The numbering trap that costs marks: PS II fires before PS I.
Photosystem II was discovered after Photosystem I — but in the actual Z-scheme of light reactions, PS II acts first. This naming-vs-reaction-order mismatch is a high-frequency NEET trap. Get it backwards and you lose 5 marks (4 + 1 negative).
Where light reactions happen. Light reactions occur on the thylakoid membranes of chloroplasts (NCERT Class 11 Biology Chapter 11, page 140). The key players are PS II (reaction centre P680), PS I (reaction centre P700), cytochrome b6f complex, and ATP synthase.
The sequence. Light hits PS II → P680 gets excited → electrons are ejected → water is split at PS II (photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂). The electrons released from water replace those lost by P680. Electrons then travel through the electron transport chain (plastoquinone → cytochrome b6f → plastocyanin) to PS I. At PS I, P700 absorbs light, boosts electrons to a higher energy level, and passes them through ferredoxin to NADP⁺ reductase, producing NADPH.
Two types of phosphorylation. Non-cyclic photophosphorylation uses both PS II and PS I — produces ATP, NADPH, and O₂. Cyclic photophosphorylation uses only PS I — electrons cycle back through cytochrome b6f, generating ATP only. No NADPH, no O₂ released.
The chemiosmotic mechanism. Proton accumulation inside the thylakoid lumen (from photolysis and electron transport) creates a gradient. H⁺ ions flow down this gradient through ATP synthase (CF₀–CF₁ complex), driving ATP synthesis.
Watch out: When a question asks "where does O₂ evolve?" or "source of electrons in Z-scheme," the answer always traces back to PS II and water photolysis — not PS I.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The reaction centre of Photosystem II has an absorption peak at which wavelength?
Answer: B. The reaction centre chlorophyll of PS II is P680, named for its absorption peak at 680 nm (NCERT Class 11 Biology Chapter 11, page 138).
Why A is wrong: A — 700 nm is the absorption peak of PS I (P700), not PS II. Swapping these values is a common confusion arising from PS I/PS II naming order (trap: PS I vs PS II reversed).
Why C is wrong: C — 660 nm is not the reaction centre wavelength for either photosystem. Chlorophyll a has an absorption peak near 663 nm in solution, but the reaction centre designation is P680.
Why D is wrong: D — 720 nm is in the far-red range and does not correspond to either PS I (700 nm) or PS II (680 nm) reaction centre.
During non-cyclic photophosphorylation, the final electron acceptor is:
Answer: A. In non-cyclic electron flow, NADP⁺ is the terminal electron acceptor; it is reduced to NADPH by NADP⁺ reductase on the stromal side (NCERT Class 11 Biology Chapter 11, page 140).
Why B is wrong: B — Cytochrome b6f is an intermediate carrier in the electron transport chain between PS II and PS I, not the terminal acceptor.
Why C is wrong: C — O₂ is a product of water splitting at PS II, not an electron acceptor. It is released, not reduced.
Why D is wrong: D — Ferredoxin is the last mobile carrier before NADP⁺ reductase, but it passes electrons onward. The final acceptor is NADP⁺.
Photolysis of water during light reactions occurs at:
Answer: C. Water splitting (photolysis) occurs at the oxygen-evolving complex (OEC) associated with PS II, located on the lumenal (inner) side of the thylakoid membrane (NCERT Class 11 Biology Chapter 11, page 139).
Why A is wrong: A — PS I does not split water. PS I (P700) receives electrons from plastocyanin, not from water directly. Confusing PS I with PS II here reflects the naming-order trap.
Why B is wrong: B — Cytochrome b6f transfers electrons between PS II and PS I but plays no role in water photolysis.
Why D is wrong: D — PS I is located on the thylakoid membrane but does not carry out photolysis. Water splitting is exclusively a PS II function.
In the Z-scheme, electrons flow in the sequence:
Answer: B. The Z-scheme electron flow is PS II → plastoquinone → cytochrome b6f → plastocyanin → PS I → ferredoxin → NADP⁺ reductase → NADP⁺. The cytochrome b6f complex sits between the two photosystems (NCERT Class 11 Biology Chapter 11, page 140).
Why A is wrong: A — This reverses the reaction order, placing PS I before PS II. Discovery order (PS I first) ≠ reaction order (PS II first). This is the classic PS I/PS II naming trap.
Why C is wrong: C — This incorrectly places cytochrome b6f after PS I. Cyt b6f mediates electron transfer between PS II and PS I, not after PS I.
Why D is wrong: D — This reverses the entire chain, starting with PS I and ending at PS II. The Z-scheme begins at PS II where water photolysis provides electrons.
Cyclic photophosphorylation involves only PS I and produces:
Answer: A. In cyclic photophosphorylation, electrons from PS I cycle back through cytochrome b6f and return to PS I. This generates a proton gradient that drives ATP synthesis only. No NADPH is formed (electrons never reach NADP⁺) and no O₂ is released (no water splitting at PS II) (NCERT Class 11 Biology Chapter 11, page 140).
Why B is wrong: B — ATP, NADPH, and O₂ are all products of non-cyclic photophosphorylation, which involves both PS I and PS II. Cyclic phosphorylation uses PS I alone.
Why C is wrong: C — NADPH is not produced in cyclic flow because electrons cycle back to PS I instead of reaching NADP⁺ reductase.
Why D is wrong: D — O₂ release requires photolysis of water at PS II. Since cyclic photophosphorylation involves only PS I, no water is split and no O₂ is evolved.
