Q.55 Which TWO of the following reactions are INCORRECT in relation to 𝐂𝟐 oxidative photosynthetic
carbon cycle in land plants?
P. 2 (Ribulose-1,5-biphosphate) +2(CO2) → 2 (phosphoglycolate) +2 (3-phosphoglycerate) +4H+
Q. Serine +𝛼-ketoglutarate → hydroxypyruvate + glutamine
R. 2 (Phosphoglycolate) +2(H2O) → 2 (glycolate) +2 Pi
S. Hydroxypyruvate +NADH+ H+ → glycerate +NAD+
(A) P and Q
(B) 𝑄 and 𝑅
(C) R and S
(D) S and P

The C2 oxidative photosynthetic carbon cycle, also called photorespiration or the C2 cycle, occurs in land plants (C3 plants) when RuBisCO’s oxygenase activity produces phosphoglycolate instead of carboxylation products. Question 55 asks which two reactions (P, Q, R, S) are incorrect in this pathway, with the correct answer being (A) P and Q.​

Correct Answer

Option (A) P and Q identifies the incorrect reactions. Reaction P wrongly shows CO2 producing phosphoglycolate (RuBisCO oxygenase uses O2, not CO2), and Q misstates serine conversion (it forms glycine + glutamate, not glutamine).​

Reaction P Analysis

Reaction P: 2 (Ribulose-1,5-bisphosphate) + 2(CO₂) → 2 (phosphoglycolate) + 2 (3-phosphoglycerate) + 4H⁺
Incorrect. In photorespiration, RuBisCO oxygenase reacts RuBP + O₂ → 3-PGA + phosphoglycolate + 2H⁺ (one molecule each per RuBP); CO₂ is not a reactant here—carboxylation uses CO₂, but oxygenation uses O₂. No stoichiometry matches 2 CO₂ yielding 2 phosphoglycolate directly.​

Reaction Q Analysis

Reaction Q: Serine + α-ketoglutarate → hydroxypyruvate + glutamine
Incorrect. In peroxisomes, serine undergoes transamination: serine + glutamate → hydroxypyruvate + glycine (via serine:glycine aminotransferase). α-Ketoglutarate forms glutamate via glutamine synthetase/glutamate synthase, but not directly producing glutamine here.​

Reaction R Analysis (Correct)

Reaction R: 2 (Phosphoglycolate) + 2(H₂O) → 2 (glycolate) + 2 Pi
Correct. In chloroplasts, phosphoglycolate phosphatase hydrolyzes each phosphoglycolate + H₂O → glycolate + Pi, matching the doubled form.​

Reaction S Analysis (Correct)

Reaction S: Hydroxypyruvate + NADH + H⁺ → glycerate + NAD⁺
Correct. In peroxisomes, hydroxypyruvate reductase reduces hydroxypyruvate to glycerate using NADH, as stated.​

C2 Cycle Overview

This cycle recycles 75% of phosphoglycolate carbon back to 3-PGA (via glycolate → glycine → serine → glycerate → 3-PGA across chloroplast-peroxisome-mitochondrion), releasing 25% as CO₂ but preventing toxicity.​