Disaccharides and Polysaccharides

James Richard Fromm


Many of the most important carbohydrates are the more complex disaccharide and polysaccharides rather than the simpler monosaccharides.

Disaccharides

When two monosaccharide molecules react or condense with each other, losing a water molecule in the process, the product is a disaccharide. Several disaccharides are common in nature, the most well-known being sucrose. Sucrose is also called cane sugar, beet sugar, table sugar, and dextrose. Sucrose is a disaccharide of glucose and fructose. Other disaccharides which are important in nature are lactose or milk sugar, which is a disaccharide of glucose and galactose, and two disaccharides of glucose with itself, maltose and cellobiose, which differ in the joining of the two glucose rings. (The different ring joinings are also distinguished by Greek letters.)

Disaccharides will react with water, especially in the presence of enzymes such as those supplied by honeybees, to form invert sugar. Invert sugar is a mixture of the two monosaccharides and is responsible for the sweet taste of honey.

Polysaccharides

A polysaccharide is a polymer of one of the monosaccharides. All of the well-known polysaccharides found in nature are polymers of glucose. Several familiar materials are polymers of glucose: starch, which is poly-alpha-glucose; cellulose, which is poly-beta-glucose; and glycogen, which is a differently branched form of poly-alpha-glucose. Cellulose is used to form plant structures since it is not soluble in water, while plants use starch for energy storage. Animals store energy as glycogen or, for longer-term storage, convert glycogen and other carbohydrates to lipids.


Previous Topic: Introduction to Carbohydrates

Next Topic: Introduction to Lipids

Return To Course Outline


Copyright 1997 James R. Fromm