Polysaccharides

• A polysaccharide is formed when more than two monosaccharides are joined together by condensation reactions
• Lots of alpha glucose molecules are joined together by glycosidic bonds to form amylose
• Polysaccharides can be broken down into their constituent monosaccharides by hydrolysis reactions
• Starch:

  Cells get energy from glucose. Plants store excess glucose as starch. Starch is a mixture of two polysaccharides of alpha-glucose - amylose and amylopectin

  • Amylose - is a long, unbranched chain of alpha glucose. The angles of the glycosidic bonds give it a coiled structure. This makes it compact, so its really good for storage because you can fit more in to a small space
  • Amylopectin - is a long, branched chain of alpha 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 is insoluble in water and does not affect the water potential so makes it a good storage unit.
• Glycogen:

  • Animal cells get energy from glucose too but animals store excess glucose as glycogen - another polysaccharide of alpha glucose
  • Its structure is very similar to amylopectin, except that it has loads 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
  • Its also a very compact molecule so its good for storage
• Cellulose:

  • Cellulose is 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
• • If you want to test for the presence of starch in a sample, you'll need to do the iodine test
  • Just add iodine dissolved in potassium iodide solution to test sample
  • If there is starch present, the sample changes from browny- orange to a dark, blue-black colour
  • Browny-orange colour indicates no starch
  • Dark, blue-black colour indicates starch