Carbohydrates

• Describe how cellulose is formed and where it is stored

  • Cellulose is a polymer of β-glucose molecules that are joined together by glycosidic bonds in a condensation reaction to form long straight chains
  • Found in the cell wall of plants
• Describe the basic Benedict’s test

  • Heat sample with Benedict’s reagent in water bath
  • If reducing sugar is present, the sample will form a brick red precipitate
• Describe how glycogen is formed and where it is stored

  • Glycogen is formed by the condensation of alpha glucose molecules
  • Stored in large amounts in liver and muscle tissues
• Describe how you would test for non reducing sugars?

  • Heat with acid then neutralise to hydrolyse any non reducing sugars into reducing sugars
  • Add benedicts reagent to solution
  • Positive result = colour change to brick red
• Describe the structure of starch and where it is stored

  • Starch consists of long branched chains of alpha glucose molecules that are linked together by glycosidic bonds during condensation reactions
  • Stored in starch grains in the cytoplasm of plant cells
• Describe how cellulose is adapted to its function as a structural component

  • Long chains of beta glucose joined together by glycosidic bonds in condensation reactions
  • Adjacent chains joined together by hydrogen bonds
  • This produces a 3D structure - microfibril
  • This provides strength and rigidity to the cell wall
  • Which prevents the cell from bursting
• Describe the test for starch

  • Add 2-3 drops of iodine/potassium iodide solution
  • If starch is present, colour change: yellow/orange to blue/black
• How is starch adapted to its function as a storage carbohydrate?

  • Large so it cannot cross the cell surface membrane and leave the cell
  • Insoluble and therefore, osmotically inactive i.e it cannot draw in water by osmosis which could lead to cell damage
  • It has a helical shape and therefore it forms a compact store
  • Branched so that glucose can easily be released from its ends for respiration
• How is glycogen adapted to its function as a storage carbohydrate?

  • Insoluble and therefore, osmotically inactive
  • Similar structure to starch but with more branches so that it can be hydrolysed more rapidly for respiration
• How are disaccharides formed and what are the word equations for each formation?

  Two monosaccharides are joined together by a glycosidic bond in a  condensation reaction ‎‏‏‎ ‎‏‏‎

  1. Glucose + Glucose → Maltose + Water
  2. Glucose + fructose → Sucrose + Water
  3. Glucose + Galactose → Lactose + Water