A-LevelBiologyEnergy and respiration9700/41 · May/June 2019 · Q410 Marks

The link reaction and Krebs cycle take place in the mitochondrion. The main stages of the link reaction and Krebs cycle are listed in Table 4.1. They are not listed in the correct order. [Table 4.1] Complete Table 4.2 to show the correct order of the stages. Three of the stages have been done for you. [Table 4.2]

(a)Complete Table 4.2 to show the correct order of the stages. Three of the stages have been done for you.[4]
(b)Outline the role of NAD in respiration in aerobic conditions.[4]
(c)(i)Carbon dioxide is removed from compounds in the link reaction and Krebs cycle by decarboxylation. State the total number of molecules of carbon dioxide removed in the link reaction and Krebs cycle for each molecule of glucose respired.[1]
(c)(ii)In a mammal, carbon dioxide diffuses from cells into the blood to be transported to the lungs. Suggest why carbon dioxide is transported in the blood mainly as hydrogen carbonate ions and not as carbonic acid.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[4] correct order letter of stage 1 E 2 I 3 F 4 A 5 G 6 J 7 B 8 K 9 C/H 10 H/C 11 D ;;;;
[4] any four from: 1 acts as, coenzyme / cofactor; 2 to dehydrogenase / in dehydrogenation ; 3 takes/carries / delivers / releases / gives, hydrogen / H (atoms); 4 to, (carrier on) ETC / inner mitochondrial membrane / cristae or for, ATP production / oxidative phosphorylation ; 5 ref. to redox / reduction and oxidation / reduced and oxidised; 6 2.5/3, ATP per reduced NAD ;
[1] 6;
[1] any one from: 1 hydrogen carbonate (ions) prevents decrease in (blood) pH ; 2 buffer system / carbonic acid dissociates / ref. to carbonic anhydrase;

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