enzyme definition

An enzyme is a substance that alters the rate of a chemical reaction without itself changing during the process.  Enzymes are essential in the correct functioning of organisms and cells.  Lacking a particular enzyme can result in a host of problems due to the lack of chain reactions that the enzyme is responsible for starting.  An example of the importance an enzyme plays is when someone is lactose intolerant; such a person lacks the enzyme in his/her stomach to break down milk sugar, causing uncomfortable symptoms to result.