Cyclic Photophosphorylation & PS I — NEET Biology
✅ Asked in NEET 2023When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons. A possible location where this could be happening is in the stroma lamellae. While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+. Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.
In cyclic photophosphorylation, only PS I operates and electrons cycle back to PS I instead of moving to NADP+, producing only ATP—not NADPH. This occurs in stroma lamellae lacking PS II and NADP reductase. Students often confuse it with non-cyclic photophosphorylation (which uses both PS I and PS II to make both ATP and NADPH). Remember: cyclic = PS I only = ATP only; non-cyclic = both photosystems = ATP + NADPH. NTA tests whether you distinguish between these pathways and understand which products each makes.
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