Carbohydrates

• All carbohydrates only contain the elements C, H, O.
• Glucose is an isomer that exists as alpha glucose or beta glucose. It is a hexose sugar (contains 6 carbon atoms in each molecule).
• The hydroxyl group faces opposite directions: downwards in alpha glucose and upwards in beta glucose.

  
• A disaccharide is formed when 2 monosaccharides join together by a glycosidic bond in a condensation reaction.
• The disaccharide maltose is formed by the condensation of two glucose molecules. It is a reducing sugar.
• The disaccharide sucrose is formed by the condensation of a glucose molecule and a fructose molecule. It is a non-reducing sugar.
• The disaccharide lactose is formed by the condensation of a glucose molecule and a galactose molecule. It is a reducing sugar.
• Polysaccharides are long chains of monosaccharides linked by glycosidic bonds.
• Starch is the storage polysaccharide of plants. It is a mixture of 2 polysaccharides of alpha glucose - amylose and amylopectin. It is compact, insoluble, and too large to diffuse through the cell membrane.
• Amylose is an unbranched helix-shaped chain of alpha glucose. - The helix shape makes it compact, so it is good for storage as more molecules can fit in.
• Amylopectin is a long, branched chain of alpha glucose. Side branches mean glucose molecules at chain ends can be easily hydrolysed by enzymes for use in respiration.
• Starch is insoluble, which means it doesn’t affect water potential, so it doesn’t cause water to enter cells via osmosis, which would make them swell.
• • Glycogen is the storage polysaccharide of animals.
  • It is a branched chain of alpha glucose. Branching enables more free ends, so allows rapid release of glucose molecules.
  • This is important as the energy demand of animals can change rapidly.
• • Cellulose is the structural polysaccharide of plants.
  • It is a long, unbranched chain of beta glucose.
  • The inverted arrangement of glucose molecules allows many hydrogen bonds to form between cellulose chains, making it strong.
  • Cellulose molecules are tightly cross-linked to form microfibres, which form fibres.
• Test for reducing sugars:

  1. Add Benedict’s reagent to a sample and heat it in a gently boiling water bath for 5 minutes.
  2. If reducing sugar is present, a coloured precipitate will form: blue (none) -> brick-red (high conc.)
• Test for non-reducing sugars:

  1. Add dilute HCl to the sample and heat it in a gently boiling water bath for 5 minutes.
  2. Neutralise the solution with sodium hydrogencarbonate.
  3. Carry out the test for reducing sugars again.
• Test for starch:

  1. Add iodine dissolved in potassium iodide solution to a sample.
  2. If starch is present, the sample turns from brown to blue-black.
• Starch as a storage molecule for plants

  • insoluble: doesn't affect water potential
  • alpha helix/branched: makes molecule compact, can fit many in a small area
  • large: cannot cross the cell membrane
  • branched: more free ends for fast breakdown/enzyme action
  • polymer of a-glucose: provides glucose for respiration
• Cellulose as a structural polysaccharide in cell walls

  • alternating inversion of b-glucose allows many hydrogen bonds to form between chains: provides strength
  • chains are tightly cross-linked to form microfibrils -> microfibrils -> strong cellulose fibres in cell walls
  • long, straight, unbranched: provides rigidity