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Biology Question 100 – NEET-UG 2026

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Photorepiration reaction catalyzed by RuBisCo is shown below : RuBP + O2 → 3-Phosphoglycerate + X Identify “X” from the given options :

Photorespiration occurs when the enzyme RuBisCO binds to oxygen instead of carbon dioxide.

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Ninja Strategy•Pathway Specificity

Recognize that the question specifically asks about photorespiration, and eliminate options that are characteristic products of other metabolic pathways like C4 photosynthesis or glycolysis.

Step 1: Understand Photorespiration✦ Active

Photorespiration is a metabolic pathway that occurs in plants when the enzyme RuBisCO binds to oxygen (O2) instead of carbon dioxide (CO2). This process is considered wasteful as it consumes ATP and NADPH without producing sugar.

Step 2: Recall the RuBisCO Oxygenase Reaction○ Expand

When RuBisCO acts as an oxygenase, it catalyzes the reaction of Ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound, with O2. The products of this reaction are 3-Phosphoglycerate (a 3-carbon compound) and 2-Phosphoglycolate (a 2-carbon compound).

RuBP (5C)+O2→RuBisCO3-Phosphoglycerate (3C)+2-Phosphoglycolate (2C)
💡 Teacher's Secret Hint

Remember that RuBisCO has both carboxylase and oxygenase activity, and photorespiration occurs due to the latter.

Step 3: Identify 'X'○ Expand

The given reaction is: RuBP + O2 → 3-Phosphoglycerate + X. Comparing this with the known photorespiration reaction, 'X' is 2-Phosphoglycolate.

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