Q. 58 Select the CORRECT combination of the following statements.
P. The cyclic electron transport chain involving PSI results in net production of both ATP
and NADPH
Q. The cyclic electron transport chain involving PSI results in net production of ATP
R. Rubisco enzyme usually converts RuBP and CO2 into 2–phosphoglycolate and
3–phosphoglycerate
S. Rubisco enzyme usually converts RuBP and O2 into 2–phosphoglycolate and
3–phosphoglycerate
(A) P, Q (B) R, S (C) Q, S (D) P, R
The correct answer is (C) Q, S.
Cyclic electron transport around PSI produces only ATP (not NADPH), and Rubisco’s oxygenase activity with O₂ produces 2-phosphoglycolate and 3-phosphoglycerate.
Introduction
Understanding the cyclic electron transport around Photosystem I (PSI) and the dual carboxylase/oxygenase activity of Rubisco is critical for solving photosynthesis MCQs in CSIR NET, GATE, and NEET exams. This question tests your conceptual clarity on ATP vs NADPH production in photophosphorylation and on how Rubisco acts differently with CO₂ and O₂ to generate either 3-phosphoglycerate alone or in combination with 2-phosphoglycolate.
Statement-wise Analysis (P, Q, R, S)
Statement P
P. The cyclic electron transport chain involving PSI results in net production of both ATP and NADPH
-
In cyclic photophosphorylation, only PSI participates; electrons excited from PSI are cycled back through the electron transport chain to PSI, generating a proton gradient used for ATP synthesis.
-
No electrons are ultimately transferred to NADP⁺, so no net NADPH is produced in cyclic electron flow, unlike linear (non-cyclic) flow which produces both ATP and NADPH.
⇒ P is incorrect because cyclic electron transport around PSI produces ATP only, not both ATP and NADPH.
Statement Q
Q. The cyclic electron transport chain involving PSI results in net production of ATP
-
In cyclic electron flow, electrons from PSI move through ferredoxin and cytochrome b₆f, back to PSI, building a proton motive force across the thylakoid membrane.
-
This proton gradient drives ATP synthase, leading to net ATP production without water splitting, O₂ evolution, or NADPH formation.
⇒ Q is correct, as cyclic electron transport involving PSI results in net ATP synthesis only.
Statement R
R. Rubisco enzyme usually converts RuBP and CO₂ into 2-phosphoglycolate and 3-phosphoglycerate
-
When CO₂ is the substrate, Rubisco catalyzes carboxylation of RuBP, adding CO₂ to RuBP to form a six-carbon intermediate that splits into two molecules of 3-phosphoglycerate (3-PGA) as the first stable products of the Calvin cycle.
-
Production of 2-phosphoglycolate occurs when Rubisco reacts with O₂, not CO₂; therefore combining CO₂ with RuBP does not yield 2-phosphoglycolate.
⇒ R is incorrect because RuBP + CO₂ → 2 × 3-phosphoglycerate, not 2-phosphoglycolate + 3-phosphoglycerate.
Statement S
S. Rubisco enzyme usually converts RuBP and O₂ into 2-phosphoglycolate and 3-phosphoglycerate
-
Rubisco has oxygenase activity: when O₂ competes with CO₂ at the active site, RuBP reacts with O₂ to give one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate.
-
This oxygenation reaction initiates photorespiration, a carbon- and energy-wasting process that requires salvage of 2-phosphoglycolate via chloroplasts, peroxisomes, and mitochondria.
⇒ S is correct, as RuBP + O₂ → 3-phosphoglycerate + 2-phosphoglycolate during Rubisco’s oxygenase reaction.
Final Evaluation of Options
| Option | Included statements | Evaluation | Correct/Incorrect |
|---|---|---|---|
| (A) | P, Q | P incorrect (no NADPH in cyclic flow); Q correct | Incorrect |
| (B) | R, S | R incorrect (CO₂ gives 2 × 3-PGA); S correct | Incorrect |
| (C) | Q, S | Q correct (ATP only in cyclic PSI); S correct (RuBP + O₂ → 3-PGA + 2-phosphoglycolate) | Correct |
| (D) | P, R | Both P and R incorrect | Incorrect |
Therefore, the correct combination is (C) Q, S.
