A-LevelBiologyEnergy and respiration9700/42 · May/June 2017 · Q19 Marks

(a) Fig. 1.1 represents the link reaction. R + coenzyme A NAD reduced NAD S + acetyl coenzyme A Fig. 1.1 With reference to Fig. 1.1:

(a)(i)name substances R and S R S[2]
(a)(ii)explain what happens to the reduced NAD.[2]
(b)The pH of the blood of an athlete decreases during a race and returns to its normal level after the race. The decrease in the pH of the blood is caused by the presence of waste products that have been excreted by cells during respiration. Name the waste products that are excreted and describe what occurs to these products to help return the pH of the blood back to a normal level.[5]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[2] R - pyruvate ; S - carbon dioxide;
[2] idea that, hydrogen(s) / protons and electrons, are released ; A (reduced NAD), oxidised / dehydrogenated at ETC / (for) oxidative phosphorylation;
[max 5] 1. lactate (produced); A lactic acid 2. (lactate) taken to liver ; 3. converted to pyruvate ; 4. (pyruvate) converted to, glucose / glycogen ; 5. carbon dioxide (produced); 6. ref. to carbon dioxide / pH, receptors; 7. (carbon dioxide) goes into alveoli ; 8. increased breathing (rate) ; 9. ref. to haemoglobin acts as a buffer for carbon dioxide ;

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