Q.56 Which of the following statements in photosynthesis are CORRECT?
P. The absorption maxima for photosystem I (PS I) and PS II are 680 nm and 700 nm, respectively
Q. Photosynthetic reaction centre contains 300 chlorophyll molecules and the release of one molecule of oxygen requires a minimum of 8 photons
R. The non-photochemical quenching of excitation energy is enhanced by the presence of zeaxanthin
S. The photochemical splitting of water occurs in PS I
(A) P, Q
(B) R, S
(C) P, S
(D) Q, R
Correct Answer
✅ Option (D): Q and R
Detailed Explanation
Photosynthesis involves light-dependent reactions occurring in Photosystem II (PS II) and Photosystem I (PS I). Each statement is evaluated below.
Statement P
“The absorption maxima for PS I and PS II are 680 nm and 700 nm, respectively.”
❌ Incorrect
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PS II has an absorption maximum at 680 nm (P680)
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PS I has an absorption maximum at 700 nm (P700)
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The statement reverses the values, making it incorrect.
Statement Q
“Photosynthetic reaction centre contains 300 chlorophyll molecules and the release of one molecule of oxygen requires a minimum of 8 photons.”
✅ Correct
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Each photosystem has ~300 antenna chlorophyll molecules
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Oxygen evolution requires 4 electrons
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Since 2 photons are needed per electron, a total of 8 photons are required to release one O₂ molecule
✔ This statement is biochemically correct
Statement R
“The non-photochemical quenching of excitation energy is enhanced by the presence of zeaxanthin.”
✅ Correct
-
Non-photochemical quenching (NPQ) protects plants from excess light
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Zeaxanthin, a xanthophyll pigment, plays a key role in dissipating excess excitation energy as heat
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This mechanism prevents photoinhibition
✔ This statement is correct
Statement S
“The photochemical splitting of water occurs in PS I.”
❌ Incorrect
-
Water splitting (photolysis) occurs in PS II, not PS I
-
It is mediated by the oxygen-evolving complex (OEC) associated with PS II
Why Option (D) Is Correct
-
Q is correct (antenna size and photon requirement)
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R is correct (role of zeaxanthin in NPQ)
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Other options include at least one incorrect statement
✔ Hence, Option (D): Q and R is the correct answer.
Quick Exam Memory Tips
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PS II → P680 → Water splitting
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PS I → P700 → NADPH formation
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8 photons → 1 O₂ molecule
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Zeaxanthin → Photoprotection (NPQ)
Exam Relevance
🔹 Frequently asked in CSIR-NET Life Sciences
🔹 Important for photosynthesis & plant physiology units
🔹 High-yield MCQ concept for competitive exams
