Pepsin is an enzyme that hydrolyses proteins (protease). Some students used pepsin from the stomach of a mammal. The activity of the pepsin was investigated by placing a small quantity of the enzyme with a known concentration of the protein albumen. Fig. 2.1 shows the progress of the enzyme-catalysed reaction that was carried out at 20°C. [Figure 2.1]

Mark scheme

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

1. [2] any two from: anything within range 0.6 to 0.8;
2. µmol dm⁻³ min⁻¹ / µmol per dm³ per min; A µmol per dm³ / min or µmol dm⁻³ / min
3. [2] ignore any explanation any two from:
4. 1 initial rate will be, lower / slower ;
5. 2 takes longer to reach the, plateau / end concentration / 10 µmol dm⁻³ ; A longer to complete the reaction / maximum concentration
6. 3 idea that the final / end, concentration of product will be, the same / 10 µmol dm⁻³; I refs to the shape of the graph
7. [2] ignore any explanation any two from:
8. 1 initial rate will be, higher / faster;
9. 2 shorter time to reach, / plateau / end concentration / 10 µmol dm⁻³ ; A shorter time to complete the reaction / maximum concentration
10. 3 idea that the final / end, concentration of product will be, the same / 10 µmol dm⁻³; I ref. to the shape of the graph
11. [2] any two from:
12. 1 allows a (valid) comparison of the, activities / AW, of the enzymes ;
13. 2 initial rate is the fastest rate;
14. 3 idea that if rate is calculated over a longer period of time than the linear part of the curve the rate will vary ;
15. 4 not limited by substrate concentration / substrate is in excess ; A 'no limiting factors' if substrate not mentioned
16. 5 not influenced by (increase in concentration of) product ;