This is because chemical reactions that occur inside living organisms can't happen without a catalyst that would make these reactions happen and control them. Catalysts, in reactions other than biochemical, generally are common inorganic substances which have uses outside of being catalyst for a reaction. For example, platinum is used to catalyse the reaction that breaks down nitrogen oxides in car exhaust, yet platinum has many other uses. Most biological catalysts, or enzymes, on the other hand are very specific. They exist and are created with only one purpose, to act as a catalyst for one specific reaction biochemical reaction.
Enzymes are proteins which are used to catalyze a specific reaction.
The exact nature of how enzymes work is not know. If is only recently that scientist have had any clue as to the mechanism of enzyme catalyzation. Made up of a complex of amino-acids, enzymes are part of every chemical reaction in living things. Examples of enzyme aided reactions include all digestion, growth and building of cells, any breakdown of substances such as vitamins, and nutrients, all reactions involving transformation of energy. Reactions are also controlled by enzymes.