(a) An investigation was carried out to measure the rate of photosynthesis at different concentrations of carbon dioxide. Two different plants, barley and sugar cane, were tested at two different temperatures, 10°C and 25°C. The results are shown in Fig. 4.1. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2017). The mark or mark type is shown in brackets.

1. [1] (a)(i) three from: 1 reference to limiting factors;
2. [1] (a)(i) 2 (limiting factor) not carbon dioxide;
3. [1] (a)(i) 3 (limiting factor possibly) light intensity/temperature ;
4. [1] (a)(ii) three from: sugar cane: ORA barley 1 (rate of photosynthesis) higher rate, at lower concentrations of CO2/initially ;
5. [1] (a)(ii) 2 levels off/becomes constant, at lower rate of photosynthesis ;
6. [1] (a)(ii) 3 levels off/becomes constant, at a lower carbon dioxide concentration;
7. [1] (a)(ii) 4 data quote to support mp2 or mp3 ; e.g. mp2 - sugar cane at 7–7.5 au and barley at 14 au OR mp3 - sugar cane at 60–70 au and barley at 500 au
8. [1] (a)(iii) four from: 1 sugar cane/C4 plants, can reduce photorespiration;
9. [1] (a)(iii) 2 correct ref. to adaptation to reduce photorespiration ; e.g. RuBP and rubisco, in bundle sheath cells/kept away from air OR tightly packed mesophyll cells
10. [1] (a)(iii) 3 enzymes in, sugar cane/C4 plants, have high(er) optimum temperature ;
11. [1] (a)(iii) 4 carbon dioxide absorbed by mesophyll cells ;
12. [1] (a)(iii) 5 carbon dioxide, fixed by PEP carboxylase/combines with PEP ;
13. [1] (a)(iii) 6 PEP carboxylase has higher affinity for carbon dioxide than rubisco ;
14. [1] (b) conditions (max two): 1 low light intensity; A at night/in the dark
15. [1] (b) 2 dry conditions ;
16. [1] (b) 3 high temperatures ;
17. [1] (b) 4 high light intensity;
18. [1] (b) 5 high wind speed/AW;
19. [1] (b) benefits (max two): 6 reduce transpiration (rate);
20. [1] (b) 7 (so) conserves water ;
21. [1] (b) 8 retains turgidity of cells ;
22. [1] (b) 9 (physical) support of plant/prevents wilting ;