All the physical processes and even the mental processes that take place in our bodies are influenced or made possible by chemical reactions. The metabolism is basically the sum of all the chemical reactions that take place in the body. People use the term metabolic rate to indicate the rate at which these chemical reactions take place.
The proper function of the metabolism vitally depends on the proper functioning of the body’s enzymes. Enzymes are proteins that catalyze (they’re catalysts) virtually all the chemical reactions of the body. Catalysts make it possible for certain chemical reactions to take place in a way they otherwise wouldn’t. For example, when one uses epoxy glue to make a repair, it is necessary to mix the two parts together before it can be used. One part is the resin and the other is the hardener. Separately, they are useless as glue because they will not harden. But when the hardener is added to the resin, a chemical reaction takes place enabling the glue to do what it is designed to do. Here the hardener is acting as the vital catalyst. In the same way, if it were not for the important catalysts of the body known as enzymes, the body could not survive.
So, at the very heart of the metabolism are the enzymes. As it turns out, how well an enzyme functions depends on its shape, and its shape depends on its structure. Enzymes are long chains of amino acids that are assembled according to the genetic code of the DNA. Depending on how the amino acids of the enzymes are arranged, the enzymes will take on a particular shape. This shape can change at different times and under different conditions. In fact, this ability to change shape in a characteristic way is precisely what gives them their function. They’re like a twisted telephone receiver cord that will untwist when you answer the phone and pull the cord tight, and then twist back into its previous shape when you put it back on the hook.