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covalent bonds, are broken and re-formed. At any one time several hundred different kinds of chemical reactions are occurring simultaneously in every cell, and any chemical can, in principle, undergo multiple chemical reactions. Both the extent to which a reaction can proceed and the rate at which it actually takes place determine which reactions actually occur in cells.
When reactants first come together — before any products have been formed — their rate of reaction is determined in part by their initial concentrations. As the reaction products accumulate, the concentration of each reactant decreases and so does the reaction rate. Meanwhile, some of the product molecules begin to participate in the reverse reaction, which re-forms the reactants. This reaction is slow at first but speeds up as the concentration of products increases. Eventually, the rates of the forward and reverse reactions become equal, so that the concentrations of reactants and products stop changing. The mixture is then said to be in chemical equilibrium.
At equilibrium the ratio of products to reactants, called the equilibrium constant, is a fixed value that is independent of the rate at which the reaction occurs. The rate of a chemical reaction can be increased by a catalyst, a substance that brings reactants together and accelerates their interactions but is not permanently changed during a reaction. This function is aptly reflected in the Chinese term for catalyst, tsoo mei, which literally means “marriage broker.” In this section, we discuss several aspects of chemical equilibria; in the next section, we examine energy changes during reactions and their relationship to equilibria. In the final section, we review the factors that determine reaction rates and how enzymes, the cell’s biological catalysts, increase the rates of biochemical reactions.