Betsy McClure, age 14, of Bessemer, Ala., for her question:
WHAT DO CARBOHYDRATES DO FOR YOU?
One of the three main classes of foods essential to the body are carbohydrates. The other two 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 the proportion of about one atom each of carbon and oxygen to two atoms of hydrogen.
Carbohydrates are made during photosynthesis, or the food making process, in green plants.
A common simple carbohydrate is a mildly sweet sugar called glucose. Glucose is present, along with other sugars, in corn syrup and honey.
Complex carbohydrates include starch, cellulose and glycogen. They consist of simpler carbohydrates joined together in long chains.
Carbohydrates actually form a large group of compounds and include all sugars and starches. One hundred percent of sugar is carbohydrate. Percentages of the carbohydrates in starches vary from 90 percent in rice to about 53 percent for white bread, about 49 percent for whole wheat bread and about 18 percent in a potato.
Glucose is quickly absorbed into the blood stream. The blood carries it to all cells of the body. Cells use glucose as fuel. Sick people who are unable to take food by mouth can often be fed with a glucose solution injected into the bloodstream.
Fructose is another simple carbohydrate that is easily used by the cells. It is found in many fruits and in honey.
Cows' milk contains about five percent of a simple carbohydrate called lactose, or milk sugar.
Starch is made up of hundreds or even thousands of glucose molecules joined end to end. Starch is found in tiny granules in such foods as beans, corn, wheat and potatoes.
In the body, starch is converted into 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 vlaue. However, certain amounts of it are useful in a person's diet because it adds bulk to food and thus helps digestion.
Even plant eaters, such as horses and cattle, cannot digest cellulose themselves. Their digestive pouches contain bacteria that breaks down cellulose for them. Plant eaters then absorb the digested products and use them as sources of energy.
Glycogen is made up of glucose molecules. Glycogen is similar to starch in structure. Humans and animals convert some of the glucose in 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.