Polysaccharides: Structure-Function Relationships

1. Starch (plant energy storage):

Composed of two components:

• Amylose (~20-30%): linear, alpha-1,4 glycosidic linkages only. Adopts a left-handed helix in solution. Gives intense blue-black colour with iodine ($I_{3}^{-}$ trapped in helix).
• Amylopectin (~70-80%): branched, alpha-1,4 along chains + alpha-1,6 at branch points (every 24-30 units).

Linkage arrows (amylopectin):

Glu --[alpha-1,4]--> Glu --[alpha-1,4]--> Glu --[alpha-1,6]--> Glu (branch)
                                                    |
                                                    v
                                            [alpha-1,4] chain continues

2. Cellulose (plant structural polysaccharide):

Linear polymer of D-glucose via beta-1,4 glycosidic linkages. The beta bond angle allows the chain to extend fully. Adjacent chains form extensive intermolecular H-bonds, creating rigid crystalline microfibrils (plant cell walls, cotton fibres).

Human digestive enzymes (amylases) are specific for alpha-linkages only → cellulose is indigestible (dietary fibre).

3. Glycogen (animal energy storage):

Like amylopectin but more highly branched: alpha-1,4 + alpha-1,6 bonds, with branch points every 8-12 glucose units. Higher branching = more non-reducing ends = faster glucose mobilisation by glycogen phosphorylase.

Comparative table: