WJEC Biology for A2 Level: Revision Workbook

Question and mock answer analysis Q & A 1 [a = AO1, b = AO2, c = AO3] A sample of calf liver was homogenised in an ice-cold isotonic buffer solution, and then centrifuged at high speed to separate the organelles. The supernatant and a sample of mitochondria were incubated with glucose at 37°C and the products identified. The experiment was repeated with both samples using pyruvate instead of glucose. The results are shown in the table: Sample Incubated with glucose Incubated with pyruvate CO 2 produced Lactate produced CO 2 produced Lactate produced Mitochondria ✗ ✗ ✓ ✗ Supernatant ✗ ✓ ✗ ✗ a) Explain why the buffer used was isotonic and suggest why it was ice-cold. (2) b) Explain the results observed. (5) c) What would you expect to see if the experiment were repeated using mitochondrial matrix? Explain your answer. (2). Lucie’s answers a) Isotonic buffer prevents lysis of mitochondria ✓ and ice-cold slows down enzymes which might damage contents. ✓ b) Mitochondria are unable to metabolise glucose, which is why neither carbon dioxide nor lactate is produced. ✓ When mitochondria are incubated with pyruvate, carbon dioxide is produced, because pyruvate can diffuse into the mitochondria and carbon dioxide is produced as a result of Link reaction, and Krebs cycle that occur within the mitochondrial matrix. ✓ The supernatant contains the enzymes present in the cytoplasm of the liver cells and so glycolysis occurs, producing lactate from glucose by anaerobic respiration. ✓ No lactate or carbon dioxide are produced when supernatant is incubated with pyruvate because the link reaction and Krebs cycle do not occur in the cytoplasm. ✓ c) The results would be the same as for the mitochondria ✓ as all the reactions, e.g. link and Krebs that produce carbon dioxide, occur in the matrix. ✓ Lucie achieves 8/9 marks MARKER NOTE The answer could have included that glycolysis does not occur in the mitochondria and therefore will not have the enzymes necessary to break down glucose. 22 | WJEC Biology for A2 Level: Revision Workbook | Unit 3 Energy, Homeostasis and the Environment Copyright: Sample material