Exam Qs

• A precipitate is produced in a positive result for reducing sugar in a Benedict's test.

  A precipitate is solid matter suspended in solution.

  A student carried out the Benedict's test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution.

  Answer:

  Filter and dry (the precipitate);

  Find mass/weight;
• Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.

  Other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose.

  Answer: Similarity

  Both contain galactose / a glycosidic bond;

  Difference

  Lactulose contains fructose, whereas lactose contains glucose;
• Glycogen and cellulose are both carbohydrates.

  Describe two differences between the structure of a cellulose molecule and a glycogen molecule.

  Answer: 

  • Cellulose is made up of B-glucose (monomers) and glycogen is made up of a-glucose (monomers);
  • Cellulose molecule has straight chain and glycogen is branched;
  • glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4-glycosidic bonds;
• Starch is a carbohydrate often stored in plant cells.

  Describe and explain two features of starch that make it a good storage molecule.

  Answer

  •  Insoluble (in water), so doesn't affect water potential;
  • Branched / coiled / (a-)helix, so makes molecule compact;
  • Polymer of (a-)glucose so provides glucose for respiration;
  • Branched / more ends for fast breakdown / enzyme action;
  • Large (molecule), so can't cross the cell membrane
• Describe the structure of glycogen.

  • Polysaccharide of a-glucose;
  • (Joined by) glycosidic bonds oR
  • Branched structure;
• Suggest how glycogen acts as a source of energy.

  • Hydrolysed (to glucose);
  • Glucose used in respiration;
• Explain the difference in the structure of the starch molecule and the cellulose molecule shown in the diagram

  • Starch formed from a-glucose but cellulose formed from B-glucose;
  • Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.

  
• Starch molecules and cellulose molecules have different functions in plant cells. Each molecule is adapted for its function.

  Explain one way in which starch molecules are adapted for their function in plant cells.

  Insoluble; Don't affect water potential;

  OR Helical;  Compact;

  OR

  Large molecule;

    Cannot leave cell.
• Explain how cellulose molecules are adapted for their function in plant cells.

  • Long and straight chains;
  • Become linked together by many hydrogen bonds to form fibrils;
  • Provide strength (to cell wall)
• student investigated the difference in the reducing sugar content of two fruit juices. He performed a biochemical test on each fruit juice using Benedict's solution. He then used a colorimeter with each test result.

  Describe how the results from the colorimeter can identify the fruit juice containing the higher sugar content.

  • Higher absorbance (has more sugar)
• Name the monomer present in cellulose.

  • glucose;
• Name the type of reaction that converts cellulose to its monomers.

  • hydrolysis 
• Describe how B-glucosemolecule differs from a molecule of a-glucose. [']

  H at top right end (instead of OH) /

  OH at bottom (carbon 1)
• Describe the structure of

  starch and explain how its structure is related to its function. [3]

  Formed from a glucose; Joined by condensation

  Between (carbons) 1 and 4 (and 1 and

  6);

  Coiled chain;

  compact;

  (Allows) storage of large amount in a small space;

  Insoluble so has no effect on osmosis/water potential;

  Branches;

  (Allows) rapid breakdown/release of glucose / hydrolysis;
• Describe the structure of cellulose and explain how its structure is related to its function. [3]

  Alternate B-glucose rotated 180°,

  long straight chains,

  Many hydrogen bonds join (polysaccharide) chains to each other

  makes microfibrils
• Describe the chemical reactions in the conversion of monomers to polymers and polymers to monomers

  Give 2 named examples of polymers and their associated monomers to illustrate your example (5)

  • Condensation reaction = joining together of many monomers to form a polymer. Bonds formed and a molecule of water
  • Hydrolysis reaction = molecule of water used to break the bonds between polymers to make monomers.
  • examples: Alpha glucose – glycosidic bonds – starch
  • Amino acids – peptide bonds – polypeptide
• Explain one way in which the structure of glycogen is adapted to its function (1)

  • Highly branched
  • So glucose rapidly released for use in respiration to release energy
  • Large molecule
  • So can’t leave the cell (storage molecule
• Explain how cellulose molecules are adapted for their function (3)

  • Long straight chains of beta glucose
  • Linked by many hydrogen bonds to form a fibril
  • This makes a strong molecule that adds strength the cell wall of a plant