STPM Biology Biological Molecules Part 7 Carbohydrate - Polysaccharides - Glycogen

1. Molecular formula: (C6H10O5)n

2. Plant stores starch and animals stores glycogen. Glycogen is referred to as "animal starch".

3. Physical properties:

• not soluble in water.
• not sweet.
• cannot crystallize.
• high molecular mass.

4. Chemical properties:

• undergoes hydrolysis to become glucose.
• reacts with iodine solution to form a purplish red color.

5. Distribution of glycogen (mainly):

• liver
• muscles
• brain

6. Structure of glycogen:

• Glycogen has branched-chain molecules. The structure of glycogen molecule is similar with amylopectin, but glycogen molecule has more branches.
• Glycogen molecules are built from α-glucose molecules that are linked together by 1,4-glycosidic bond. Branches are linked by 1,6-glycosidic bonds.
• The chain are coiled into helix.

Glycogen structure

7. Functions:

• Glycogen is storage carbohydrate in animals.
• Source of energy for animals.

Properties and structure of glycogen related to its function

1. Glycogen is a storage compound. It is due to:

• It is not soluble in water. Thus, stored glycogen does not change the osmotic pressure of the organ.
• Glycogen molecules are compact. A large mass can be stored in a small space.

2. Glycogen is a source of energy for animals. Glycogen can carry out this function because it can undergo hydrolysis to form glucose. Glucose is the substrate for respiration.