Cells therefore use catalysts called enzymes to accelerate many metabolic reactions.
This also allows cells to control these reactions by regulating the enzymes which catalyze them. At one extreme, cells can choose whether or not to make a particular enzyme. Therefore, in multicellular organisms different cell types will have different sets of enzymes. At the other extreme cells can fine-tune the activity of an enzyme by modifying it in various ways such that the activity of a particular enzyme within a given cell may vary widely depending upon the environmental conditions. This is how organisms can regulate their metabolism.