Margaret Bell, age 14, of Prescott, Ariz., for her question:

WHAT IS AN ENZYME?

An enzyme is a protein molecule that speeds up chemical reactions in animals and plants. All living cells make enzymes, but enzymes are not alive.

Enzymes function by altering other animal or plant molecules. They combine with the altered molecules to form a complex molecular structure in which chemical reactions take place. The enzyme, which remains unchanged, then separates the product by the reaction.

Enzymes thus serve as catalysts. A single enzyme molecule can perform its entire function a million times a minute. Chemical reactions occur thousands or even millions of times as fast in the presence of enzymes as they would without them.

More than 1,000 different kinds of enzymes are found in the human body. Each one performs a specific job.

Some break down complex substances into simpler ones. Others build complex compounds from simpler ones. Most remain in the cells where they were formed, but some do their work elsewhere.

Two Greek words meaning "in yeast" were combined to form the word "enzyme." Many scientists who studied enzyme reactions in the 1800s thought they were caused by yeast enzymes.

First to isolate a pure enzyme in the form of crystals was an American biochemist, James Sumner, who made his discovery just 63 years ago. In 1926, he extracted the enzyme urease from beans.

In 1969, scientists created the first man made enzyme, ribonuclease, from amino acids. This enzyme breaks down ribonucleic acid into molecules of other amino acids.

Enzymes play important roles in digesting food. The salivary glands produce an enzyme called salivary amylase, which splits carbohydrates into simpler chemicals. Then an enzyme called pepsin is secreted by the walls of the stomach, which acts on proteins. Next, pancreatic lipase is secreted by the pancreas into the small intestine, where it breaks down fats.

Doctors use medicines containing enzymes to help clean wounds, dissolve blood cells, relieve certain forms of leukemia and check allergic reactions to penicillin. They also diagnose a number of diseases by measuring the amount of various enzymes in blood and other body fluids. Such diseases include anemia, cancer, leukemia and heart and liver ailments.

Manufacturers use enzymes to make products such as detergents. Enzymes are also used in to make antibiotics, beer, bread, cheese, coffee, meat tenderizers, vinegar, vitamins and many other products.

In the future, scientists say that enzymes will be used to change raw sewage into useful products. They might also be used to help get rid of spilled oil that harms oceans and lakes.

Enzymes are too small to be seen even with the most powerful light microscope.