Carbohydrates

• Structure of Glucose


  Glucose is a hexose sugar - monosaccharide with six carbon atoms in each molecule.

  There are two types (isomers) - alpha α and beta β

  
• Benedict's Test for Sugars - Reducing Sugars

  1. Reducing sugars include all monosaccharides (e.g. glucose) and some disaccharides (e.g. maltose & lactose)
  2. Add benedict's reagent (which is blue) to a sample and heat it in a water bath that's been brought to boil.
  3. If the test's positive it will form a coloured precipitate (solid particles suspended in the solution).
  4. The higher the concentration of the reducing sugar, the further the colour change goes - this can be used to compare the amount of reducing sugar in different solutions.
• Benedict's Test for Sugars - Non-Reducing Sugars

  1. If the result of the reducing sugars test is negative, there could still be a non-reducing sugar present. To test for non-reducing sugars first you have to break them down into monosaccharides.
  2. You do this by getting a new sample of the test solution, adding dilute hydrochloric acid and carefully heating it in a water bath that's been brought to boil. You then neutralise it with sodium hydrogencarbonate. Then carry out the benedict's test for a reducing sugar.
  3. If the test's positive for sugar it will form a coloured precipitate.
• Amylase is produced in saliva and pancreatic juice. Amylopectin is more easily digestible than amylose because its branches make it easier for amylase to access the glycosidic bonds between glucose units.
• Polysaccharides form when more than two monosaccharides are joined together by condensation reactions.
• Starch (Main energy storage in plants)

  1. Cells get energy from glucose. Plants store excess glucose as starch (when plants need more glucose for energy, it breaks down starch to release the glucose)
  2. Starch is a mixture of two polysaccharides of alpha glucose - amylose and amylopectin.
  3. Starch is insoluble in water and doesn't affect water potential, so it doesn't cause water to enter cells by osmosis, which would make them swell (makes it good for storage).
• Structure of Amylose

  -A long, unbranched chain of α-glucose.

  -The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact so it's really good for storage because you can fit more in to a small space.
• Glycogen and Starch are formed by the condensation of α-glucose
• Cellulose is formed by the condensation of β-glucose
• Structure of Amylopectin


  -long, branched chain of α-glucose.

  -side branches allow the enzymes that break down the molecule to get at the glyosidic bonds easily(so glucose can be released quickly)

  
• Glycogen


  -animals store excess glucose as glycogen (another polysaccharide of α-glucose)

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

  -Compact molecule good for storage

  
• Cellulose


  -made of long, unbranched chains of β-glucose (when β-glucose molecules bond they form straight cellulose chains)

  -The cellulose chains are linked together by hydrogen bonds to form strong fibers called microfibrils

  -The strong fibers mean cellulose provides structural support for cells (e.g. in plant cell walls)

  
• Iodine Test for Starch

  • Add iodine dissolved in potassium iodine solution to the test sample
  • Positive sample changes from browny-orange to a dark, blue-black colour