Question

The thylakoid membrane becomes damaged, causing the inside of the thylakoid to mix with the stroma. Which of the following processes is likely to be most affected by this damage?

• A. The reduction of ${\mathrm{NADP}}^{+}$ to NADPH
• B. Sunlight absorption by the chloroplasts
• C. The movement of electrons from photosystem II (PSII) to photosystem I (PSI)
• D. ATP synthesis

Answer 1

The correct option is D ATP synthesis

If the membrane of the thylakoid is damaged and the lumen is in contact with the stroma, then the direct effect will be on the electrochemical proton gradient.

Synthesis of NADPH does not require the development of a proton gradient. PS I absorbs light of wavelength 700nm, its electron in its reaction centre P700 chlorophyll a molecule gets excited and is expelled out which is received by a modified chlorophyll molecule $A_0$, $A_1$, Fe-S centre, ferredoxin (Fd) and finally to ferredoxin ${\mathrm{NADP}}^{+}$ reductase (FNR) which passes on the electron to ${\mathrm{NADP}}^{+}$ which gets converted to NADPH.

Chloroplasts will keep absorbing sunlight despite damage to thylakoids. Light absorption is independent of thylakoid structural integrity.

Movement of electrons from PS II to PS I occurs through electron carriers which might not be affected by damaged thylakoids.

But the synthesis of ATP requires the development of a proton gradient across the thylakoid membrane i.e high proton concentration in the lumen and low in the stroma. If the thylakoid membrane becomes damaged, causing the inside of the thylakoid to mix with the stroma, there will be no electrochemical proton gradient development. This will affect the production of ATP.