b -D-fructose. Since the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non reducing sugar. Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (– .
°) is more than dextrorotation of glucose (+ . °), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar . (ii) Maltose : Another disaccharide, maltose is composed of two a -D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II).
The free aldehyde group can be produced at C1 of second glucose in solution and it shows reducing properties so it is a reducing sugar. (iii) Lactose : It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of b -D-galactose and b -D-glucose. The linkage is between C1 of galactose and C4 of glucose.
Free aldehyde group may be produced at C- of glucose unit, hence it is also a reducing sugar. Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. These are the most commonly encountered carbohydrates in nature. They mainly act as the food storage or structural materials.
(i) Starch : Starch is the main storage polysaccharide of plants. It is the most important dietary source for human beings. High content of starch is found in cereals, roots, tubers and some vegetables. It is a polymer of a -glucose and consists of two components— Amylose and Amylopectin .
Amylose is water soluble component which constitutes about - % of starch. Chemically amylose is a long unbranched chain with - a -D-(+)-glucose units held together by C1– C4 glycosidic linkage. Amylopectin is insoluble in water and constitutes about - % of starch. It is a branched chain polymer of a -D-glucose units in which chain is formed by C1–C4 glycosidic linkage whereas branching occurs by C1–C6 glycosidic linkage.
. . Polysaccharides (ii) Cellulose : Cellulose occurs exclusively