Oligosaccharides

Glycosidic linkage. A sugar molecule can combine with an identical or a different type of a sugar molecule. The linkage between two monosaccharide sugar molecules is called a glycosidic linkage or glycosidic bond.
 
This process is condensation and is in fact dehydration synthesis. The reverse process in which the molecule is cleaved with the incorporation of the elements of water is called hydrolysis.

Disaccharides are the most important of oligosaccharides. They occur as constituents of both plant and animal cells. A disaccharide is formed by condensation of two monomers of monosaccharides with elimination of one molecule of water. The important disaccharides are lactose, maltose and sucrose.

Lactose or milk sugar occurs in the milk of mammals and is synthesized in the mammary glands. It may be also be present in the urine during pregnancy. Souring of milk takes place when bacteria found in milk convert lactose into lactic acid.



Maltose or malt sugar is found in germinating seeds and malt. It is also produced during the digestion of starch by α – amylase. Maltose on hydrolysis yields two glucose molecules. This shows that it is made up of two glucose units.

Cellobiose, a disaccharide formed during the hydrolysis of cellulose, is identical with maltose except that it has a β – 1,4 glycosidic linkage.

Isomaltose, which is formed during the hydrolysis of certain polysaccharides, also resembles maltose, except that it has an α – 1,6 linkage.

Surcose or cane sugar is widely distributed in higher planet, although the commercial sources are solely sugar cane and sugar beets. Sucrose consists of one D – glucose unit and one D – fructose unit.

Trisaccharides have three monosaccharide units and the general formula C18H32O16. Raffinose consists of three monosaccharide units D-glucose, D-fructose and D-galactose. Among the other trisaccharides are mannotriose, rabinose, rhamminose, gentianose and melezitose.