Carbohydrates

• Carbohydrates contain carbon, oxygen and hydrogen.

  they are organic compounds

  Cx(H20)y
• There are three main groups:

  • monosaccharides
  • disaccharides
  • polysaccharides
• Monosaccharides:

  • dissolve easily in water to form a sweet solution
  • general formula (CH20)n where n is the number of carbons

  e.g

  trisoses = 3 carbons

  pentoses = 5 carbons

  hexoses = 6 carbons
• Glucose is a monosaccharide
• glucose

  • a hexose
  • made up of a chain of atoms long enough to close upon itself and form a stable ring structure
  • an energy source
  • soluble so an be transported in water
• Glucose exists in two structurally different forms:

  alpha glucose and beta glucose

  therefore known as an isomer; possess the same molecular formula but different arrangement of atoms
• Alpha glucose:

  OH below the ring

  Beta glucose:

  OH above the ring
• Polysaccharides are formed from two isomers of glucose:

  starch - alpha glucose

  glycogen - alpha glucose

  cellulose - beta glucose
• Monosaccharides combine = condensation reaction occurs. Bond formed = glycosidic bonds
• Beta glucose = cellulose = very strong, chain and held together by intermolecular forces
• Polysaccharides

  • not sugars (insoluble)
  • e.g starch, glycogen and cellulose
  • no longer affects water potential in the cell
• Amylose:

  • long unbranching chains
  • 1,4 glycosidic bonds
  • formed by condensation reactions
  • forms a helix and stabilised by hydrogen bonds making it compact and less soluble
• Amylopectin:

  • 1,4 linked alpha glucose molecules form chains
  • shorter
  • branches from 1,6
• Starch:

  • a mixture of two substances; amylose and amylopectin
  • starch granules are insoluble in water
  • used for storage in plants
  • starch grains build up in chloroplasts or storage organs such as potato tubers
• Glycogen:

  • the form in which carbohydrate is stored in the animal body
  • alpha glucose molecules - 1,4 and 1,6 links
  • more branched than amylopectin so more compact so needs less space to be stored.
• Starch & glycogen

  • starch and glycogen are energy storage molecules
  • take up little space due to their compact shapes
  • very large molecules = insoluble
  • help prevent too high concentrations of glucose in cells so water potential is not affected
• Cellulose:

  • polymer of about 10,000 beta glucose molecules in a long unbranched chain
  • many chains run parallel to each other and have cross linkages (hydrogen bonds) between them = increased stability
• structure of cellulose:

  • to join together one beta glucose molecules mast be rotated 180 degrees relative to each other
  • molecules become tightly cross-linked with eachother to form bundles called micro-fibrils.