NEETBotanyPhotosynthesis in Higher Plants
In the stroma lamellae of chloroplasts, only Photosystem I is functional and cyclic photophosphorylation occurs. Under these conditions, how is the proton gradient across the thylakoid membrane primarily generated?
Options
- ABy the splitting of water molecules on the inner side of the membrane.
- BBy the activity of NADP reductase enzyme located on the stroma side of the membrane.
- CBy the transfer of electrons through an H-carrier, which transports protons from the stroma into the lumen.
- DA proton gradient is not established because Photosystem II is absent in stroma lamellae.
Correct answer
C. By the transfer of electrons through an H-carrier, which transports protons from the stroma into the lumen.
Step-by-step solution
During cyclic photophosphorylation in the stroma lamellae, Photosystem II and the NADP reductase enzyme are absent. Therefore, there is no splitting of water (which normally adds protons to the lumen) and no reduction of NADP^+ (which normally removes protons from the stroma). However, ATP is still synthesised, which requires a proton gradient. This gradient is established solely by the cyclic flow of electrons through the electron transport chain. As electrons pass through the H-carrier (plastoquinone), it removes