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Give an account of the structure and functions of polysaccharides in living organisms.

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Introduction

Give an account of the structure and functions of polysaccharides in living organisms. The commonly occurring polysaccharides starch, glycogen and cellulose are all polymers of glucose. Starch is a mixture of two polymers of alpha-glucose, amylose and amylopectin. Amylose is a linear polymer and amylopectin is a branched polymer. The proportions of each vary from plant to plant, but usually amylose is the minor component at about 10-30% of the starch. Amylose is a glucan (glucose polymer) with 1,4 glycosidic bonds. Amylose takes on a helical shape with each coil having six monomers per turn and it is held together by hydrogen bonds between the groups attached to the carbon atoms. ...read more.

Middle

It is insoluble, so preventing it from moving out of the cells it is stored in and into others and also making it strong and not involved in most chemical reactions because most take place in liquid. Starch is unique among polysaccharides because it occurs naturally in grains, which are found in amyloplasts, which are present in the cytoplasm of plant cells. Glycogen is also a polysaccharide made up of the monomer alpha-glucose and is very similar to amylopectin. It has both 1,4 and 1,6 glycosidic bonds, short chains and is a very branched molecule, with the branches occurring every 8 - 12 residues, therefore forming a very compact structure. ...read more.

Conclusion

The hydrogen bonding between the chains forms cross links, which make the polysaccharide very strong, and prevents access from water so that cellulose is very resistant to hydrolysis and is a good structural component. The chains can be held together, up to 2000 at a time, to form a microfibril. The tensile strength of cellulose, which enables it to resist pulling forces, makes an important structural component in cell walls. The microfibrils are embedded in a matrix, which is a mixture of pectins and hemicelluloses. Cellulose is very difficult to digest and very few animals make the enzyme cellulase, which allows the cellulose to be broken down into glucose. Some animals have to depend on cellulose-digesting bacteria to break the polysaccharide down to release the important nutrients. ...read more.

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