Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) is an important enzyme involved in the light independent stage (Calvin cycle) of photosynthesis. It fixes carbon by combining carbon dioxide with RuBP. In certain situations, the active site of rubisco becomes occupied by a sugar phosphate, making the enzyme inactive. Rubisco can become active again in the presence of another enzyme, rubisco activase.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

1. [2] 1 covalent / peptide ;
2. 2 hydrogen ;
3. 3 ionic / electrostatic ;
4. 4 disulfide ;
5. 5 hydrophobic interactions ;
6. 6 Van der Waals ;
7. 4 correct = 2 marks
8. 2/3 correct = 1 mark
9. [1] remove sugar phosphates from, active site / rubisco / enzyme ; A breaks down sugar phosphate
10. [3] any three from:
11. 1 RuBP and rubisco in bundle sheath cells ;
12. 2 mesophyll cells surround the bundle sheath cells ;
13. 3 stops, air / oxygen, getting to bundle sheath cells ;
14. 4 (so) oxygen does not, combine with RuBP / react with rubisco
15. or
16. (so) carbon dioxide, combines with RuBP / reacts with rubisco;
17. [2] any two from:
18. 1 increase in (rate of), light dependent stage / photophosphorylation / photolysis ;
19. 2 (so) increase in oxygen produced ;
20. 3 leads to an increase in oxygen to carbon dioxide ratio ;
21. 4 favours reaction with oxygen (ref. to rubisco);
22. 5 more stomata open ;
23. [2] any two from:
24. 1 less RuBP to combine with carbon dioxide / less carbon fixation;
25. 2 less, GP / TP ;
26. 3 ref. to reduction in (rate of), Calvin cycle / light independent stage ;