18 7 Lipids and carbohydrates are used as respiratory substrates. The respiration of lipids generates more ATP than the respiration of carbohydrates. (a) The table shows bonds that may be found in carbohydrates and lipids. Which box in each row shows whether the bond may be found in these molecules? (3) Bond Molecule that bond may be found in carbohydrate only lipid only both carbohydrate and lipid neither carbohydrate nor lipid covalent ester hydrogen (b) The diagram shows some steps in the respiration of carbohydrates and lipids. Glycogen Starch Glucose Pyruvate Acetyl coenzyme A Fatty acids Lipid Krebs cycle (i) Where in a cell does the conversion of pyruvate to acetyl coenzyme A take place? (1) A cytoplasm B matrix C stroma D tonoplast 19 Turn over (ii) Explain why lipids can be respired only in aerobic conditions. 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(c) The electron transport chain uses hydrogen ions to generate ATP. Explain why the respiration of lipids generates more ATP than the respiration of carbohydrates. 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(d) The respiratory quotient (RQ) can indicate which substrate is being used in respiration. The formula used to calculate RQ is RQ volume of carbon dioxide produced volume of oxygen used = (i) State why the RQ for carbohydrate is 1.0. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 20 (ii) The table shows the formula for calculating the volume of oxygen used and the volume of carbon dioxide produced in the respiration of lipids. The number of carbon atoms in the lipid is represented by n. Volume of oxygen used / a.u. Volume of carbon dioxide produced / a.u. 4.5n + 3.75 3n + 6 Calculate the RQ for a lipid containing 21 carbon atoms. (2) Answer .............................................................. (e) Explain why an insect has an RQ of 1.0 at rest and an RQ of 0.7 during flight. 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(Total for Question 7 = 15 marks)