Darrell Wheeler, age 13, of Helena, Mt., for his question

WHAT DOES A CARBOHYDRATE DO?

A carbohydrate is one of the three main classes of foods essential to the body. The others are proteins and fats. Carbohydrates provide living things with much of the energy they need to operate muscles and nerves and to build and repair plant and body tissues.

Carbohydrates contain the chemical elements carbon, hydrogen and oxygen. These elements are in carbohydrates in about the proportions of one atom each of carbon and oxygen to two atoms of hydrogen.

Carbohydrates form a large group of compounds and include all sugars and starches. The sugar used to sweeten food and drink is a carbohydrate. So is cellulose, the substance that makes up a large part of the cell walls of plants and glycogen, a kind of starch that is found in animals.

A common simple carbohydrate is a mildly sweet sugar called glucose. Glucose is present, along with other sugars, in corn syrup.

Glucose is rapidly absorbed into the blood stream. The blood carries it to all cells of the body. Cells use glucose as fuel.

Sick persons who are unable to take food by mouth can be "fed" with a glucose solution injected into the blood stream.

Fructose, another simple carbohydrate, also is easy to use by cells. It has the same chemical formula as glucose, but its atoms are arranged differently. Both fructose and glucose are abundant in honey.

Table sugar is sucrose. This carbohydrate has a molecule of glucose linked to a molecule of fructose. Brown sugar and maple sugar are mainly sucrose.

Complex carbohydrates include starch, cellulose and glycogen. They consist of simple carbohydrates joined together into long chains.

Starch is made up of hundreds or thousands of glucose molecules joined end to end. Starch occurs in tiny granules or grains in such foods as beans, corn, potatoes and wheat. In the body, starch is converted into a sugar called maltose, which is converted into glucose.

Cellulose is similar to starch because it is made up of many glucose molecules joined in chains. But, cellulose cannot be digested by the human body and has no food value. However certain amounts of it are useful in a person's diet because it adds bulk to food and thus helps digestion.

Glycogen is made up of glucose molecules. Glycogen is similar to starch in structure. Human beings and animals convert some of the glucosein their blood to glycogen and store it in the liver, muscles or other tissues.

The liver of a well fed animal may contain more than five percent of glycogen.Some of this glycogen is changed back into glucose when the body needs energy quickly.