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  Polysaccharides, Biology tutorial

Introduction:

The majority of carbohydrates found in nature found as polysaccharides. They signify third class of carbohydrate types also known as glycans. Hydrolysis of polysaccharides yields completely monosaccharides or products associated to monosaccharides, most often D-Glucose. Though, D-mannose, Dgalactose, D-fructose, D - arabinose and D-glucoronic acid, D. galacturonic acid, D - glucosanmine, sialic acids and uronic acids also found as constituents of polysaccharides. Different polysaccharides vary not only in constituent monosaccharide composition but vary also in molecular weight and other structural features. Therefore, some polysaccharides are linear whereas some are highly branched.

Types of polysaccharides:

Though there are different indices which can be utilized in classifying polysaccharides, the most extensively utilized index is product of hydrolysis of polysaccharides, whether they are similar in which case they are known as homopolysaccharides or whether products of hydrolysis are different in which case known as heteropolysaccharides.

Homopolysaccharides:

These are polysaccharides which on hydrolysis provide only one kind of monomeric units. There are different kinds of homopolysaccharides that comprise: cellulose, starch and glycogen.

i) Cellulose:

Cellulose is unquestionably the most plentiful carbohydrate and the most abundant organic compound in world, comprising 50% or more of all carbon in vegetation. It is linear homopolysaccharide made up of Dglucopyranose units related by b (1-4) linkages. On partial hydrolysis of cellulose, a diasaccharide cellobiose is generated while on complete hydrolysis of cellulose, glucose units are generated.

1061_Repeating cellobiose unit of cellulose.jpg

ii) Starch:

Starch is the polymer of glucose units connected in a 1-4 linkages. It acts as nutritional reservoir in plants. Repeating diasaccharide unit in starch thus is maltose.

723_Starch.jpg

Native starches are the mixture of 2 compounds which are separable from each other, Amylose and Amylopectin. Amylose is the component which is thought to be long unbranched chain of glucose linked together by 1-4 bonds and amylopectin that is a branched chain polysaccharide. Glucose residue which is situated at each point of branching is substituted on carbon 4 and carbon 6. Isolation of a 1-6 diasaccharide, isomaltose, from products of incomplete hydrolysis of amylopectin proves substitution of branch points.

iii) Glycogen:

Glycogen is another hompolysaccharide of glucose. It is also storage polysaccharides of animals which acts as a source of fuel, providing similar purpose as starch. It is like amylopectin in that it is branched polysaccharide. It is though different from amylopectin in that its branch point takes place every 8 - 10 units of glucose. Glycogen is hydrolysable by α and β - amylases to yield maltose, glucose and limit dextrins.

Heteropolysaccharides:

These are class of carbohydrate made up of repeating monomeric unit which are different. Several kinds of heteropolysaccharides do exist.

i) Pectins:

These are heteropolysaccharides comprising of arabinose, galactose and galactouronic acid.

ii) Hyaluronic acid:

This is heteropolysaccharide comprising of repeating units of D-glucoromic acid and N-acetyl D glucosamine. Monosaccharides are joined together by β (1 - 3) bonds to create diasaccharides that is joined by β (1 - 4) bond to next repeating unit. It is soluble in water and creates viscous solution.

2330_hyaluronic acid.jpg

iii) Chondroitin:

This is also heteropolysaccharide like hyaloronic acid in composition except that amino sugar is N-acetyl D-galactosammine not N-acetyl D-glucosamine sulfate esters are found at C-4 or C-6 of amino sugar of chondroitin forming it chondroitin- 4-sulfate or chondroitin-6-sulfate respectively.

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