Q.i) What is photophosphorylation.

Photophosphorylation makes ATP using light energy; the cyclic route uses only PS I and recycles its electron, producing ATP alone (no NADPH, no oxygen).

i) Photophosphorylation: The synthesis of ATP from ADP and inorganic phosphate, driven by the energy captured from light during the light-dependent reactions of photosynthesis, occurring across the thylakoid membrane using the proton gradient generated by the electron transport chain.

ii) Cyclic photophosphorylation: This occurs when only Photosystem I (PS I, reaction centre P700) is active - typically in the stroma lamellae, which lack Photosystem II and NADP reductase. Light excites an electron in P700, which is passed to the primary electron acceptor and then down an electron transport chain (through cytochrome b6-f complex and plastocyanin) that pumps protons into the thylakoid lumen, building a proton gradient used by ATP synthase to make ATP. Instead of being handed to NADP+ (as in the non-cyclic/linear pathway), the electron is returned back to the P700 reaction centre of PS I itself, completing a cycle. Because the electron is recycled back to PS I rather than reducing NADP+, cyclic photophosphorylation produces ATP only - i …