The induced-fit hypothesis and the lock-and-key hypothesis are used to describe the mode of action of enzymes. Radish plants contain the enzyme peroxidase that catalyses the breakdown of hydrogen peroxide. Students investigated the effect of increasing the concentration of hydrogen peroxide on the activity of peroxidase extracted from radish. The results of their investigation are shown in Fig. 4.1. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [3] idea of shape needs to be in the answer for mp2 and mp4 \ 'substrate changes shape'
2. any three from
3. 1 active site is not (fully) complementary to substrate ;
4. 2 active site, changes shape / moulds around, to fit the substrate ;
5. A conformational change for shape change
6. 3 enzyme-substrate complex / ESC, forms ;
7. 4 active site returns to original shape on release of product ;
8. 5 AVP;
9. e.g. change of shape (to give complementary fit) lowers activation energy / puts strain on bonds / AW
10. ref. to binding site / catalytic site (of active site)
11. [3] A peroxidase for enzyme
12. any three from
13. rate of reaction increases as substrate concentration increases to 0.33 – 0.35 mmol dm⁻³
14. 1 (up to 0.33 to 0.35 mmol dm⁻³) some active sites are not occupied ;
15. 2 more collisions between, enzyme / active site, and substrate molecules ;
16. A more collisions lead to increase in formation of enzyme-substrate complexes
17. rate of reaction remains constant above 0.33-0.35 mmol dm⁻³
18. 3 all active sites, occupied / saturated ;
19. 4 correct reference to limiting factor for slope ; A limits / limiting
20. e.g. at low substrate concentrations, substrate concentration limiting / enzyme concentration not limiting
21. 5 correct reference to limiting factor for plateau ; A limits / limiting
22. e.g. at high substrate concentrations, enzyme concentration limiting / substrate concentration not limiting
23. [2] idea that determined Vmax / maximum rate / ref. to 5.6 (µmol min⁻¹) ;
24. A if Vmax shown on graph
25. Km is the substrate concentration at half the Vmax ;
26. [3] any three from
27. in respiring tissue
28. 1 (carbonic anhydrase) catalyses the reaction between water and carbon dioxide in, red (blood) cells / erythrocytes ;
29. 2 to form carbonic acid, which dissociates to form HCO₃¯ and H⁺ ;
30. A from equation A use of word ion(s)
31. 3 HCO₃¯, moves / passes / diffuses (through membrane proteins from red blood cells) into plasma ;
32. 4 (activity of enzyme) maintains (steep) concentration gradient for diffusion of carbon dioxide into red blood cells ;
33. 5 idea that so helps to remove large quantities of carbon dioxide ;
34. in lungs
35. 6 (carbonic anhydrase) converts HCO₃⁻ back into carbon dioxide so can be, excreted / removed / exhaled ;
36. if described earlier A catalyses reverse reaction so carbon dioxide can be, excreted / AW