Carbohydrates

The carbohydrates are the compounds which provide energy to living cells. They are compounds of carbon, hydrogen and oxygen with a ratio of two hydrogens for every oxygen atom. The carbohydrates we use as foods have their origin in the photosynthesis of plants. They take the form of sugars, starches, and cellulose.

The name carbohydrate means "watered carbon" or carbon with attached water molecules. Many carbohydrates have empirical formuli which would imply about equal numbers of carbon and water molecules. For example, the glucose formula C₆H₁₂O₆ suggests six carbon atoms and six water molecules.

Sugars

The sugars are the carbohydrates which are used directly to supply energy to living organisms. A key group of the sugars have the molecular formula C₆H₁₂O₆. This group includes glucose, which may exist in either straight-chain or ring forms. Others are fructose, galactose, and mannose. Such sugars are called monosaccharides.

Pairs of ring-form sugars can link to form disaccharides such as common table sugar (sucrose), lactose, and maltose. More complicated linked structures form polysaccharides. An important polysaccharide for human metabolism is glycogen, which serves for energy storage in the liver and skeletal muscles.

The pentose sugars ribose and deoxyribose are important in the structure of nucleic acids like DNA and RNA.

Starches

Starches are carbohydrates in which 300 to 1000 glucose units join together. It is a polysaccharide which plants use to store energy for later use. Starch forms in grains with an insoluble outer layer which remain in the cell where it is formed until the energy is needed. Then it can be broken down into soluble glucose units. Starches are smaller than cellulose units, and can be more readily used for energy. In animals, the equivalent of starches is glycogen, which can be stored in the muscles or in the liver for later use.

Foods such as potatoes, rice, corn and wheat contain starch granules which are important energy sources for humans. The human digestive process breaks down the starches into glucose units with the aid of enzymes, and those glucose molecules can circulate in the blood stream as an energy source. Tillery, et al. point out an interesting example of this enzyme-catalyzed breakdown process. If you chew on a piece of bread for a while, it will begin to taste sweet because the enzymes in saliva are already beginning to break down the starch into glucose, a sugar.

Cellulose

Cellulose is a form of carbohydrate in which some 1500 glucose rings chain together. It is the chief constituent of cell walls in living organisms. Wood is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth.

Cellulose molecules tend to be straight chains, and the fibers which result from collections of cellulose molecules have the strength to form the supporting structures of plants. Even though human digestion cannot break down cellulose for use as a food, animals such as cattle and termites rely on the energy content of cellulose. They have protozoa and bacteria with the necessary enzymes in their digestive systems. Cellulose in the human diet is needed for fiber.

Comparison of Starch and Cellulose

Both starches and cellulose are carbohydrates which are classified as polysaccharides since they are composed of chains of glucose molecules. While they are similar, starches can be used as energy sources by the human body while cellulose cannot. Enzymes are important in the metabolism of foods, and these enzymes are very specific. They are somewhat like keys which will fit the geometry of the starch bonds, but not those of the cellulose bonds.