Polysaccharides

POLYSACCHARIDES

Carbohydrates which contain more than 10 monosaccharide units are known as polysaccharides. Example : Starch, cellulose, glycogen, inulin etc.

1. HOMOPOLYSACCHARIDES

Starch:

a) Source:

Plant materials such as roots, tubers, stem, vegetables, fruits and cereals are the main sources of starch.

b) Structure:

Starch is the nutritional reservoir in plants. Starch is a homopolysaccharide consists of only α-D-glucose. The two chief constituents of starch are (i) Amylose (15-20%) and (ii) Amylopectin (80 - 85%).

Amylose forms the inner portion of the starch grain and is soluble in water. It is linear, non-branched polymer of glucose. The glucose residues are united by α(1-4) linkage. The molecular weight of amylose is 60,000.

Amylopectin forms the outer covering of the starch grain and is insoluble in water. It is a highly branched polymer of glucose. The glucose residues are united by α(1- 4) linkages in the chains and by α (1 - 6) at the branch points. Its molecular weight is 2,00,000. It is like glycogen except its lower degree of branching.

c) Hydrolysis of starch :

Starch is hydrolysed both by acids and enzymes. Both amylose and amylopectin are rapidly hydrolysed by α-amylase which is secreted by salivary glands and pancreas. α-Amylase acts upon starch and hydrolyses it into finally maltose molecules.

Uses : Starch is used as

(i) as a food material.

(ii)for the manufacture of glucose and alcohol.

(iii) in paper industry.

(iv) in textile industry.

(v) in printing. to prepare starch acetate, nitrostarch etc.

(vii) for making adhesives.

(viii) as an indicator.

Glycogen :

a) Source :

Glycogen is the carbohydrate reserve in animals; hence often referred as animal starch. It is present in the high concentration in liver, muscle and brain.

b) Structure :

Glycogen is very large, branched polymer of glucose residues. The structure of glycogen is similar to that of amylopectin with more number of branches. The glucose units in glycogen are linked by α(1-4) glycosidic bonds and α-(1-6) glycosidic bonds at branching points. Branching occurs about once in 10 units. The molecular weight (upto 1 × 10⁸) and the number of glucose units (upto 25,000) vary in glycogen depending upon the source.

Uses :

·     Excess carbohydrate in the body are deposited as glycogen.

·     Animal glycogen is used as food.

2. HETEROPOLYSACCHARIDES (HETEROGLYCANS)

Glycosaminoglycans :

·     Glycosaminoglycans are otherwise known as mucopolysaccharides.

·     These are heteroglycans made up of repeating unit of aminosugars and uronic acids.

·     Because of the presence of charged groups (carboxyl group, sulphate group, acetylated amino group), they attract water molecules and so they produce viscous solutions.

·     Some of the mucopolysaccharides are found in combination with protein to form mucoproteins (or) mucoids (or) proteoglycans. They contain 95% carbohydrate and 5% protein.

·     Examples : i) hyaluronic acid ii) heparin iii) chondroitin sulpahte iv) keratan sulphate v) dermatan sulphate.

(i) Heparin:

·     It is a mucopolysaccharide present in liver, lung, spleen, kidney, and blood.

·     It is a blood anticoagulant.

·     Heparin is composed of alternating units of N-sulpho-glucosamine-6 sulphate and L-iduronate-2-sulphate.

·     These two molecules are held together by α(1-4) glycosidic bond.

·     Its molecular weight 20,000.

ii) Hyaluronic acid :

·     It is a mucopolysaccharide present in syanovial fluid, vitreous humor of eyes, cartilage tissues, loose connective tissues and in bacteria.

·     It consists of repeated units of α-glucuronic acid and N-acetylglucosamine.

·     These two molecules are held together by α(1-3) glycosidic bond.

·     It is an unbranched chain polymer.

·     Its solutions are viscous and hence acts as lubricant and shock absorbent in joints.

·     In tissues, it acts as a barrier and permits the metabolites to pass through but not the bacteria and other infectious agents.

·     Hyaluronic acid contains about 250-25,000 disaccharide units, held by α-(1-4) glycosidic bonds with a molecular weight upto 4 million.

·     The α(1-4) linkage in hyaluronic acid is cleaved by the enzyme hyaluronidase. This enzyme is present in high concentration in testes, seminal fluid and certain snake venom.