Biology · Photosynthesis in Higher Plants · NEET
| Reaction centre | P680 (chlorophyll a) | P700 (chlorophyll a) |
| Absorption peak | 680 nm | 700 nm |
| Splits water / photolysis | Yes (water-splitting complex here) | No |
| Releases oxygen | Yes | No |
| Order of action in Z-scheme | First | Second |
| Final electron output | Passes electrons on to carriers | Reduces NADP+ to NADPH |
| Cyclic photophosphorylation | Not involved | Involved (only PS I) |
The photosystems were named in the order scientists discovered them, not the order they work in. In the non-cyclic pathway (Z-scheme), electrons start at PS II, travel through carriers, and end at PS I. So PS II acts first, PS I acts second. NCERT states this directly: they are named in the sequence of their discovery, and not in the sequence in which they function.
Only Photosystem II (PS II) splits water. The oxygen-evolving (water-splitting) complex is attached to PS II. When water splits, it gives electrons (to refill P680), protons (H+), and oxygen. PS I does NOT split water and does NOT release oxygen. A very common NEET trap is to link the water-splitting complex with PS I, which is wrong.
P700 is the reaction centre chlorophyll a of Photosystem I; it absorbs light best at 700 nm. P680 is the reaction centre chlorophyll a of Photosystem II; it absorbs best at 680 nm. Both are single chlorophyll a molecules, but they sit in different photosystems and absorb at different wavelengths. The 'P' stands for pigment and the number is the absorption peak in nanometres.
Photosystem I is the one whose electrons finally reduce NADP+ to NADPH. Electrons excited from P700 pass to ferredoxin and then to NADP+, forming NADPH on the stroma side. PS II does not make NADPH directly; it starts the electron flow and makes oxygen. So: PS II = water splitting and O2, PS I = NADPH.
No. Cyclic photophosphorylation uses only Photosystem I (P700). Electrons leave P700, go around a loop, and come back to P700, making only ATP (no NADPH, no O2). Non-cyclic photophosphorylation uses BOTH PS I and PS II and makes ATP, NADPH and oxygen. This 'which system is used' point is a frequent NEET question.
The reaction centre in PS II has an absorption maxima at
Which pigment has an absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700)?
Find the incorrect statement(s) about photosynthesis: A. The water splitting complex is associated with PS I. B. C4 plants use the C3 pathway of CO2 fixation as the main biosynthetic pathway. C. In C4 plants, photorespiration does not occur. D. C3 plants exhibit 'Kranz' anatomy. E. ATP synthesis in chloroplast occurs through chemiosmosis.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
PS II splits water and releases oxygen and has reaction centre P680 (680 nm). PS I has reaction centre P700 (700 nm) and finally passes electrons to NADP+ to make NADPH. PS II acts first in the Z-scheme, PS I second.
Because it was discovered first. NCERT clearly says the photosystems are named in the order of their discovery, not the order in which they function during the light reaction.
Only Photosystem I (P700). It makes only ATP, no NADPH and no oxygen. Non-cyclic photophosphorylation uses both PS I and PS II.
The reaction centre in both photosystems is a single molecule of chlorophyll a. The other pigments (chlorophyll b, carotenoids, xanthophylls) act as accessory light-harvesting pigments in the antenna.
Photosystem II produces oxygen through the splitting of water (photolysis). PS I never releases oxygen.
