Polysaccharides: Starch and Glycogen

Polysaccharides are complex carbohydrates. They are large molecules, or polymers, consisting of chains of monosaccharides linked together by glycosidic bonds.

Some polysaccharides have metabolic functions and others have structural functions in cells and organisms.

Starch and glycogen are both examples of carbohydrates that are involved in the metabolism of an organism. They are both storage polysaccharides that can store and release glucose as necessary.

Both starch (plants) and glycogen (animals) are made of chains of α-glucose.

Starch is a mixture of two different polysaccharides, amylose (which forms coiled molecules) and amylopectin (a branched molecule). In amylopectin, the bonds between glucose molecules within a branch are α-1,4 glycosidic bonds but at branching points, the bonds are α-1,6 glycosidic bonds.

Glucose is a polar molecule – hydrogen bonds can form between the O^δ- on C2 of one glucose molecule and C3^δ+ of the next glucose molecule in the chain. As a result, the amylose molecule coils up to form a helix.

This makes starch a compact molecule that is less soluble in water – ideal properties for storage of glucose. Also, because starch is insoluble it does not affect the water potential of the cell in which it is stored. This means that starch is osmotically stable.

Glycogen is similar to amylopectin but is even more branched due to glycosidic bonds forming between OH groups on C1 and C4 but also C1 and C6. Glycogen can form granules in cells and act as a carbohydrate/energy store.

The branches in amylopectin and glycogen make them better for the release of glucose. This is because there are more ‘ends’ where glycosidic bonds can be hydrolysed and glucose released, which can be used in respiration to produce ATP.

The structures of these molecules are shown in the diagram.