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5. The two diagrams below show nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), two nucleotides used in respiration. (a) State two chemical features which these two molecules have in common and one difference between the two molecules. [3] Features in common. (i)............................................................................................. (ii)............................................................................................. Page 10 of 51 WJEC/CBAC 2016
Difference (b) Substrate level phosphorylation (SLP) is the simplest, oldest and least-evolved way to make ATP. In substrate level phosphorylation, ATP is made during the conversion of an organic molecule from one form to another. Energy released during the conversion is used to synthesise the high energy bond of ATP. (i) Describe the position of the 'high energy bond of ATP' referred to in the paragraph above. [1] (ii) Suggest why SLP is referred to as the 'simplest and oldest way to make ATP'. [2] The diagram below shows glycolysis, the link reaction and Kreb's cycle. Page 11 of 51 WJEC/CBAC 2016
(iii) Draw two arrows labelled A and B on the diagram opposite to show the two steps where there is conversion of an organic molecule from one form to another and SLP occurs. [2] (iv) Give the number of ATP molecules made by SLP at each of the conversions shown in part (iii) per glucose molecule in aerobic respiration. Page 12 of 51 WJEC/CBAC 2016
[2] (c) (i) Where does the link reaction occur in cells? [1] (ii) Name the two types of enzyme involved in the link reaction. [1] Page 13 of 51 WJEC/CBAC 2016
6. (a) Skeletal muscle fibres can respire aerobically and anaerobically. (i) State precisely where in a muscle fibre aerobic respiration and anaerobic respiration take place. [1] Aerobic......................................................................................... Anaerobic....................................................................................... (ii) State when a muscle fibre would respire anaerobically. [1] (b) (i) Describe the process by which lactic acid (lactate) is produced in muscle fibres. [4] (ii) Suggest why the lactic acid is removed from the muscle fibre. Page 14 of 51 WJEC/CBAC 2016
[1] (c) What is meant by the term 'oxygen debt'? [1] (d) Other organisms can respire anaerobically. What are the terms used to describe the following? [2] I. bacteria which only survive in the absence of oxygen II. bacteria which grow in the presence or absence of oxygen Page 15 of 51 WJEC/CBAC 2016
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9. The detailed diagram below shows the link reaction and Krebs cycle. Most of the intermediates involved are named. (a) (i) Using the diagram above, and your own knowledge, determine how many carbons there are in the following Krebs cycle intermediates. [1] citrate............................................................................. α-ketoglutarate............................................................................. succinate............................................................................. Page 21 of 51 WJEC/CBAC 2016
(ii) Explain precisely how you have arrived at these figures. [2] (b) Briefly describe how reduced FAD and reduced NAD are used to create an electrochemical gradient. [4] In patients with a suspected mitochondrial disorder a minimally invasive tissue biopsy provides the best opportunity to examine mitochondrial function, freezing the sample immediately after collection. In addition to activity measurements of individual enzymes, analysis of mitochondrial respiration and ATP production rates are performed. This includes the measurement of mitochondrial oxygen consumption in the presence of different substrates, such as pyruvate and α-ketoglutarate. Analysis may show increased levels of a Krebs cycle intermediate, such as malate and succinate. Defects in mitochondrial ATP synthesis may lead to high lactate levels in blood. J Inherit Metab Dis. 2011 April; 34(2): 283-292 (c) Name the two enzyme types involved in the conversion of pyruvate to acetyl CoA [2] Page 22 of 51 WJEC/CBAC 2016
Using the diagram opposite, the text above and your own knowledge, answer the following questions. (d) Suggest a suitable tissue to examine mitochondrial function and explain why you have chosen this tissue with respect to patient safety. [2] (e) What could be deduced if the oxygen consumption was low with the pyruvate as a substrate but high with α-ketoglutarate as a substrate? [2] (f) What could be deduced if there was a build up of any one of the Krebs cycle intermediates? [1] (g) Explain why there is a raised blood lactate level in many patients with mitochondrial disease. [2] Page 23 of 51 WJEC/CBAC 2016
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11. The diagram below shows the link reaction and the Krebs Cycle. The number of carbon atoms present in some of the molecules is shown. (a) State precisely where in the cell the reactions of the Krebs Cycle take place. Page 26 of 51 WJEC/CBAC 2016
[2] (b) (i) On the diagram opposite, use arrows marked CO² to show the points where carbon dioxide is removed. [2] (ii) Name the process by which the carbon dioxide is removed. [1] (iii) Describe briefly what happens to a molecule of carbon dioxide removed in this way in a human. [3] (c) The role of the Krebs Cycle and glycolysis is to generate reduced NAD to be used in ATP manufacture. Describe the way in which reduced NAD is produced in the Krebs Cycle. [3] Page 27 of 51 WJEC/CBAC 2016
(d) Reduced NAD is also produced during glycolysis. Explain what happens to the reduced NAD under anaerobic conditions and why this is essential for glycolysis to continue. [2] 12. For each stage of the respiratory cycle shown in the table below use ticks ( ) to indicate which statements are correct. Page 28 of 51 WJEC/CBAC 2016
