Trypsin is an enzyme which catalyses the hydrolysis of casein, a protein found in milk. Milk that contains casein has a cloudy, white appearance. As the casein is hydrolysed by trypsin, the milk changes in appearance to a clear (transparent), colourless solution. A student carried out an experiment to investigate the effect of enzyme concentration on the rate at which trypsin hydrolyses casein. The student added a solution of trypsin to a sample of milk and recorded the time taken for the milk to become transparent. The student repeated the experiment with different concentrations of trypsin. All other variables were kept constant. Fig. 5.1 shows the results from the experiment. [Figure 5.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

1. 5(a)(i) correct answer as a whole number 34(%);
2. 5(a)(ii) allow enzyme for trypsin allow substrate for casein any five from:
3. 1 at lower concentrations there are not enough, active sites / enzymes;
4. 2 idea of substrate molecules cannot enter active site until product released ;
5. 3 as concentration increases, increased number of active sites ;
6. 4 idea that as concentration increases there are more successful collisions between enzyme and, substrate / casein ;
7. 5 (so) allows increase in (number of) enzyme-substrate complexes (formed per unit time);
8. 6 increased rate of casein breakdown as enzyme concentration increases ;
9. 7 ref. to casein reaction e.g. milk becomes transparent as solution contains amino acids ;
10. 8 AVP; e.g. depends on how many active sites available at any one time ref. to enzyme-substrate complex formation per unit time at high concentration substrate concentration is becoming the limiting factor when clear, total hydrolysis of casein has occurred
11. 5(b)(i) any one from: immobilising the enzyme may have altered the, tertiary structure of the enzyme / shape of the active site;
12. suggestion that material may not be inert and may have an inhibitory effect at lower temperatures ;
13. immobilising may have covered, part of the active site / active site of some enzymes ;
14. idea that trypsin free in solution has increased chance of collision with substrate ;
15. immobilising changes the optimum temperature of the enzyme ;
16. 5(b)(ii) any two from: immobilisation, protects / stabilises / AW, the, enzyme / trypsin;
17. from (thermal) denaturation ; ora enzyme free in solution denatured
18. maintains shape of active site;
19. immobilisation may result in lower, kinetic energy / vibration, of the enzyme molecules compared to free (so protects from denaturation) ; ora
20. prevents the breaking of bonds holding the tertiary structure (of immobilised enzyme molecule); ora
21. ref. to, hydrogen / ionic bonds ;