Some students decided to find the respiratory quotient (RQ) of germinating pea seeds. Pea seeds germinate best at temperatures between 13°C to 18°C. Table 1.1 shows the main biological molecules in pea seeds that could be used to release energy for seed germination. Table 1.1 biological molecule g per 100 g of dry mass carbohydrate 68 protein 25 fat 1 The students used the respirometer shown in Fig. 1.1. [Figure 1.1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [1] (approximately) 1 ; A value in 0.94 to 1 range to allow for mixed CHO and protein
2. [7] (a)(ii) any 7 from:
3. 1 idea of adding a fixed, number / mass, of peas to respirometer (below the shelf with soda lime);
4. 2 idea of a control containing dead (sterilised) peas / glass beads of same mass ;
5. 3 idea of inserting bung with tubing with clip and glass tubing with scale so that_the apparatus is airtight / AW;
6. 4 idea of dipping glass tube into coloured liquid ;
7. 5 idea of placing respirometer tube into a water-bath at a stated temperature in range 13°C to18°C;
8. 6 idea of (opening clip and) leaving to equilibrate ;
9. 7 idea of closing the clip and marking the position of dye and idea of starting stop-clock immediately;
10. 8 idea of measuring movement of dye on the scale, for 5 minutes / at timed intervals ;
11. 9 idea of opening clip and resetting the coloured liquid ;
12. 10 idea of doing minimum of three measurements in sequence and taking a mean;
13. 11 idea of removing carbon dioxide absorbent ;
14. 12 idea of again measuring movement of dye on the scale, for 5 minutes / at timed intervals ; must be linked to mp11
15. 13 low risk experiment
16. or
17. 14 medium risk experiment and carbon dioxide absorbent is harmful and wear gloves ;
18. 15 AVP;
19. [3] (b) any 3 from:
20. 1 find radius of glass tubing and use πr² (to find surface area);
21. 2 multiply πr² by distance moved by liquid with carbon dioxide absorbent to find volume of oxygen used ;
22. 3 multiply πr² by distance moved by liquid without carbon dioxide absorbent ;
23. 4 add together the volume with carbon dioxide absorbent and volume without carbon dioxide absorbent (to find volume of carbon dioxide released) ; A marks on a correct formula e.g. oxygen consumed = πr² × distance moved with carbon dioxide absorbent = 2
24. [3] (c)(i)
25. 1 idea of replacing the sodium hydroxide by (a named) oxygen absorbent/replacing air with nitrogen ;
26. 2 idea of setting dye position, close to the tube containing peas / half way along the tube ;
27. 3 look for liquid moving away from the peas (showing that carbon dioxide is produced in anaerobic conditions) ;
28. [1] (c)(ii) trial 2 and the result, is anomalous / does not fit the pattern ;