13. Respiration results in the production of ATP in cells. Production of ATP in the mitochondrion is catalysed by an enzyme and requires energy supplied by a proton gradient. The diagram below represents a model of the ATP synthetase complex. (a) (i) On the diagram above, complete the equation for the production of ATP. [1] (ii) Name parts W and Z shown on the diagram above. [2] W... Z... Page 29 of 51 WJEC/CBAC 2016
(b) The proton gradient can be maintained as long as reduced NAD is available in the mitochondrion. Explain the reasons for reduced NAD being required to maintain a proton gradient. [2] (c) The diagram below shows one stage of respiration in which reduced NAD is produced. (i) Name chemicals P and Q. Page 30 of 51 WJEC/CBAC 2016
[1] P... Q... (ii) Where in the cell do the reactions shown in the diagram above occur? [1] (iii) Outline the pathway for the production of triose phosphate in glycolysis. [3] (d) (i) The reaction labelled R on the diagram on the previous page occurs in humans when there are anaerobic conditions in a tissue. Explain the biochemical reasons for carrying out the reaction, despite the fact that lactate is toxic in high concentrations. [3] Page 31 of 51 WJEC/CBAC 2016
(ii) Reaction R sometimes occurs in muscle fibres when a short burst of very rapid ATP production is needed. Suggest a reason for this. [1] Page 32 of 51 WJEC/CBAC 2016
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Examiner's Comments 1. A well answered question in general. However, some confusion was seen between mitochondria and chloroplasts and in identifying the regions where different stages of respiration takes place; a number identified substance W as ATP rather than carbon dioxide and quite a few were unable to identify where substrate-level phosphorylation occurs. This comment originally referred to question 6 on paper 1074/01 (11/01/2013) 2. A substantial majority of candidates were awarded all three marks for part (a). In part (a) (i) some candidates failed to label the base or labelled it incorrectly. In part (a) (ii) the most common errors were 'andenine triphosphate' and 'andenosine triose phosphate.' In part (b) (i) there were some very poor drawings despite only having to copy them from part (a). Most candidates were able name an intracellular energy requiring process for part (b) (ii). In part (c) the table was correctly completed by fewer candidates than expected, apart from not getting the numbers of molecules right, many candidates did not describe both matrix and mitochondria for precise location. Parts (c) (ii) and (iii) were generally well answered. There was some confusion with anaerobic respiration in animals in part (d) with 'lactic acid' being given quite often. This comment originally referred to question 1 on paper 1074/01 (13/06/2011) 3. There were some excellent responses to this question with candidates from a number of centres having a good grasp of biochemical details. It was particularly pleasing to see good responses to (d) although many failed to appreciate the importance of the term 'per turn of the cycle' in the stem of the question. In (d) (iii) very few knew that the electron carrier system involving NAD has three pumps whereas that involving FAD has two pumps. This comment originally referred to question 6 on paper 1074/01 (24/01/2011) 4. Part (d) was poorly answered with the majority of candidates having little understanding of respiration energy budgets. This comment originally referred to question 4 on paper 1074/02 (24/01/2011) Page 47 of 51 WJEC/CBAC 2016
5. Often gained full/almost full marks, especially since it was largely recall. Part (a) was synoptic and tested nucleotide structure with some common sense since they could use the description in the stem of the question. It routinely gained full marks, as did (b)(iii), (iv) and (c). Part b)(i) proved difficult for some to describe and many resorted to diagrams which were credited. Part(ii) seemed to be an 'all or nothing' response. Either scripts referred to the fact that it did not need oxygen (in glycolysis) or involve an ETC/electrochemical gradient/the process of chemiosmosis/stalked particles/mitochondria and got both marks, or they simply thought that it was used by simple organisms, had been discovered first and was therefore the oldest and got nothing. This comment originally referred to question 6 on paper 1074/01 (10/01/2014) 6. Candidates were able to identify where aerobic and anaerobic respiration takes place in a muscle fibre and how lactic acid is produced in muscle fibres. Knowledge and understanding of the oxygen debt was poor. This comment originally referred to question 5 on paper 1074/02 (13/06/2014) 7. (a) (i) Most candidates understood that glycolysis takes place in the cytoplasm but a significant number thought that it was matrix or 'cytoplasm' in the mitochondria. (ii) Candidates had a very sound understanding of the stages of glycolysis and the stages where dehydrogenation, phosphorylation and splitting of hexose take place. (iii) Most candidates correctly stated that the net gain of ATP when one molecule of glucose is broken down was two. (iv) There was a very good understanding of the process of anaerobic respiration in muscle. (b) (i) There was a very good understanding of what happens to fats under aerobic conditions. Most candidates correctly stated that fats were broken down into fatty acids and glycerol and correctly stated how these were used, several candidates correctly stated that there was more hydrogen and linked this to chemiosmosis. (ii) Most correctly stated that carbon dioxide was the form that carbon was excreted from the body, there were some very interesting alternatives but alas incorrect. Page 48 of 51 WJEC/CBAC 2016
