Polysaccharides

Polysaccharides are the polymers of monosaccharides. Normally, only glucose can be the monomer.

There are several types of polysaccharides, as starch, amylose, glycogen and cellulose.

Starch is a long chain of glucose molecules. They form a spiral in water and present the property of a gel. When iodine is added, iodine will go into the center of the coil and form numerous metallic bonds with the lone pair electrons along the starch molecule. The lone pair electrons revolve and absorb light. So, the mixture would become a dark blue color.

The formation of the dark blue colour is inhibited by :

(1)

the excessive heat. The excessive heating would unfold the spiring coil and force out iodine from the compound. Once iodine is no longer inside the spiring coil, the mixture would loss the dark blue colour.

(2)

the presence of alcohol. In the presence of alcohol, iodine cannot enter the center of the spiring coil. So, it cannot form metallic bonds and fail to produce the dark blue colour. This is the reason why we need to use the I²-KI solution instead of the alcholic solution of iodine.

Amylose is different from starch in that its molecule is smaller.

Glycogen is the much branched structure. When iodine is added, glycogen would form the red-brown colour.

Cellulose is different from the others in that it is composed of beta glucose. (Glucose molecules in starch, amylose and glycogen are alpha glucoses.) The hydroxyl groups on the first and fourth carbon of an alpha glucose are on the same side. When two alpha glucose molecules form the glycosidic bond, it is formed readily and called the alpha glucosidic bond. Higher animals can have digestive enzymes to break down the alpha glycosidic bond. So, higher animals can digest starch, amylose and glycogen.

In beta glucose, the hydroxyl groups on the first carbon and the fourth carbon are on opposite sides. When they are connected together, glucose molecules must be inverted alternately in order to help to form the bond. This type of bond is called a beta glycosidic bond. No higher animal can have digestive enzyme to break down the beta glycosidic bond. So, no higher animal can digest cellulose. Only bacteria, fungi and some protozoans can produce cellulase and digest cellulose. Higher animals would use other means for the cellulose digestion. As termits use the protozoans inside their guts to digest cellulose. Ruminants use the 4 chambered stomach for its digestion. Bacteria inside their omasum (a chamber of the stomach) can secrete cellulase. Other mammals use the symbiotic bacteria inside their caecum to digest it. So, heribvores have a larger caecum, but carnivores have a shorter caecum.

Polysaccharides are, normally, insoluble in water. Also, it is dependent on the molecular size. The larger the molecule, the smaller the solubility. They are odorless and tasteless. The qualitative test is by the use of I₂-KI solution. If there is no alcohol and the solution is not too hot, the mixture would display a dark blue colour.

(27.08.2005)

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