The proton gradient for ATP synthesis in chloroplasts is generated across the thylakoid membrane with higher H⁺ concentration in the:
Answer: D. H⁺ ions accumulate in the thylakoid lumen from water photolysis (releases H⁺ inside) and from electron transport through cytochrome b6f (pumps H⁺ from stroma to lumen). This creates a higher H⁺ concentration in the lumen, and the gradient drives ATP synthase (NCERT Class 11 Biology Chapter 11, page 140).
Why A is wrong: A — The stroma has lower H⁺ concentration (higher pH). H⁺ flows from lumen to stroma through ATP synthase. Confusing lumen and stroma reverses the gradient direction.
Why B is wrong: B — The cytoplasm is outside the chloroplast entirely and plays no role in the light-reaction proton gradient.
Why C is wrong: C — The outer chloroplast membrane is not involved in the light reaction proton gradient. The chemiosmotic gradient is across the thylakoid membrane specifically.
During non-cyclic electron flow, for every two molecules of water split, identify the correct combination of products:
Answer: D. 2H₂O → 4H⁺ + 4e⁻ + O₂. The 4 electrons travel through the chain; each pair reduces one NADP⁺ to NADPH, giving 2 NADPH total. The 4H⁺ are released into the lumen (NCERT Class 11 Biology Chapter 11, page 139). This requires tracking both the water-splitting stoichiometry and the electron-to-NADPH ratio.
Why A is wrong: A — This undercounts the H⁺ released. Photolysis of 2H₂O releases 4H⁺ into the lumen, not 2.
Why B is wrong: B — This correctly identifies 1 O₂ and 4 H⁺ but undercounts NADPH. Four electrons from two water molecules reduce two NADP⁺ (2e⁻ per NADPH), not one.
Why C is wrong: C — Two molecules of water yield only one O₂, not two. The reaction is 2H₂O → O₂ + 4H⁺ + 4e⁻.
If a herbicide specifically blocks electron transfer from PS II to plastoquinone, which of the following would be directly affected first?
Answer: C. Blocking electron flow from PS II to plastoquinone halts the non-cyclic chain. Water splitting at PS II continues momentarily but electrons cannot leave, so O₂ evolution effectively stops and NADP⁺ reduction ceases — NADPH production is directly and immediately affected. Calvin cycle and starch synthesis are downstream consequences, not the first effect (NCERT Class 11 Biology Chapter 11, pages 139–140).
Why A is wrong: A — Cyclic photophosphorylation uses only PS I and does not depend on electron transfer from PS II to plastoquinone. It would not be directly blocked by this herbicide.
Why B is wrong: B — CO₂ fixation in the Calvin cycle would eventually be affected (it needs ATP and NADPH), but this is a secondary downstream effect, not the first direct consequence of the block.
Why D is wrong: D — Starch synthesis depends on the Calvin cycle, which depends on NADPH and ATP. This is two steps removed from the site of the block and would not be the first effect.
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Given
A student is asked: "In the Z-scheme of light reactions, which photosystem splits water and what is the immediate electron acceptor from this photosystem?"
Required
Identify (a) the photosystem responsible for water photolysis and (b) the first mobile electron carrier that accepts electrons from it.
Concept
The Z-scheme describes non-cyclic electron flow. Water is the primary electron donor at PS II. After P680 is excited and loses an electron, the electron passes to pheophytin and then to the first mobile carrier, plastoquinone (PQ). The naming trap (PS II was discovered second but acts first) is the primary source of wrong answers.
Formula/Rule
Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂ (at PS II, lumenal side).
Electron path from PS II: P680* → pheophytin → plastoquinone (QA → QB).
Substitution/Application
(a) Water photolysis → PS II (P680), NOT PS I.
(b) First mobile electron acceptor from PS II → plastoquinone.
Calculation
No numerical calculation. This is a recall + concept-application question. The reasoning step is: discovery order (PS I named first) ≠ reaction order (PS II acts first). Photolysis is tied to PS II because the oxygen-evolving complex is physically associated with the PS II complex.
Final answer
(a) Photosystem II (P680) splits water.
(b) Plastoquinone is the immediate mobile electron acceptor from PS II.
Common trap
Reversing PS I and PS II. Students recall that PS I was discovered first and assume it acts first in the electron transport chain. In NEET questions, options offering "PS I splits water" or "PS I → plastoquinone" exploit this exact confusion. Counter: reaction order is PS II → PS I; the numbers refer to discovery order, not sequence.
Similar NEET-style question
"During light reactions, electrons from water first enter which photosystem, and what is the terminal electron acceptor of non-cyclic electron flow?"
Expected answer: PS II; NADP⁺.
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PS II (P680) → e⁻ → PQ → cyt b6f → PC → PS I (P700) → ferredoxin → NADPH. Photolysis of H2O (Mn cluster) releases O2. Cyclic photophosphorylation (PS I only) → ATP only. Non-cyclic → ATP + NADPH + O2.
-- NCERT Class 11 Biology, Chapter 11, p. 139These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Negative Marking
Light reactions: water splits at PS II (P680) FIRST, electrons flow PS II → PS I (P700). PS II numbered AFTER PS I in discovery, but reaction order is PS II → PS I.
Question on Z-scheme order, water photolysis, electron source.
Discovery order ≠ reaction order. Photolysis at PS II; PS II reduces PS I.
More in Plant Physiology: 4 exam traps and mistakes · 1 question pattern from its other lessons.
3 questions from NEET 2022, 2023, 2026. Answers verified against NTA official keys.
Which pigment has absorption peak at 700 nm in the photosynthetic reaction centre PS I (P700) ?
The reaction centre in PS II has an absorption maxima at
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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