Polysaccharides are formed when a large number (hundreds to even thousands) of monosaccharide molecules join together with the elimination of water molecule. Thus, polysaccharides may be regarded as condensation polymers in which the monosaccharides are joined together by glycosidic linkages. Some important polysaccharides are:

1.   Cellulose                 2.         Starch

3.   Glycogen                4.         Gums and

5.   Pectins

Starch

It is a polymer of glucose.  Its molecular formula is (C6H10O5)n where the value of  n (200 – 1000) varies from source to source. It is the chief food reserve material or storage polysaccharide of plants and is found mainly in seeds, roots, tubers, etc. Wheat, rice, potatoes, corn, bananas etc., are rich sources of starch.

Starch is not a single compound but is a mixture of two components –  amylose (10 to 20%) and amylopectin (20 to 80%). Both amylose and amylopectin are polymers of  α-D-glucose.

Amylose is a linear polymer of α-D-glucose. It contains about 200 glucose units which are linked to one another through a-linkage involving C1 of one glucose unit with C4 of the other as shown below:

Amylopectin, on the other hand, is a highly branched polymer. It consists of a large number (several branches) of short chains each containing 20-25 glucose units which are joined together through a-linkages involving C1 of one glucose unit with C4 of the other. The C1 of terminal glucose unit in each chain is further linked to C6 of the other glucose unit in the next chain through C1 – C6 α-linkage. This gives amylopectin a highly branched structure as shown below.-

 

Hydrolysis: Hydrolysis of starch with hot dilute acids or by enzymes gives dextrins of varying complexity, maltose and finally D-glucose. Starch does not reduce Tollen’s reagent and Fehling’s solution.

Uses:  It is used as a food. It is encountered daily in the form of potatoes, bread, cakes, rice etc. It is used in coating and sizing paper to improve the writing qualities. Starch is used to treat textile fibres before they are woven into cloth so that they can be woven without breaking. It is used in manufacture of dextrins, glucose and ethyl alcohol. Starch is also used in manufacture of starch nitrate, which is used as an explosive.

Illustration 1. Nature of glycosidic linkage present in amylopectin                                             

                        (A)  α – 1, 6 linkage                                (B)  α – 1, 4 linkage

                        (C)  β – 1, 4 linkage                                 (D)  both (A) and (B) 

Solution: (B)

Amylopectin is a branched chain saccharides consist of α – D – glucose unit join straight chain by α, 1, 4 linkage & both the straight chain are ionized α – 1, 6 linkage

Illustration 2.

A.

B.

C.

D.

Solution:

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CBSE Class 12 Chemistry Biomolecules All Topic Notes CBSE Class 12 Chemistry All Chapters Notes

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