A-LevelBiologyPhotosynthesis9700/42 · Oct/Nov 2022 · Q312 Marks

Photosynthesis is an energy transfer process that takes place in chloroplasts of plant cells. [Figure 3.1, Figure 3.2]

(a)(i)Fig. 3.1 shows a diagram of a chloroplast. Using the letters A, B, C or D, identify the structure that: • contains genes that code for some of the enzymes used in photosynthesis • is the site of synthesis of some of the enzymes used in photosynthesis.[2]
(a)(ii)Explain how grana are adapted for their specific role in photosynthesis.[4]
(b)(i)An experiment was carried out to investigate the effect of changing light conditions on the pH of the chloroplast stroma. Scientists followed pH changes in chloroplast stroma using fluorescent chemicals that can be used as pH indicators. • Chloroplasts were isolated from cells. • A suspension of chloroplasts was prepared and kept in the dark for 180 seconds. • The chloroplasts were exposed to a period of light of fixed intensity for 240 seconds, then returned to dark conditions. • The pH of chloroplast stroma was continuously measured and recorded. Fig. 3.2 shows the results of this experiment. Describe the results shown in Fig. 3.2.[3]
(b)(ii)Discuss how the results in Fig. 3.2 support that chemiosmosis occurs during photophosphorylation.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[2]
D;
B;
[4] any four from:
1 thylakoid stacks or discs / many thylakoids, for, increased or large, surface area;
2 increased or large, surface area maximises the absorption of light (energy) / AW ; Ignore trap (light energy)
mp 3-5 thylakoid membranes/grana contain
3 photosynthetic / named / primary / accessory, pigments to absorb light (energy);
4 electron carriers / ETC, to, transfer / release, energy (from excited electrons);
5 photosystems / antenna complex and reaction centre, are light harvesting structures ;
6 thylakoid space or lumen to, form proton gradient/have high concentration of protons ;
7 thylakoid membrane is (relatively) impermeable, to maintain the proton gradient (for chemiosmosis) ;
8 ATP synthase to make ATP ;
9 contains oxygen-evolving complex / OEC, for the photolysis of water;
[3] any three from:
1 the pH is higher in the light than in the dark ora or the pH increases (from dark) to light or the pH decreases (from light) to dark;
2 the pH, increases sharply / AW, when changed to light ;
3 the pH, levels off / AW, in light ;
4 the pH decreases, gradually / less steeply, when returned to the dark or decrease in pH does not return to original (dark) pH ;
5 ref. to fluctuations anywhere / described ;
6 comparative figures to support ; (mp1) dark(1) ~ pH 7.31–7.6 vs light ~ 7.52–7.61 vs dark (2) ~ 7.51–7.38 (mp2) from pH 7.36–7.56 in 36 s (1/2 squares and each square 24 s) (mp4) from pH 7.56–7.4 in 288 s (12 squares)
[3] any three from:
1 the stroma has higher pH when H+ ions, move, out (of stroma) / into thylakoid, space / lumen ;
2 (leads to) increased H+ concentration in the thylakoid, space / lumen;
3 (then) H+ diffuse back (into stroma) through ATP synthase ;
4 AVP; e.g. idea not to support theory