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 13, page 248). 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?
During non-cyclic photophosphorylation, the final electron acceptor is:
Photolysis of water during light reactions occurs at:
In the Z-scheme, electrons flow in the sequence:
Cyclic photophosphorylation involves only PS I and produces:
The proton gradient for ATP synthesis in chloroplasts is generated across the thylakoid membrane with higher H⁺ concentration in the:
During non-cyclic electron flow, for every two molecules of water split, identify the correct combination of products:
If a herbicide specifically blocks electron transfer from PS II to plastoquinone, which of the following would be directly affected first?
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⁺. ---
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, Ch. 13, p. 248These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
C3: 3-PGA (3-carbon) is first stable product. C4: oxaloacetate (4-carbon) is first stable product.
Question asks first stable product of CO2 fixation in C3/C4 plant.
Pathway name = carbons in first product. C3 → 3C; C4 → 4C.
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.
Root cause: concept gap
Mnemonic: C3 → 3-PGA (3-carbon); C4 → OAA (4-carbon). Hatch-Slack pathway is C4.
Root cause: concept gap
Glycolysis is in CYTOPLASM (cytosol). Krebs cycle in mitochondrial matrix; ETC in inner membrane.
Root cause: term confusion
RuBisCO = ribulose-1,5-bisphosphate CARBOXYLASE/OXYGENASE. Carboxylase activity drives Calvin; oxygenase causes photorespiration.
38 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Which of the following statements about RuBisCO is true?
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
The reaction centre in PS II has an absorption maxima at
Which micronutrient is required for splitting of water molecule during photosynthesis?
Which hormone promotes internode/petiole elongation in deep water rice?
Which of the following combinations is required for chemiosmosis?
Which one of the following plants does not show plasticity?
What amount of energy is released from glucose during lactic acid fermentation?
The first stable product of CO fixation in sorghum is 2
Which of the following statements is correct ? cambial ring
flÎÁh ∑§Ë ¬˝Á∑˝§ÿÊ •Áœ∑§Ã◊ Á∑§‚ ŒÊÒ⁄UÊŸ „Ê ÃË „Ò? 114. The process of growth is maximum during :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
photosynthesis pathway product confusion
C3 and C4 pathways share the Calvin cycle; students who cannot anchor the first stable product in each pathway invert the carbon counts: 3-PGA (3-carbon) is the C3 first stable product, OAA (4-carbon) is the C4 first stable product. RuBisCO has dual carboxylase and oxygenase activity; students who recall only one role select incorrect statements about its function, including denying that it causes photorespiration.
ps i ps ii assignment reversal
PS I and PS II are numbered in order of discovery, not order of action in the Z-scheme. PS II acts first (water photolysis, O2 release, P680 reaction centre at 680 nm); PS I acts second (P700 reaction centre at 700 nm, NADPH production). Students who use Roman numeral order assign water photolysis to PS I and attribute the 700 nm absorption maximum to PS II.
respiration compartment swap
Glycolysis feeds the mitochondrial Krebs cycle, creating a mental link: students place glycolysis in the mitochondrial matrix. The anchor -- glycolysis occurs in the cytosol because it predates mitochondria and operates in anaerobes -- is lost under time pressure.
rubisco function confusion
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) has dual activity: carboxylase drives CO2 fixation in the Calvin cycle; oxygenase drives photorespiration. Questions give multiple statements about RuBisCO and ask which set is correct; students who have not anchored the dual-function name omit or deny the oxygenase role, selecting option sets that describe it as carboxylase only.
assertion reason unchecked reason
A/R questions in plant physiology present a true Assertion (e.g., ATP is used at two steps in glycolysis) alongside a Reason that sounds mechanistically plausible but may be factually imprecise or not the correct explanation. Students accept the Reason without independently checking whether its specific biochemical claim is accurate.
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