STPM Biology Biological Molecules Part 6 Carbohydrate - Polysaccharides - Starch

1. General formula: (C6H10O5)n

2. Distribution:

• Leaves
• Storage organs
• Seeds

3. Physical properties:

• Not soluble in water.
• Cannot crystallize.
• Not sweet.
• Has high molecular mass.

4. Chemical properties:

• Can undergo hydrolysis to form maltose.
• Reacts with iodine to form dark blue complex.

5. Functions:

• Provides energy.

Structure

1. Starch consists of 2 types of components:

• Amylose (20%)
• Amylopectin (80%)

2. Amylose has unbranched-chain molecules. Amylose molecule consists of α-glucose molecules that are linked by 1,4-glycosidic bonds. Amylose molecule is coiled into a helix.

Amylose

3. Amylopectin has branch-chain molecules. Same with amylose, amylopectin molecule consists of α-glucose molecules that are linked by 1,4-glycosidic bonds. But the branches are linked by 1,6-glycosidic bonds. The chains are coiled into helix.

Amylopectin

Comparison between amylose and amylopectin

Properties and structure of starch related to its function

1. Starch is a storage compound. Starch can carry out this function because:

• It is not soluble in water. Hence, stored starch do not change the osmotic concentration of the organ.
• Starch molecules are compact. A large mass can be stored in a small space.

2. Starch is a source of energy for organism. Starch can undergo hydrolysis to form glucose. Glucose is the substrate for respiration.