Exam Qu's

• The molecule shown in the diagram is a part of a peptidoglycan molecule. Explain why a peptidoglycan cannot be classified as a true polysaccharide. (1)

  It contains nitrogen

  
• NAG and NAM can exist as alpha or beta isomers. Identify which isomers of NAG and NAM are found in this molecule. Explain your answer. (1)

  It contains Beta isomers as the glycosidic bonds forms with OH which is on top in C1

  
• Peptidoglycan molecules can be cross-linked by the formation of peptide bonds between NAM molecules in adjacent chains. Describe how the structure of cellulose molecules enables cross-linking and explain why cross-linking of molecules of the polymer shown in the diagram would result in a stronger cell wall. (3)

  Celluose molecules rotate 180 degrees to form a glycosidic bond.

  Parallel chains are held by hydrogen bonds.

  Hydrogen bonds are weaker than peptide bonds.

  
• Name the bond that would join these amino acids and with reference to the structure of the molecules, explain why two different dipeptides could be formed. (2)

  Peptide bonds

  The OH (hydroxyl group) can be lost from either methionine or glycine to form a bond

  
• State if this is a transverse or a longitudinal section. Explain your answer. (2)

  Longitudinal

  Rings of lignin can be seen OR cells are rectangular

  
• Explain why a group of xylem vessels form a tissue rather than an organ. (1)

  A tissue is an aggregation of similar cells that perform a similar function.

  
• With reference to the polar nature of water molecules, explain how the loss of water from leaves enables water to be transported upwards through xylem vessels. (4)

  1. There is cohesion between water molecules
  2. Hydrogen bonds form between the positive hydrogen of one water and the negative oxygen of another
  3. Evaporation of water puts tension on the xylem as pressure is reduced at the top
  4. This means water is pulled up the xylem in a transpiration stream
  5. Adhesion is the attraction of water molcules to the xylem
• Describe one use of nitrate (1)


  For production of amino acids/Proteins/ATP/Nucleic acids
• Prove the hypothesis by using the information in the table. Hypothesis 1. Nitrate is transported from xylem to phloem by diffusion through plasmodesmata. (2)

  There is higher concentration of nitrate in the xylem than the phloem so nitrate moves by diffusion down a concentration gradient.

  Plamodesmata provide channels for movement.

  
• Prove the hypothesis by using the information in the table. Hypothesis 2. Phosphate is transported from xylem to phloem by active transport and must cross a cell membrane in the process. (3)

  There is a lower concnetration of phophate in the xylem than the phloem so it moves by active transport against the concnetration gradient using ATP.

  Through carrier proteins embedded in the cell membrane.

  
• Chlamydomonas lives in freshwater. Explain why the volume of water released from a cell of Chlamydomonas increases as the water potential of the external solution increases. (4)

  1. Water moves into cell
  2. By osmosis
  3. down a water potential gradient from a hypotonic solution outside the cell to a hypertonic solution inside the cell
  4. As the external water potential increases, the water potential gradient increases
  5. So the rate of osmosis increases = more water enters cell
  6. So the vacuole must empty to get rid of excess water

  
• The cytoplasm of Chlamydomonas was found to contain an enzyme that partially digests the chromosomes. Explain how the presence of this enzyme could be a reason for the results obtained for the whole Chlamydomonas extract. (2)

  The chromosomes are broken down into smaller pieces.

  This means they travel different distances in the centrifuge.

  
• To improve the quality of data obtained using this method, the scientists added an inhibitor of the enzyme that digests the chromosomes of Chlamydomonas. Suggest how this change would affect the data obtained.

  Less DNA chromosomes are digested.

  Fewer fragments

  Different peaks for each chromosome

  
• Explain why keeping the whole Chlamydomonas extract on ice in a buffer would also improve the quality of data. (2)

  Ice: reduces kinetic energy of the enzyme

  Buffer: Maintains pH

  
• Both mitochondria and chloroplasts are believed to have evolved from prokaryotes. What evidence does the graph provide that could support this hypothesis? Explain your answer (2)

  Single peak for both organelles

  Means they contain only one chromosome which could be in the form of plasmids

  
• What are the two major biochemical components present in all viruses? (1)

  • Protein
  • DNA/RNA
• Using information from the diagrams, explain how Ebola virus particles enter a host cell. (3)

  Ebola binds to the complementary receptors on the cell membrane.

  The membrane engulfs the virus by endocytosis/phagocytosis to form a vesicle.

  
• In one technique, known concentrations of glucose and reagents are prepared leading to a colour change. The absorbance is then measured using a colorimeter. Why is it not possible to use the standard curve for glucose determination in a whole blood sample? (1)


  Blood is red so a colorimeter reading cant be obtained
• Modern medicine uses immobilised enzymes, in devices called biosensors, to detect blood glucose levels. This involves the use of glucose oxidase as the enzyme and glucose as the substrate. Give two advantages of using immobilised enzymes for blood glucose monitoring. (2)


  Only glucose will give a result/enzyme specific (to glucose)

  Detects very low concentrations in biosensor

  Reading is more accurate /enzyme is more stable
• The experiment was repeated using the same range of sucrose concentrations and sweet potato but there was no decrease in mass at any sucrose concentration. Explain why there was no decrease in mass at any of the sucrose concentrations. (1)


  Sweet potato has a very high sugar concentration and a very low water potential so water will always move in
• Describe how you would modify the method to determine the ψtissue of sweet potato.

  • Increase the sugar concentration
  • until the point where water leaves the potato
• An experiment was set up as shown below with the addition of alkaline sodium carbonate causing the milk to turn pink. Phenolphthalein is a pH indicator which is pink in alkaline conditions and colourless when in acid conditions. Why would this experiment not be suitable for assessing the effect of different pH values on lipase? (1)

  • Result depends on a pH change so you cant chnage pH
  • Acidic pH's woul be colourless so a colour chnage wouldnt be visable
  • pH buffer would maintain pH and prevent chnages in colour
• One drug used to treat obesity acts as a competitive inhibitor of lipase. It prevents approximately 30% of the lipid being absorbed, thereby reducing energy (calorie) intake. Describe how this drug would lead to weight loss. (2)

  • Stops lipids from being hydrolsed and absorbed.
  • This reduces the calorie intake
  • Body uses its stored fat as energy
• One drug used to treat obesity acts as a competitive inhibitor of lipase. With reference to the action of this drug, explain why people tended to regain the weight when they stopped taking it. (1)

  If a person continues to eat a fatty diet without the inhibitor, lipids can be broken down and be absorbed causing weight gain.
• Explain the process of transcription including the roles of the enzymes involved. (3)

  • DNA helicase breaks the hydrogen bonds within DNA, unzipping the double helix
  • RNA polymerase aligns RNA nucleotides opposite the DNA template strand to form complementary base pairs
  • RNA polymerase joins the nucleotides together
  • RNA polymerase separates at a stop signal
  • mRNA leaves the nucleus after introns are removed
• Explain why it is important that the RNA which is finally translated, consists only of exons. (2)

  • Introns do not code for the amino acid sequence
  • If they were included, a different protein structure would be produced