Biology · Photosynthesis in Higher Plants · NEET
| Photosystems used | Only PS I (P700) | Both PS II (P680) and PS I (P700) |
| Electron path | Electron cycles back to PS I | Open path, electron ends on NADP+ |
| Products | Only ATP | ATP + NADPH + H+ |
| Water splitting / O2 | No photolysis, no oxygen | Water split, oxygen released |
| Likely site | Stroma lamellae (no PS II, no NADP reductase) | Grana lamellae (have both PS I and PS II) |
| When it occurs | Only PS I active; light beyond 680 nm; extra ATP needed | Normal light, both photosystems active |
No. In cyclic photophosphorylation only PS I works and the excited electron is cycled back to PS I through the electron transport chain instead of going to NADP+. So the product is only ATP. NADPH + H+ is made only in non-cyclic photophosphorylation, where the electron finally reaches NADP+ and reduces it. This one point (only ATP vs ATP + NADPH) is the most common NEET question on this topic.
Oxygen comes from the splitting of water (photolysis), and water splitting is linked to PS II. Cyclic photophosphorylation does not use PS II, so water is not split and no oxygen is released. Non-cyclic photophosphorylation uses PS II, splits water, and therefore releases oxygen. Remember: no PS II means no water splitting means no O2.
Only PS I (with reaction centre P700) is used in cyclic photophosphorylation. Non-cyclic photophosphorylation uses both PS II (P680) and PS I (P700) working in a series. A very common NEET trap is the statement 'cyclic photophosphorylation involves both PS I and PS II' - this is wrong.
According to NCERT, a likely site is the stroma lamellae (the stroma thylakoid membranes). The stroma lamellae have PS I only; they lack PS II and lack the NADP reductase enzyme. The grana lamellae have both PS I and PS II, so non-cyclic photophosphorylation happens on the grana thylakoids.
Cyclic photophosphorylation occurs when only PS I is functional. NCERT also states it happens when only light of wavelength beyond 680 nm is available (because PS II needs shorter wavelengths to work). It also runs to supply the extra ATP that the Calvin cycle needs, since the cycle uses more ATP than NADPH per CO2 fixed.
Which of the following statements is incorrect?
Which out of the following statements is incorrect?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cyclic uses only PS I and makes only ATP. Non-cyclic uses PS II and PS I together (Z-scheme) and makes ATP, NADPH and oxygen.
Yes. Non-cyclic uses PS II, which splits water (photolysis). This water splitting releases oxygen. Cyclic photophosphorylation does not split water, so it releases no oxygen.
The Calvin cycle needs more ATP than NADPH (3 ATP and 2 NADPH per CO2). Cyclic photophosphorylation makes the extra ATP needed to balance this, since it produces ATP without producing NADPH.
In non-cyclic photophosphorylation the final electron acceptor is NADP+ (forming NADPH). In cyclic photophosphorylation there is no final external acceptor - the electron returns to PS I, so it keeps cycling.
No. Cyclic photophosphorylation does not use PS II and does not split water, so no oxygen is released. This is a frequent NEET assertion-reason and single-statement trap.
