Q.100 Botany ⭐ 360° Option Elimination Active

NEET 2026 Botany Question 100 Solution

Topic: Photosynthesis in Higher Plants · Official NTA Booklet Code 50

Question

Photorespiration reaction catalyzed by RuBisCo is shown below: RuBP + O₂ → 3-Phosphoglycerate + X. Identify "X" from the given options:

✓

Verified Against NTA Final Answer Key

Option (2) : 2-Phosphoglycolate

📚 Photosynthesis in Higher Plants Medium

📋 Given Parameters & Problem Setup

Reaction RuBP + O₂ → 3-Phosphoglycerate (PGA) + X

Enzyme RuBisCO acting as an oxygenase

Substrate Ribulose-1,5-bisphosphate (RuBP, 5-carbon compound)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Carbon Balance in Photorespiration: 5C = 3C + 2C

⏱️ Save ~50 seconds

5-carbon RuBP splits into one 3C (PGA) and one 2C (Phosphoglycolate)!

1 Count carbon atoms: RuBP has 5 carbons.

2 In presence of O₂, RuBisCO oxygenates RuBP.

3 The products must total 5 carbons: One 3-carbon molecule is 3-Phosphoglycerate (PGA).

4 The remaining molecule X must have 5 - 3 = 2 carbons!

5 A 2-carbon phosphorylated intermediate is 2-PHOSPHOGLYCOLATE!

6 Option (2) is 2-Phosphoglycolate! Pick Option (2) in 10 seconds flat!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Photosynthesis verbatim: When RuBisCO binds O₂, RuBP binds with O₂ to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon) in a pathway called photorespiration.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Understand the Dual Activity of RuBisCO

RuBisCO (Ribulose bisphosphate carboxylase-oxygenase) is the most abundant enzyme in the biosphere. Its active site can bind both CO₂ and O₂ competitively: - When CO₂/O₂ ratio is high, it acts as a carboxylase, fixing CO₂ into two 3C molecules (3-PGA) in the Calvin cycle. - When O₂ concentration is high and CO₂ is low, it acts as an oxygenase, initiating the photorespiratory C₂ cycle.

\text{RuBisCO: Carboxylase Activity vs Oxygenase Activity}

• Photorespiration does not produce ATP or sugars and wastes fixed carbon.

Step 2

Formulate the RuBisCO Oxygenase Reaction

In the chloroplast stroma, RuBisCO oxygenates RuBP (5-carbon) with molecular oxygen to cleave it into one 3-carbon molecule and one 2-carbon molecule:

\text{RuBP (5C)} + \text{O}_2 \xrightarrow{\text{RuBisCO (oxygenase)}} \text{3-Phosphoglycerate (3C)} + \text{2-Phosphoglycolate (2C) } [\mathbf{X}]

• 3-Phosphoglycerate (PGA) enters the Calvin cycle.
• 2-Phosphoglycolate is dephosphorylated to glycolate and transported to peroxisomes and mitochondria for carbon salvage.

Step 3

Identify Molecule X from the Options

1. Phosphoenolpyruvate (PEP): 3-carbon CO₂ acceptor in C₄ pathway. 2. 2-Phosphoglycolate: 2-carbon photorespiratory product (X). 3. Oxaloacetate (OAA): 4-carbon organic acid in C₄ pathway / TCA cycle. 4. Malate: 4-carbon dicarboxylic acid in C₄ pathway.

\mathbf{X} = \text{2-Phosphoglycolate}

• Matches Option (2).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Phosphoenolpyruvate (PEP) is the primary CO₂ acceptor in C₄ plants, not a product of RuBP oxygenation.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing C₄ cycle substrates with photorespiratory products.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

Exact match with the canonical photorespiration reaction: 5C RuBP + O₂ → 3C PGA + 2C Phosphoglycolate.

💡

Why this is airtight: None; textbook mastery of the C₂ photorespiration pathway.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Oxaloacetate (OAA) is a 4-carbon acid produced by PEP carboxylase in C₄ photosynthesis.

⚠️

Student Fallacy (Why candidates pick this trap): Mistaking C₄ first stable product for photorespiratory product.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Malate is a 4-carbon transport acid in C₄ plants.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing C₄ malate shuttle with photorespiration.

⚠️

NTA Trap Alert: Confusing Photorespiration (C₂) with C₄ Photosynthesis

Students frequently confuse products of photorespiration (Phosphoglycolate, 2C) with products of C₄ Hatch-Slack cycle (Oxaloacetate and Malate, 4C).

🛡️ How to avoid the -1 mark: Photorespiration is the C₂ cycle: it creates the 2-carbon compound Phosphoglycolate!

📖

Core NEET Syllabus Standard

NEET Biology Core Syllabus · Photosynthesis in Higher Plants Standard Benchmark

Core Scientific Principle Tested:

"Oxygenation of RuBP by RuBisCO yields one molecule of 3-PGA and one molecule of 2-phosphoglycolate."

Syllabus Focus: Photorespiration - RuBisCO oxygenase reaction and C₂ cycle

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Oxygen hits RuBP: 3 carbons go to PGA, 2 carbons leave as Phosphoglycolate!

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NEET 2026 Botany Question 100 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Carbon Balance in Photorespiration: 5C = 3C + 2C

Step-by-Step Rigorous Workflow & Mathematical Proof

Understand the Dual Activity of RuBisCO

Formulate the RuBisCO Oxygenase Reaction

Identify Molecule X from the Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Photorespiration (C₂) with C₄ Photosynthesis

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Photosynthesis in Higher Plants

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Enzymatic Properties of RuBisCO (Carboxylation of RuBP)

Substrates and Energy Intermediates for the Dark Reaction (Calvin Cycle)

ATP and NADPH Consumption per CO2 Fixed in the C3 Pathway

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