Enzymes act as CATALYSTS. This means that they lower activation energy in chemical reactions so that they can occur at the temperature cells exist in. For instance, human cells must have all their chemical reactions take place at 98.6 degrees Farhrenheit.

The following graph shows how activation energy is decrease with enzymes (+Enzyme) in comparison to without enzymes (-Enzyme).

In this graph, "A" represents the reactants and "B" represents the products of a chemical reaction.