CAIE A-Level Biology A2 13.1.10 During Photophosphorylation Questions

Explain events during photophosphorylation using evidence from each question and mark scheme.

Exam points

Explain events during photophosphorylation.

[Maximum number: 3]

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.

Fig. 3.2

Discuss how the results in Fig. 3.2 support that chemiosmosis occurs during photophosphorylation.

any three from:
1 the stroma has higher pH when $\mathrm{H}^{+}$ ions, move, out (of stroma) / into thylakoid, space / lumen ;
2 (leads to) increased $\mathrm{H}^{+}$ concentration in the thylakoid, space / lumen ;
3 (then) $\mathrm{H}^{+}$ diffuse back (into stroma) through ATP synthase ;
4 AVP ;
e.g. idea not to support theory

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CAIE A-Level Biology A2 13.1.10 During Photophosphorylation Questions question 1