Carbohydrates are a widely diverse group of compounds that are common in nature. Chemically, carbohydrates are molecules composed of carbon, hydrogen, and oxygen – usually in the same ratio as that found in water (H2O). Typical carbohydrates are composed of strings or chains of monosaccharides – that is, chains of individual sugars. A monosaccharide (mono = one, saccharide = sugar) is the smallest carbohydrate unit. The type of monosaccharides in the chain, length of the chain, and method of linking all determine the carbohydrate composition. A listing of some of the saccharides include:
- Monosaccharides – single molecules, usually with 5 or 6 carbons
- pentoses – sugars with 5 carbons, including arabinose, xylose, ribose
- hexoses – sugars with 6 carbons, including glucose, fructose, galactose, mannose
- Disaccharides – sugars containing 2 monosaccharides
- sucrose – glucose + fructose
- maltose – glucose + glucose
- lactose – glucose + galactose
- trisaccharides – combinations of three monosaccharides
- polysaccharides – combinations of a large number of monosaccharides into complex threedimensional forms.
- glycogen – three-dimensional glucose strings produced by the liver and stored in the liver and muscles. Glycogen serves as a glucose reserve for animals.
- Starch – three-dimensional strings of glucose stored in plants. Like glycogen, starch is one of the glucose reserves used in plants. It is highly digestible by animals and is a ready source of glucose.
- Cellulose – Linear chains of glucose produced by plants. The glucose in cellulose is linked together differently than starch, which changes the properties of the molecule. Enzymes produced by mammals cannot digest cellulose. However, some bacteria do produce cellulase, the enzyme that breaks down cellulose.
- Lignin – the polysaccharide that comprises the woody parts of plants. Cobs, hulls, and the woody portions of trees and shrubs all contain this complex carbohydrate. Lignin is largely indigestible and is therefore unavailable to animals. Some classify lignin in a separate category of compounds due to the complexity of the chemical structure.
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Sugars generally don’t exist in nature in linear chains. Instead, they normally exist in a ring structure, which may be in an alpha or beta form, depending on how the ring is formed. The two monosaccharides in lactose (glucose and galactose) are combined into a disaccharide using a specific type of chemical linkage. This linkage (?-1,4-galactosidic linkage) joins the two monosaccharides to form one disaccharide. There are other types of chemical linkages (?-1,4- linkage) which is used to join other disaccharides together. Polysaccharides are huge molecules, consisting of huge, sometimes complex structures of monosaccharides. Polysaccharides include starch, cellulose and lignin.
Starch is comprised of large strings of glucose molecules. There are many types of starches with different overall structures. However, the two major units of starch are amylose and amylopectin. Amylose is composed of linear chains of glucose joined by ?-1,4-glucosidic linkages. On the other hand, Amylopectin is comprised of glucose molecules connected into branched structures by ?-1,6-glucosidic linkages. These linkages allow amylopectin to branch into far more complex structures than simple linear chains of glucose. Cellulose is the primary structural component of plants. It is found primarily in the cells walls and is a primary component of the fiber component of animal feeds.
The structure of cellulose is similar to that of amylose in starch – that is, linear chains of glucose, except that the glucose molecules are joined by ?-1,4-glucosidic linkages. This linkage is the reason that humans cannot digest cellulose. Lignin is actually a class of compounds that provides a woody structure to cell walls. The characteristics of lignin vary depending on the plant species, maturity and method of determination. However, lignin is clearly important to nutrition, as it is the component that limits the digestibility of fiber sources such as hay. Plant cell walls are complex arrangements of cellulose, hemicellulose and lignin. This contributes significantly to the overall digestibility of the fiber. The proportion of each component depends on the species and age of the plant. Chemical determination of structural carbohydrates of plants normally includes determination of acid or neutral detergent fiber.