(iii) Some candidates did state that increased levels of oxygen would be required when fats are used as a respiratory substrate and that more carbon dioxide would be produced but many candidates were unable to give a valid suggestion as to why fats were not used in muscles as the main source of energy. This comment originally referred to question 2 on paper 1074/02 (11/01/2013) 8. (a) Anaerobic respiration. Before attempting an essay question it is advisable that candidates plan out their answers. Many failed to do this and so did not appreciate the important points required by the question. Although there were some good responses many candidates described only the breakdown of pyruvic acid into lactic acid. In failing to describe the pathway of conversion of glucose to pyruvate they were unable to achieve the maximum marks. Many concentrated only on the removal of lactic acid. (b) Fermenter. It was pleasing to see some really good, detailed answers to this question. This comment originally referred to question 8 on paper 1074/02 (24/01/2011) 9. This was designed to test the understanding of respiratory knowledge and proved to be an excellent discriminating question. Only those who really understand respiration gained all marks and their scripts were a real pleasure to mark - it is to be hoped they might put this knowledge and ability to use in their future careers. Part (a) routinely gained the mark for the number of carbons in the intermediates but, despite being asked for a precise explanation of how these figures were derived could not explain their logic. Many answers referred to 6, 5 and 4 carbon compounds and not the named intermediates in the diagram and had citrate being decarboxylated not isocitrate. Parts (b) and (c) routinely gained full marks by those who knew the work. Part (d) gave some unusual suggestions ranging from kidneys, small intestine, skin and tongue. Only the best answers gained both marks with muscle, and the reasons why this was a good tissue to choose. Parts (e), (f) and (g) required analysis of the information given and the use of the Krebs cycle diagram. Some superb answers, but many scripts showed a poor grasp of metabolic pathways and the use of enzymes to control them. Far too many simply stated that Page 49 of 51 WJEC/CBAC 2016
oxygen was not available and this led to the results stated. How people with mitochondrial disease can survive at all if they can only respire anaerobically was not considered. This comment originally referred to question 6 on paper 1074/01 (13/06/2014) 10. A substantial majority of candidates were awarded all three marks for part (a). (a) (i) Some candidates failed to label the base or labelled it incorrectly. (ii) The most common errors were 'andenine triphosphate' and 'andenosine triose phosphate.' (b) (i) There were some very poor drawings despite only having to copy them from part (a). (ii) Most candidates were able name an intracellular energy requiring process for part (b) (ii). (c) (i) The table was correctly completed by fewer candidates than expected, apart from not getting the numbers of molecules right, many candidates did not describe both matrix and mitochondria for precise location (ii)&(iii) Were generally well answered. (d) There was some confusion with anaerobic respiration in animals, with 'lactic acid' being given quite often. This comment originally referred to question 1 on paper 1074/02 (13/06/2011) 11. Part (a) was well answered but for (b)(i) very few candidates scored full marks because they did not know that carbon dioxide is given out between pyruvate and acetyl Co-A. The term decarboxylation was well known but candidates often struggled to describe the removal of carbon dioxide from the body. Few mentioned that it diffused from the mitochondria though many referred to its transportation in the blood with few mentioning it diffusing into the alveoli before being exhaled. In answer to part (c) few candidates mentioned dehydrogenation via a series of intermediates. Many talked about the removal of hydrogen without specific reference to hydrogen atoms or protons. The answers to part (d) often referred to the production of lactate though often failed to mention the regeneration of NAD. This comment originally referred to question 2 on paper 1074/02 (11/06/2013) Page 50 of 51 WJEC/CBAC 2016
12. Most candidates demonstrated an impressive knowledge of all stages of the respiratory cycle, the only common error was to assume that oxygen is used in the link reaction, Krebs cycle and Oxidative phosphorylation and that ATP is produced in the link reaction. This comment originally referred to question 5 on paper 1074/02 (10/01/2014) 13. Apart from a very occasional 'cell membrane' for W and 'cytoplasm' for Z all sub-parts of part (a) were well answered. The role of NAD was well understood in part (b) though there was some confusion over oxidation and reduction. In part (c)(i), the vast majority of candidates were awarded the mark, however a very small number reversed the ATP/ADP. Most candidates were able to state that the reactions take place in the cytoplasm in part (c)(ii).the early steps of glycolysis were fairly well understood in part (c)(iii), although a significant number of candidates failed to pick up all three marks. Part (d)(i) proved to be a challenge with relatively few candidates scoring all three marks. QWC was an issue and a significant number of candidates made no attempt at an answer. Likewise, the mark for part (d)(ii) was awarded relatively infrequently, where it was it was usually for describing low oxygen conditions. This comment originally referred to question 5 on paper 1074/01 (11/06/2013) Page 51 of 51 WJEC/CBAC 2016