Examples include:
- glycogen - three dimensional strings of glucose that is produced by the liver and stored in 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 glucoses in cellulose are linked together differently than starch, that changes the properties of the molecule. Cellulose cannot be digested by enzymes produced by mammals. 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.
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 very large molecules, consisting of very large, 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. Amylopectin on the other hand, 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 the 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 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 species and age of the plant. Chemical determination of structural carbohydrates of plants normally includes determination of acid or neutral detergent fiber.