Photosynthesis

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12
Question 2
Hard
a.

A student makes the following statement: 'Plants produce all the ATP they need through the light-dependent stage of photosynthesis, so they do not need to carry out respiration.'

Explain why the student's statement is not correct.

[3]
b.

Scientists researching how chloroplasts produce ATP during the light-dependent stage of photosynthesis proposed a model:

  • Hydrogen ions (H+\text{H}^+H+) are actively transported (pumped) into the thylakoid space inside the thylakoid membrane, as shown in Fig. 1.1.
  • The ions become highly concentrated within this space.
  • The ions diffuse back out into the stroma through ATP synthase, generating ATP.

To test this model, the scientists treated isolated chloroplasts with a chemical uncoupler that makes the thylakoid membrane highly permeable (leaky) to hydrogen ions, as shown in Fig. 1.2.

Diagram showing two thylakoid membranes. Fig. 1.1 (untreated chloroplast) represents a thylakoid with hydrogen ions (H+) highly concentrated inside the thylakoid lumen, diffusing out into the stroma only through a membrane-bound ATP synthase protein. Fig. 1.2 (treated chloroplast) shows the thylakoid lumen with pores in the membrane, allowing hydrogen ions to leak directly through the membrane, bypassing ATP synthase.

The result of their experiment showed that when the uncoupler was present, ATP production fell to zero.

Explain how this result shows that their model is correct.

[2]

Photosynthesis Questions

  1. GCSE
  2. /Biology
  3. /Photosynthesis