These are the carbohydrates which are made of large number of monosaccharides units. They are naturally occurring condensation polymers in which large numbers of monosaccharide units are joined together through glycosidic linkages. They are colourless, tasteless amorphous powders. They play a vital role in plant and animal life. Let us study three important polysaccharides starch, cellulose and glycogen.
   
Starch, (Amylum), (C₆H₁₀O₅)_n

It is abundant source of carbohydrate in human diet. It is a major food reserve material of plants and occurs mainly in seeds, fruits, tubers and roots of the plants. The important sources of starch are wheat, rice, maize, potatoes, legumes and other vegetables.

Starch is the polymer of D-(+)-glucose. It consists of two components:
   
Amylase, the water soluble fraction, and a linear polymer of D-(+)-glucose units. It gives blue colour with iodide.
   
Amylopectin, the water insoluble fraction, and consists of linear as well as branched chain polymers of D-(+)-glucose.

Natural starch contains approximately 15-20% of amylose and 80-85% of amylopectin does not give blue colour with iodide. A molecule of amylose may contain 100-1000 glucose units whereas a molecule of amylopectin may contain 2000-3000 glucose units. In amylose as well as amylopectin the D-glucose units are linked through -glycosidic linkages between C-1 of one glucose unit and C-4 of the next glucose unit. Branching in amylopectin occurs through C₁-C₆ glycosidic linkage.

The section of amylose as well as amylopectin has been given follows:

On hydrolysis with dilute acids or enzymes, starch breaks down into maltose and finally D-glucose.
                             

   
Cellulose, (C₆H₁₀O₆)

Cellulose occurs exclusively in plants. It is the most abundant organic substance in plant kingdom. It is a chief structural material of cell walls of all the plants. It is also the chief component of cotton, wood and jute. Wood contains 45-50% while cotton contains 90-95% cellulose. It may be noted that over 50% of the total cellulose. It may be noted that over 50% of the total organic matter of the living world is cellulose.

Structurally, cellulose is a linear polymer of D-glucose units joined by -glycosidic units’ linkage between C-1 of one glucose unit and C-4 of the next glucose unit as shown below:

The chains are held together by hydrogen bonds between glucose units of adjacent stands. The chains are so arranged as to constitute the bundles. The lends rigidity to its structure.

It may be noted that amylose as well as cellulose are formed by condensation of D-glucose units are joined together, in amylose, by -glycoside linkage while in cellulose D-glucose units are joined together by -glycoside linkage.

Cellulose does not reduce Tollen’s reagent or Fehling’s solution. It is not fermented by yeast. Cellulose cannot be hydrolysed easily. However, when it is heated with dilute sulphuric acid, under pressure, it undergoes hydrolysis and yields D-glucose.

Cellulose forms many useful products when treated with suitable chemical reagents. Some of the important products are:

Celluloid, which is used in films.

Gun cotton, which is an explosive.

Cellulose acetate is used in plastics, for wrapping films and nail polishes.

Methyl cellulose is used in cosmetics, paste and for fabric sizing.

Ethyl cellulose is used in plastic coats and films.

On treatment with concentrated sodium hydroxide, cellulose forms a translucent mass which imparts a silky lustre to cotton. The process is called mercerization and cotton so produced is called mercerized cotton.
   
Glycogen

Glycogen is the principal reserve of carbohydrates in animals. The molecular structure of glycogen is similar starch. However, one main difference between glycogen and amylopectin is that average chain length in glycogen is 10^-14 while in amylopectin it is 25-30 glucose units.

Glycogen is present in liver, muscles and brain. When the body needs glucose, enzymes break glycogen into glucose.

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