A catalyst is something added to a chemical reaction in order to speed it up. The reactants have to reach a certain energy level before they can react, and a catalyst can speed up the process. It reduces the amount of energy needed for the reaction. For example, chlorophyll is a catalyst that speeds up photosynthesis. Enzymes are also catalysts, found in living cells. They are used in reactions that are involved in digestion, cell construction and reproduction. If we did not have enzymes, we would be unable to produce glucose at the speed our body needs it, so these are essential to life.
What do you add if you want to slow a reaction down? You use a negative catalyst. If a chemical reaction, such as food spoiling, is occurring, it can be slowed down, by things like fruit freshener. Negative catalysts, or inhibitors, help keep the reactants apart, or bond with other reactants so the reaction will not take place.