WJEC Biology for A2: Study and Rev Guide

Exam tip It is important that you refer to stages in respiration and state exactly where they occur. Lucie’s answer Ceri’s answer a) Isotonic buffer prevents lysis of mitochondria. ✓ b) Mitochondria are unable to metabolise glucose which is why neither carbon dioxide or 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. ✓ a) Buffer maintains a constant pH. ✗  b) Mitochondria cannot produce carbon dioxide or lactate. ✗  Mitochondria release carbon dioxide when incubated with pyruvate, because pyruvate is hydrolysed during Link, and Krebs cycle. ✓  The supernatant is made from cell cytoplasm which is where glycolysis occurs, so lactate is produced by anaerobic respiration. ✓  . Examiner commentary Good answer from Lucie.  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. Lucie achieves 5/6 marks Examiner commentary  Ceri has confused isotonic buffer with pH buffer.  This is purely descriptive, there is no explanation.  Ceri mentions the reactions but could have included where they take place, i.e. mitochondrial matrix.  Ceri did not explain why carbon dioxide and lactate are not produced when the supernatant is incubated with pyruvate, i.e. because the link and Krebs cycle do not occur in the cytoplasm. Ceri achieves 2/6 marks Q & A 2 Respiration 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 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. (1) b) Explain the results observed. (5) 170 A Level Biology: Study and Revision Guide