Carbohydrates

Carbohydrates are a group of naturally occurring compounds that are essential sources of energy for all living things. Carbohydrates are manufactured by green plants, which use them for energy and to build their cell walls. Animals are able to both use and store the energy that carbohydrates contain. Carbohydrates include such organic compounds as sugars, starches, and cellulose.

Chemically, carbohydrates are natural compounds of carbon, hydrogen, and oxygen and were described as "carbon hydrates" by the chemists who first encountered them (since they mistakenly thought that carbohydrates were simply a compound of carbon and water). We now know that a carbohydrate molecule is made up of equal numbers of carbon and oxygen atoms (the smallest units of elements), with twice as many hydrogen atoms. Almost all the energy consumed by living things comes from carbohydrates that are manufactured by plants.

CARBOHYDRATES PRODUCED BY PHOTOSYNTHESIS

Plants manufacture carbohydrates using the process known as photosynthesis. Because plant cells contain chloroplasts inside their cells, plants are able to use the sunlight absorbed by their leaves that they combine with the water from the soil and the carbon dioxide from the air to make a simple form of sugar called glucose, which is a carbohydrate. Glucose is packed with energy and plants use it to make cellulose (their building

material). What they do not use, they store as starch in the form of seeds, roots, or fruits. Animals that eat plants take in their carbohydrates and reverse a plant's metabolic process. This occurs by the process of digestion in which animals break down complex carbohydrates into the original glucose that animals are then able to absorb into their bloodstream. The animals' bloodstream then carries the glucose to every cell in the body and cells turn the glucose into energy by a process known as oxidation, in which glucose combines with oxygen and releases energy. In a sort of reverse process, an animal turns a plant into the energy that the plant "captured" from the sun.

TYPES OF CARBOHYDRATES

There are three major types of carbohydrates: monosaccharides, disaccharides, and polysaccharides. As the names imply, each is more complex than the other. Monosaccharides are the simplest of the carbohydrates since they consist of a single carbohydrate unit that cannot be broken down into anything simpler. The three most common sugars in this group are glucose, fructose, and galactose. Glucose is found in fruits and vegetables; fructose is found in honey and some fruits and vegetables; and galactose is found in milk. Disaccharides are more complex since they consist of two joined monosaccharides. The three most nutritionally important disaccharides are sucrose (ordinary table sugar), maltose (found in sprouts), and lactose (found in the milk of mammals). Polysaccharides are made up of ten or more monosaccharides and are often highly complex chains of long molecules. Carbohydrates are stored in the form of polysaccharides (as starch in plants and as glycogen in animals).

THE IMPORTANCE OF CARBOHYDRATES

Much has been learned about the role carbohydrates play in our diet and health. For instance, it was once believed that avoiding carbohydrates like bread, potatoes, and pasta would keep a person slim. Scientists now know that this is not the case since not all carbohydrates are created equal and that the type of carbohydrate eaten is more important than the total amount consumed. Low-fiber carbohydrates are digested very quickly and behave like sugar, giving a quick peak in energy. On the other hand, high-fiber carbohydrates like whole grains are digested more slowly, providing a steady and gradual source of energy. Athletes have learned that continuous exercise, such as running a marathon, can burn up all of the body's supply of glycogen in about two or three hours. After that the body must start converting its stored fat into glucose. Since this is a fairly slow process, the athlete operating on stored fat becomes fatigued. To overcome this, some athletes whose competition lasts longer than ninety minutes began the practice of "carbohydrate loading" before competition. Beginning a week before competition, athletes consume an increasing amount of carbohydrate-rich foods daily.

The importance of carbohydrates to all living things cannot be overemphasized since it is the primary source of energy for plants and animals. We have learned that "starchy foods," such as potatoes, which were once thought to be inadequate for humans in large amounts, are in fact a necessary part of every meal. The U. S. Food and Drug Administration now recommends that people's carbohydrate consumption should be increased and fat consumption should be reduced.

[See alsoFruits; Photosynthesis ]