Up to this point, we have focused on the spontaneity and direction of chemical reactions. Now we address their rate. As mentioned earlier, a spontaneous reaction is not necessarily a fast one. For example, the breakdown of glucose into carbon dioxide and water is spontaneous with a negative ΔG, but the rate of the reaction is close to zero and the breakdown of glucose is imperceptible. However, glucose is readily broken down inside cells all the time. How is this possible? The answer is that chemical reactions in a cell are accelerated by chemical catalysts.

The rate of a chemical reaction is defined as the amount of product formed (or reactant consumed) per unit of time. Catalysts are substances that increase the rate of chemical reactions without themselves being consumed. In biological systems, the catalysts are usually proteins called enzymes, although, as we saw in Chapter 2, some RNA molecules have catalytic activity as well. Enzymes can increase the rate of chemical reactions dramatically. Moreover, because they are highly specific, acting only on certain reactants and catalyzing only some reactions, enzymes play a critical role in determining which chemical reactions take place from all the possible reactions that could occur in a cell. In this section, we discuss how enzymes increase the rate of chemical reactions and how this ability gives them a central role in metabolism.