Polysaccharides

Starch - Description

• Cells get energy from glucose

• Plants store excess energy as starch (when a plant needs more glucose it breaks down starch to release it)

• Starch is a mixture of two polysaccharides of alpha glucose - amylose and amylopectin

Amylose

• Long unbranched chain of alpha glucose

• The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder.

• Structure makes it compact, so really good for storage - can fit more into a small space.

Starch - properties

• Insoluble in water, so doesn’t affect water potential - prevents water from entering from osmosis, making the cell swell. This makes it good for storage.

Glycogen

• Animals store excess glucose as glycogen

• A polysaccharide of alpha glucose

• Similar structure to amylopectin, expect many more side branches coming off it.

• Lots of branches (terminal ends) - stored glucose can be released quickly, this is important for energy release in animals.

• Also compact molecule - good for storage.

Amylopectin

• Long branched chain of alpha glucose

• It’s side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily,

  Glycosidic bonds are hydrolysed to release glucose

  - this means glucose can be released quickly, More terminal ends available for enzyme action

Cellulose

• Made of long, unbranched chains of beta-glucose.

• When beta-glucose molecules bond, they form straight cellulose chains.

• Cellulose chains are linked together by hydrogen bonds, to form strong fibres called microfibrils - these strong fibres mean cellulose provides structural support for cell (i.e plant cell wall)