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Polysaccharides

Polysaccharides are formed when a many-hundreds or even thousands- of monosaccharide units 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.

The most important polysacharides are cellulose and starch. Both are obtained from plant source. These are synthesised by plants using water and carbondioxide through photosynthesis. Both cellulose and starch are made up of D- glucose units. Hence their general formula is (C₆H₁₂O₆)_n. Where n is the natural number and it denotes the number of glucose units present in the polysaccharide

Polysccharide	Monosaccharide units
Cellulose	Glucose nits linked together by beta-linkages.
Starch	Glucose nits linked together by alpha-linkages.
Glycogen	Glucoses units linked together linearly by α(1→4) glycosidc bonds
Pectin	1,4-linkedα-D-galactosyluronic acid

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 α-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 α-linkages involving C1 of one glucose unit with C4of 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 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 of Starch : 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.

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Cellulose

Cellulose a polysaccharide made up of D-glucose units condensed through β(1→4)-glycosidic bonds. It is a straight chain polymer: unlike starch, no coiling occurs. Cellulose is found primarily in the cell wall of plants and it is hydrophilic in nature i.e. insoluble in water.Complete hydrolysis of cellulose yields d- glucose units. It is the major component of wood and plant fibers like cotton. Cellulose is non-reducing sugar. Its molecular weight is very high (250000 to 1000000 or more). There are about 1500 glucose units per molecule of cellulose.

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