Assistant Professor Department of Botany Introduction Polysaccharides consist of repeating units of monosaccharides or their derivatives held together by glycosidic bonds Polysaccharides are linear as well as branched polymers ID: 1048688
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1. PolysaccharidesAmbarish BhuyanAssistant ProfessorDepartment of Botany
2. Introduction Polysaccharides consist of repeating units of monosaccharides or their derivatives, held together by glycosidic bonds. Polysaccharides are linear as well as branched polymers.They are of two types:-homopolysaccharides and heteropolysaccharidesHomopolysaccharides: these on hydrolysis yield only a single type of monosaccharide. Ex- starch, glycogen, cellulose, chitin, pectin, inulin etc.Heteropolysaccharides: These on hydrolysis yield a mixture of a few monosaccharides or their derivatives. Ex- Mucopolysaccharides
3. StarchStarch is the carbohydrate reserve of plants which is the most important dietary source of higher animals, including man.High content of starch is found in cereals, roots, tubers, vegetables etc. Starch is a homopolymer composed of D-glucose units held by α-glycosidic bond.Starch is a white soft amorphous powder and lacks sweetness. It is insoluble in water, alcohol and ether at ordinary temperature.
4. Starch consists of two polysaccharide components- water soluble amylose(15-20%) and a water insoluble amylopectin (80-85%). Chemically, amylose is a long unbranched chain with 200-1000 D-glucose units held by α(1→4) glycosidic linkages. Amylopectin is a branched chain with α(1→6) glycosidic bonds at the branching points and α(1→4) linkages everywhere else.
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6. Glycogen is the major reserve food in animals and is often called animal starch. Glycogen is stored in the liver and muscles of animals.Glycogen is a branched-chain polysaccharide and resembles amylopectin very much in structure, rather than amylose, but has somewhat more glucose residue per molecule and about one-and-a-half times as many branching points. Also the chains are shorter and hence the molecule is even more highly branched and more compact.Glycogen
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8. Inulin It is the storage form of carbohydrate in the members of the family – Compositae, such as dahlias, artichokes etc. Inulin is stored in the tubers of the dahlia and artichoke and in the roots of dandelion. It is also found in onion and garlic.Inulin has a molecular weight of about 5000 and consist of about 30-35 fructose units per molecule. It is formed in the plants by eliminating a molecule of water from the glycosidic OH group on carbon atom 2 of one β-D-fructose unit and the alcoholic OH group on carbon atom 1 of the adjacent β-D-fructose unit. In inulin, the fructose residues are thus, joined together by β(2→1) glycosidic bonds. On hydrolysis, however, inulin also yields 2 glucose units, one located somewhere in the centre and the other at the reducing end of the chain.
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10. Cellulose Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom. It is a predominent constituent of plant cell wall. Cellulose is totally absent in animal kingdom.Cellulose is composed of β-D-glucose units linked by β(1→4) glycosidic bonds. Hydrolysis of cellulose yields a disaccharide cellobiose, followed by β-D-glucose .Cellulose, though not digested, has great importance in human nutrition. It is a major constituent of fibre, the non-digestable carbohydrate. The function of dietary fibre include decreasing the absorption of glucose and cholestrol from intestine.
11. Cellulose is a fibrous, tough, white solid, insoluble in water but soluble in ammonical supric hydroxide solution. It gives no colour with iodine and lack sweetness.
12. PectinPectins are found as intercellular substances in the tissues of young plants and are especially abundant in ripe fruits such as guava, apples and pears. They are the components of middle lamella found between the cell wall and adjacent cells.Pectin is a polysaccharide of α-D-galacturonic acid where some of the free carboxyl groups are either partly or completely, esterified with methyl alcohol and others are combined with calcium or magnesium ions.
13. ChitinChitin is probably the most abundant polysaccharide of nature after cellulose.it is also one of the most abundant bioploymer on the earth. It is found in fungi but principally among the arthropods( crabs and insects).Chitin is closely related to cellulose. Here the alcoholic OH group on carbon atom 2 of β-D-glucose units is replaced by an N-acetylamino group. It is , thus , a linear polymer of N-acetyl-D-glucosamine units joined together by β(1→4) glycosidic linkages.
14. Structure of ChitinN-acetyl-D-glucosamine
15. MucopolysaccharidesMucopolysaccharides are heteroglycans made up of repeating units of sugar derivatives, namely amino sugars and uronic acids. these are more commonly known as glycosaminoglycans (GAG). Acetylated amino groups, beside sulfate andcarbonyl groups are generally present in GAG structure.The presence of sulfate and carbonyl groups contributes to acidity of the molecules, making them acid mucopolysaccharide. Some of the mucopolysaccharides are found in combination with proteins to form mucoproteins or proteoglycans. The important mucopolysaccharides include hyaluronic acid, heparin, dermatan sulfate etc.
16. Heparin is an anticoagulant that occurs in blood, lungs, liver, kidney etc. Heparin is composed of alternating units of N-sulfo-D-glucosamine-6-sulfate and glucuronate 2-sulfate
17. Hyaluronic acid has the least complicated structure among mucopolysaccharides. It is a straight-chain polymer of D-glucuronic acid and N-acetyl-D-glucosamine alternating in the chain.
18. Functions of PolysaccharidesDepending on their structure, polysaccharides can have a wide variety of functions in nature. Some polysaccharides are used for storing energy, some for sending cellular messages, and others for providing support to cells and tissues.
19. Storage of EnergyMany polysaccharides are used to store energy in organisms. While the enzymes that produce energy only work on the monosaccharides stored in a polysaccharide, polysaccharides typically fold together and can contain many monosaccharides in a dense area. Further, as the side chains of the monosaccharides form as many hydrogen bonds as possible with themselves, water cannot intrude the molecules, making them hydrophobic. This property allows the molecules to stay together and not dissolve into the cytosol. This lowers the sugar concentration in a cell, and more sugar can then be taken in. Not only do polysaccharides store the energy, but they allow for changes in the concentration gradient, which can influence cellular uptake of nutrients and water.
20. Cellular CommunicationMany polysaccharides become glycoconjugates when they become covalently bonded to proteins or lipids. Glycolipids and glycoproteins can be used to send signals between and within cells. Proteins headed for a specific organelle may be “tagged” by certain polysaccharides that help the cell move it to a specific organelle. The polysaccharides can be identified by special proteins, which then help bind the protein, vesicle, or other substance to a microtubule. The system of microtubules and associated proteins within cells can take any substance to its destined location once tagged by specific polysaccharides. Further, multi-cellular organisms have immune systems driven by the recognition of glycoproteins on the surface of cells. The cells of a single organisms will produce specific polysaccharides to adorn its cells with. When the immune system recognizes other polysaccharides and different glycoproteins, it is set into action, and destroys the invading cells.
21. Cellular SupportBy far one of the largest roles of polysaccharides is that of support. All plants on Earth are supported, in part, by the polysaccharide cellulose. Other organisms, like insects and fungi, use chitin to support the extracellular matrix around their cells. A polysaccharide can be mixed with any number of other components to create tissues that are more rigid, less rigid, or even materials with special properties. Simply by rearranging the structure, polysaccharides can go from storage molecules to much stronger fibrous molecules.
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