NEET 2025 Zoology Question 171 Solution

Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) is the most abundant enzyme on Earth, located in the chloroplast stroma of autotrophs:

• • It possesses bifunctional catalytic capacity: it can act as both a carboxylase and an oxygenase.
• • Under normal physiological conditions, RuBisCO exhibits a significantly higher affinity for $\text{CO}_2$ than for $\text{O}_2$.

The primary physiological reaction catalyzed by RuBisCO:

• • Carboxylation is the most crucial step of the Calvin ($C_3$) cycle.
• • RuBisCO catalyzes the addition of one molecule of $\text{CO}_2$ to the 5-carbon acceptor ribulose-1,5-bisphosphate (RuBP).
• • The unstable 6-carbon intermediate immediately cleaves into two molecules of 3-phosphoglyceric acid (3-PGA), the first stable product of $C_3$ photosynthesis.
• • Hence, statement (4) is undeniably TRUE.

Analyzing why options 1, 2, and 3 are scientifically incorrect:

• • Statement (1): RuBisCO is inactive in the dark; it requires light-driven stromal alkalinization and $Mg^{2+}$ influx.
• • Statement (2): NCERT explicitly clarifies that RuBisCO has a much greater affinity for $\text{CO}_2$ than for $\text{O}_2$; oxygenase activity (photorespiration) occurs only when $\text{CO}_2:\text{O}_2$ ratio decreases.
• • Statement (3): Photolysis of water ($2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$) is catalyzed by the water-splitting complex associated with Photosystem II on the luminal side of the thylakoid membrane, completely independent of RuBisCO.

The only true statement is that RuBisCO catalyzes the carboxylation of RuBP, matching Option (4).