A-LevelBiologyEnergy and respiration9700/42 · Oct/Nov 2024 · Q67 Marks

6 (a) DCPIP is an indicator and can be used to determine the rate of respiration of organisms such as yeast. [Figure 6.1]

(a)(i)State the category of indicators to which DCPIP belongs.[1]
(a)(ii)Describe the colour change that occurs in DCPIP during experiments to determine the rate of respiration.[1]
(b)(i)An investigation was carried out to determine the effect of temperature on the rate of respiration of yeast. • A suspension of yeast cells was added to a test-tube containing glucose solution. • A further four test-tubes were set up in the same way. • One test-tube was placed in a water-bath at 10°C for 5 minutes. • DCPIP was added to the test-tube and the time taken for the DCPIP to change colour was measured. • The experiment was repeated using the other test-tubes at 20 °C, 30°C, 40°C and 50 °C. The results are shown in Fig. 6.1. State two variables that need to be kept constant in this experiment.[2]
(b)(ii)Explain the results shown in Fig. 6.1.[3]

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Mark scheme

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

redox ;
blue to colourless ;
any two from:
1 volume / mass / concentration, of yeast suspension ;
2 volume / mass / concentration, of glucose (solution) ;
3 volume / concentration, of DCPIP ;
4 AVP ; e.g. time when DCPIP is added / pH
any three from:
1 the less time taken (for DCPIP) to change colour the higher the rate of respiration ; ora
rate increases up to 40 °C
2 due to increase in, kinetic energy / KE ;
3 due to more enzyme-substrate complexes formed or due to more (successful / effective) collisions between enzymes and substrates or more enzyme catalysed reactions ;
rate decreases after 40 °C
4 due to denaturation of (named respiration) enzyme ;
5 further detail ; e.g. active site shape change / optimum temperature 40 °C

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