No living organisms are stable. They are in a continual state of chemical change. All the time that we are alive we are continually dying and being reborn. About three million of our blood cell die while we read a sentence, and at the same time three million more cells are made. The word metabolism (derived from Greek that means с change) is used to signify the chemical changes which take place in living organisms. By using of suitable measuring instruments it is possible to follow the blood cells in the body and to find out how long they live. This work has revealed that the average life of each blood cell is about a fortnight. Even bones are much less stable than we might expect. It is clear that since the living body is in a state of constant change, the chemical changes must be precisely controlled. We now know that each of the many thousands of chemical reactions which take place in a living body is controlled by a particular enzyme. Enzymes are organic catalysts which take part in the chemical processes occurring in living bodies. The change of one form of sugar into another, a reaction which is common in living organisms, is a simple instance of the action of an enzyme In words this means that in the presence of water a molecule of sucrose may split into two smaller molecules, one of glucose and the other of fructose. These are sugars which each have the same molecular formula but the atoms are arranged differently in each sugar. A great number of .enzymes bring about hydrolysis-that is a chemical reaction in the presence of water. The process of digestion is brought about by hydrolyzing enzymes. Another group of enzymes brings about oxidation's. i.e. reaction during which pairs of hydrogen atoms are removed from molecules and are combined with oxygen atoms to make water. It seems probable that a single cell may contain a thousand or more different enzymes which bring about a similar number of chemical reactions.