Carbohydrates- polysaccharides; starch, glycogen and cellulose

Polysaccharides

• Carbohydrates

• Made from large numbers of their monomers (monosaccharides)

• Formed when more than 2 monosaccharides are joined together by condensation reactions forming glycosidic bonds

Structure of polysaccharides

• Can be formed from glucose monomers that are joined by 1-4 or 1-6 glycosidic bonds

• The numbers 1-4 and 1-6 refer to the carbon number of the 2 molecules that have joined to form the bond

Amylose (form of starch) and cellulose

Formed by straight unbranched chains of glucose monomers with 1-4 glycosidic bonds

Amylopectin ( form of starch) and glycogen

They are branched polysaccharides with 1-6 linkages at the branch points

Starch

• Plants store excess glucose as starch

• Mixture of 2 polysaccharides of alpha-glucose: amylose and amylopectin

Amylose

It is a long, unbranched chain of a-glucose. The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact, so it is really good for storage because you can fit more into a small space

Amylopectin

It is a long, branched chain of a-glucose. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly

Starch solubility

It is insoluble in water and does not affect water potential, so it does not cause water to enter cells by osmosis, which would make them swell. This makes it good for storage

Iodine test for starch

• Add iodine dissolved in potassium iodide solution to the test sample

• if there is starch present, the sample changes from browny-orange to a dar, blue-black colour

Glycogen

• Animals store excess glucose as glycogen, which is a polysaccharide of alpha-glucose, and it is stored in the liver

• It has a very similar structure to amylopectin, but it has more side branches coming off it

• Loads of branches means that stored glucose can be released quickly, which is important for energy release in animals

• it is a very compact molecule, so it is good for storage

Cellulose

• Made of long, unbranched chains of beta-glucose

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

• The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils

• The strong fibres mean cellulose provides structural support for cells