What Are Disaccharides and Oligosaccharides

Most carbohydrates in nature contain more than one monosaccharide unit. Those that contain two units are called disaccharides, those that contain three units are called trisaccharides, and so forth. The more general term, oligosaccharide, is often used for carbohydrates that contain from 6 to 10 monosaccharide units. Carbohydrates containing larger numbers of monosaccharide units are called polysaccharides. 

In a disaccharide, two monosaccharide units are joined by a glycosidic bond between the anomeric carbon of one unit and an —OH of the other. Sucrose, lactose, and maltose are three important disaccharides. 

Sucrose (table sugar) is the most abundant disaccharide in the hiological world. It is obtained principally from the juice of sugarcane and sugar heets. In sucrose, carhon 1 of a-D-glucopyranose bonds to carbon 2 of D-fructofuranose by an a-1,2-glycosidic bond  

Because the anomeric carbons of both the glucopyranose and fructofuranose units are involved in formation of the glycosidic bond, neither monosaccharide unit is in equilibrium with its open-chain form. Thus, sucrose is a nonreducing sugar. 

Lactose, the principal sugar present in milk, accounts for 5 to 8% of human milk and 4 to 6% of cow s milk. This disaccharide consists of D-galactopyranose, bonded by a )3-l,4-glycosidic bond to carbon 4 of D-glucopyranose  

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