Q.42 Botany ⭐ 360° Option Elimination Active

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NEET 2020 Botany Question 42 Solution

Topic: Photosynthesis in Higher Plants · Official NTA Booklet Code E1

Question

The oxygenation activity of RuBisCo enzyme in photorespiration leads to the formation of:

(1)2 molecules of 3-C compound
(2)1 molecule of 3-C compound
(3)1 molecule of 6-C compound
(4)1 molecule of 4-C compound and 1 molecule of 2-C compound

✓

Verified Against NTA Final Answer Key

Option (2) : 1 molecule of 3-C compound

📚 Photosynthesis in Higher Plants Easy

📋 Given Parameters & Problem Setup

Enzyme Activity Oxygenase activity of RuBisCO in photorespiration

Substrate RuBP (5-Carbon compound) + O2

Question Objective Determine the molecular yield and carbon composition of the products

⚡

The 30-Second Exam Hall Shortcut: ⚡ "RuBP (5C) + O2 -> 1 PGA (3C) + 1 Phosphoglycolate (2C)!"

⏱️ 35 seconds

During photorespiration, RuBisCO oxygenates RuBP (5C) into ONE 3-carbon molecule (3-PGA) and ONE 2-carbon molecule (phosphoglycolate). Look at the options: Option (2) specifies "1 molecule of 3-C compound". Mark Option (2) in 10 seconds!

1 Carboxylation: 1 RuBP (5C) + CO2 (1C) -> TWO molecules of 3-PGA (3C each).

2 Oxygenation: 1 RuBP (5C) + O2 -> ONE 3-PGA (3C) + ONE Phosphoglycolate (2C).

3 Thus, oxygenation leads to the formation of 1 molecule of 3-C compound (Option 2).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Plant Physiology: Photosynthesis in Higher Plants (Photorespiration)

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Recall Dual Activity of RuBisCO

RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) has an active site that can bind both CO2 and O2 competitively. In C3 plants, when O2 concentration is high and CO2 is low, RuBisCO acts as an oxygenase.

Step 2

Stoichiometry of Oxygenation Reaction

In the oxygenase reaction: $\text{RuBP (5C)} + \text{O}_2 \xrightarrow{\text{RuBisCO}} \text{3-PGA (3C)} + \text{Phosphoglycolate (2C)}$ Instead of forming two molecules of 3-PGA as in normal carboxylation, it yields exactly ONE molecule of a 3-C compound (3-PGA) and ONE molecule of a 2-C compound (2-phosphoglycolate).

Step 3

Evaluate the Options

Option (1) describes carboxylation (2 molecules of 3-C compound). Option (2) correctly states "1 molecule of 3-C compound". Options (3) and (4) propose stoichiometries incompatible with the 5-carbon skeleton of RuBP.

Step 4

Conclude Option 2

Therefore, the oxygenation activity of RuBisCO enzyme directly results in the formation of 1 molecule of a 3-carbon compound.

⭐

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; two molecules of 3-PGA are produced during carboxylation (CO2 fixation) in the Calvin cycle, not during oxygenation.

⚠️

Student Fallacy (Why candidates pick this trap): Habitually remembering the standard Calvin cycle carboxylation step and overlooking that oxygenation is specified.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

Factually exact; oxygenation of 5-C RuBP yields 1 molecule of 3-PGA (3-C) along with 1 molecule of 2-phosphoglycolate (2-C).

💡

Why this is airtight: Some students hesitate because the 2-C product is not mentioned, but the statement itself is completely true and correct.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; no stable 6-C compound is formed; even the transient 6-C intermediate of carboxylation hydrolyzes immediately.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming RuBP (5C) joins with something to yield an intact hexose.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; 4 + 2 = 6 carbons, but RuBP contains only 5 carbons (plus O2 adds zero carbons).

⚠️

Student Fallacy (Why candidates pick this trap): Failing to balance carbon conservation (5C cannot yield 4C + 2C).

⚠️

NTA Trap Alert: Carbon Conservation Check

RuBP has 5 carbons! When reacting with O2 (which has 0 carbons), total carbon must remain 5: 3 carbons (3-PGA) + 2 carbons (phosphoglycolate) = 5 carbons. Never pick an option totaling 6 carbons!

🛡️ How to avoid the -1 mark: Count carbons: 5C RuBP + O2 -> 3C (PGA) + 2C (phosphoglycolate). Exactly 1 molecule of 3-C compound is formed!

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Core NEET Syllabus Standard

Botany · Photosynthesis in Higher Plants Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Photorespiration: RuBisCO Oxygenase Activity and Products

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Botany Question 42 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "RuBP (5C) + O2 -> 1 PGA (3C) + 1 Phosphoglycolate (2C)!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Recall Dual Activity of RuBisCO

Stoichiometry of Oxygenation Reaction

Evaluate the Options

Conclude Option 2

360° Option Elimination Engine Key Feature

NTA Trap Alert: Carbon Conservation Check

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Photosynthesis in Higher Plants

RuBisCO Oxygenase Catalysis & Identification of 2-Phosphoglycolate

Reaction Centre Chlorophyll a in Photosystem I (P700)

Chronological Sequence of Classic Photosynthesis Experiments

Stoichiometry of the Calvin Cycle (C3 Pathway) per Glucose Formed

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