Carbohydrates

• Monosaccharides:

  • alpha glucose
  • beta glucose
  • fructose
  • galactose
• Disaccharides:

  • Maltose - alpha glucose X2
  • Sucrose - alpha glucose & fructose
  • Lactose - alpha glucose & galactose
• Polysaccharides:

  • Glycogen - alpha glucose
  • Starch - alpha glucose
  • Cellulose - beta glucose
• Disaccharides and polysaccharides are joined by glycosidic bonds.
• Starch

  Polysaccharide made up of alpha glucose joined by glycosidic bonds.

  Unbranched chain is wound into a tight coil.

  Storage molecule.

  Major energy source in most diets.

  Insoluble and large molecules so does not affect water potential and cannot diffuse into cells.

  Very compact molecules so a lot can be stored in a small space.

  Can be easily hydrolysed into alpha glucose which is used in respiration.
• Glycogen

  Polysaccharide made up of alpha glucose joined by glycosidic bonds.

  Glucose is stored as glycogen in muscles and the liver.
• Cellulose

  Polysaccharide made up of beta glucose joined by glycosidic bonds.

  Structural function.

  Every other beta glucose molecule is inverted.

  Straight chain molecule.

  Strong but flexible, so does not burst and helps plant maintain shape.
• Test for reducing sugars:

  • Add Benedict's solution to food sample and heat for 5 minutes.
  • Positive result will be indicated by a brick red precipitate.
• Test for non-reducing sugars:

  • If test for reducing sugar doesn't show positive result add dilute HCl to sample.
  • Boil for 5 minutes
  • Neutralise with NaHCO3
  • Test pH
  • Re-rest using Benedict's solution
• All monosaccharides are reducing sugars. Some disaccharides such as maltose are reducing sugars.
• All polysaccharides are non-reducing sugars. Some disaccharides such as lactose and sucrose are non-reducing sugars.