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Components of Cells
The Macromolecules


Polysaccharides are very large, high molecular weight biological molecules that are almost pure carbohydrate. They are constructed by animals and plants from simpler, monosaccharide molecules, by joining together large numbers of the simpler molecules using glycosidic bonds (-O-). In some of the largest polysaccarhide structures there can be 10,000 individual units joined together. There is a large diversity of polysaccharide form; they can differ in the type of sugar, the connections between the sugars and the complexity of the overall molecule.

Sometimes known as glycans, there are three common and principal types of polysaccharide, cellulose, starch and glycogen, all made by joining together molecules of glucose in different ways.

It has been estimated that 50% of the world's organic carbon is found in one molecule; cellulose. This molecule is synthesized, stored, modified and used as a building material by plants. It is certainly the most abundant of all the polysaccharides.

In the cellulose molecule the individual glucose monosaccharides are all linked to one another in the form of a long, long, linear chain. The carbon atom number 1 (C1) in one sugar is linked to the fourth carbon atom (C4) of the next sugar in an extended array.

All the glucose molecules in cellulose have the beta-configuration at the C1 atom, so all the glycosidic bonds that join the glucose molecules together are also of the beta type. This means that the cellulose molecule is straight, and many such molecules can lay side by side in a parallel series of rows.

Tiny forces called hydrogen bonds hold the glucose molecules together, and the chains in close proximity. Although each hydrogen bond is very, very weak, when thousands or millions of them form between two cellulose molecules the result is a very stable, very strong complex that has enormous strength.


Starch, a word that comes from old English and means to stiffen, is also a polysaccharide made in plants. It is primarily an energy storage molecule, or fuel, for the plant and for its seeds.

If the starch molecules are gently broken down by acid hydrolysis, the disaccharide maltose is produced, indicating that the glucose molecules in starch are also joined together by linking the C1 carbon of one sugar to the C4 carbon of the next sugar in the sequence. However, in this case, the glucose molecules are joined using alpha-glycosidic bonds.

However, these molecules are not straight or totally linear. At intervals along the starch molecule there are branches produced by another kind of glycosidic link between the C1 carbon on one sugar and the C6 carbon on the next sugar.

When stored starch granules are removed from plants and placed in water they swell and release two types of material; amylose and amylopectin

Amylose is the simpler of the types of molecule and is largely linear chains of C1-to-C4 glysosides, several thousand units in length. Amylopectin is more complex and these molecules are branched using a combination of C1-to-C4 bonds and C1-to-C6 bonds about every 25 glucose units along the chain. Such large, complex molecules do not dissolve well in water.

Glycogen is also made by linking together glucose molecules. Like starch, it is used by animals to store sugar and provide energy. It is similar to amylopectin in structure, but branched with a C1-to-C6 glycosidic bond about every ten glucose units.

© 2004, Professor John Blamire