produce more energy than they consume). Ex. Cells break down sugars into CO₂ and H₂ O.

Anabolism – enzyme-regulated energy requiring reactions. The building of complex organic molecules from simpler ones. These reactions are called anabolic or biosynthetic and they are generally dehydration synthesis reactions (reactions that release water), and they are endergonic (consume more energy than they produce). Ex. Formation of proteins from amino acids, nucleic acids from nucleotides, polysaccharides from simple sugars)

These reactions generate the materials for growth. This coupling of energy requiring and energy-releasing reactions is made possible through the molecule adenosine triphosphate (ATP). ATP stores energy derived from catabolic reactions and releases it later to drive anabolic reactions and perform other cellular work.

Fig. 1. ATP molecule

ATP – an adenine, a ribose and 3 phosphate groups. When terminal phosphate group is split from ATP, ADP is formed, and energy is released to drive anabolic reactions.