28 10 Respiration occurs in all healthy living cells. (a) The diagram shows part of the Krebs cycle and the electron transport chain. two CO2 citrate acetyl CoA succinate reduced NAD reduced FAD electron transport chain (i) Which row shows the number of carbon atoms in citrate and succinate? (1) Number of carbon atoms in citrate succinate A 2 4 B 5 4 C 6 4 D 6 8 (ii) Which of the following is transferred to a molecule of FAD to form reduced FAD? (1) A two oxygen atoms B two hydrogen atoms C one oxygen atom and one hydrogen atom D one water molecule 29 Turn over (iii) Explain the need for reduced NAD to be oxidised in a mitochondrion. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iv) A mutation in the gene that codes for the enzyme succinate dehydrogenase stops the conversion of succinate into citrate. Which row states the change in concentration of citrate and reduced FAD as a result of this mutation? (1) Concentration of citrate Concentration of reduced FAD A decreases decreases B decreases increases C increases decreases D increases increases 30 (b) There is a link between the methylation of certain regions of DNA and the risk of developing cancer. The graph shows the relationship between the level of methylation of these regions of DNA and the risk of developing cancer in different parts of the body by the age of 70. Level of DNA methylation / a.u. Risk of developing cancer 0.13 0.15 0.17 0.19 0.21 0.23 lung pancreas bladder liver breast colon cerebrum (i) Draw a line of best fit on the graph to identify any correlation between the independent variable and the dependent variable. (1) (ii) An investigation studied the effect of age on the mean level of DNA methylation. In this investigation, the null hypothesis was rejected at the 5% significance level. Explain what is meant by the phrase: the null hypothesis was rejected at the 5% significance level for this investigation. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 31 *(iii) Scientists have found that a high level of succinate leads to the inhibition of an enzyme called TET. The function of TET is to remove methyl groups from DNA. Explain how a mutation in the gene for succinate dehydrogenase can increase the risk of developing cancer. 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(Total for Question 10 = 14 marks) TOTAL FOR PAPER = 100 MARKS 32 BLANK PAGE