Structure of Di-Saccharides and Polysaccharides
Disaccharides are formed by the condensation of two monosaccharide molecules. These monosaccharides join together by the loss of a water molecule between one hydroxyl groups on each monosaccharide. Such a linkage, which joins the monosaccharide units together, is called glycoside linkage. If two α-glucose molecules are joined together, the disaccharide maltose is formed
Similarly, sucrose (the common sugar) consists of one molecule of glucose and one molecule of fructose joined together. Lactose (or milk sugar) is found in milk and contains one molecule of glucose and one molecule of galactose. If a large number of monosaccharide units are joined together, we get polysaccharides. These are the most common carbohydrates found in nature. They have mainly one of the following two functions- either as food materials or as structural materials. Starch is the main food storage polysaccharide of plants. It is a polymer of α-glucose and consists of two types of chains- known as amylose and amylopectin. Amylose is a water soluble fraction of starch and is a linear polymer of α-D-glucose. On the other hand amylopectin is a water insoluble fraction and consists of branched chain of α-D-glucose.
The carbohydrates are stored in animal body as glycogen which is also a polymer of α -glucose and its structure is similar to amylopectin.
Cellulose is another natural polysaccharide which is the main component of wood and other plant materials. It consists of long chain of ß-D-glucose molecules.
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