Liver contains the enzyme catalase. It can break down hydrogen peroxide which is harmful to cells into water and oxygen. Enzymes are organic catalysts and can speed up chemical reactions. Two molecules of hydrogen peroxide are broken down into two molecules of water and a molecule of oxygen gas. Both water and oxygen are harmless to cells and therefore, the presence of catalase allows cells to break down the toxic substance hydrogen peroxide before cells become poisoned. The enzyme catalase is extremely important as it can convert millions of molecules of hydrogen peroxide to water and oxygen each second. This enzyme is found in nearly all organisms that are exposed to oxygen.