Q.106 Botany ⭐ 360° Option Elimination Active

Vault ↗

NEET 2021 Botany Question 106 Solution

Topic: Photosynthesis in Higher Plants · Official NTA Booklet Code M1

Question

The first stable product of CO2 fixation in sorghum is:

(1)Pyruvic acid
(2)Oxaloacetic acid
(3)Succinic acid
(4)Phosphoglyceric acid

✓

Verified Against NTA Final Answer Key

Option (2) : Oxaloacetic acid

📚 Photosynthesis in Higher Plants Easy

📋 Given Parameters & Problem Setup

Plant Type Sorghum (C4 monocot cereal)

Biochemical Parameter First stable product of CO2 fixation

⚡

The 30-Second Exam Hall Shortcut: ⚡ Sorghum = C4 Plant $\implies$ 4-Carbon OAA (Oxaloacetic Acid)

⏱️ Save ~20 seconds

Sorghum is a classical C4 plant (along with Maize and Sugarcane). Primary CO2 fixation in mesophyll cells is catalyzed by PEP carboxylase yielding the 4-carbon organic acid Oxaloacetic acid (OAA) as the first stable product! In C3 plants, it is 3-PGA. Match Option (2)!

1 Identify the plant: Sorghum (*Sorghum bicolor*) is a tropical monocot that utilizes the C4 Hatch-Slack pathway.

2 Primary acceptor in mesophyll is Phosphoenolpyruvate (PEP, 3-carbon).

3 Reaction:

\text{PEP} + \text{HCO}_3^- \xrightarrow{\text{PEPcase}} \text{Oxaloacetic acid (OAA, 4-carbon)}

4 Match Option (2).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Photosynthesis in Higher Plants on C4 Pathway.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Classification of Sorghum as a C4 Plant

Plants exhibiting the Kranz anatomy and the Hatch-Slack pathway are termed $C_4$ plants. Classical examples include maize, sugarcane, and sorghum.

Step 2

Initial Carboxylation Reaction in Mesophyll Cells

The primary acceptor of $CO_2$ is phosphoenolpyruvate ( $PEP$ ), a 3-carbon compound present in the mesophyll cells. The enzyme responsible is $PEP$ carboxylase ( $PEPcase$ ):

\text{Phosphoenolpyruvate (3C)} + \text{HCO}_3^- + \text{H}_2\text{O} \xrightarrow{\text{PEP carboxylase}} \mathbf{\text{Oxaloacetic acid (OAA, 4C)}} + \text{H}_3\text{PO}_4

Step 3

Subsequent Metabolism of OAA

OAA (oxaloacetate) is rapidly reduced to other 4-carbon dicarboxylic acids like malic acid or aspartic acid in the mesophyll cells before transport to bundle sheath cells.

⭐

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 1

✗ Incorrect

Verdict & Scientific Reason:

Pyruvic acid (pyruvate, 3C) is formed after decarboxylation in bundle sheath cells and transported back to regenerate PEP.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the regenerated precursor with the initial carboxylation product.

2 2

✓ 100% Correct

Verdict & Scientific Reason:

Oxaloacetic acid (OAA, 4C) is the initial, first stable product of CO2 fixation in C4 plants like sorghum.

💡

Why this is airtight: None. Fundamental textbook fact.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Succinic acid is a 4-carbon intermediate of the Krebs cycle in cellular respiration, not the product of photosynthetic CO2 fixation.

⚠️

Student Fallacy (Why candidates pick this trap): Mistaking a citric acid cycle intermediate for a photosynthetic pathway intermediate.

4 4

✗ Incorrect

Verdict & Scientific Reason:

3-Phosphoglyceric acid (3-PGA, 3C) is the first stable product in C3 plants (Calvin cycle), not in C4 plants like sorghum.

⚠️

Student Fallacy (Why candidates pick this trap): Overlooking that sorghum is a C4 plant, thereby reflexively picking 3-PGA (the C3 product).

⚠️

NTA Trap Alert: Plant Identity Overlook: Sorghum is C4, Not C3

Candidates who fail to recognize Sorghum as a C4 plant automatically choose 3-Phosphoglyceric acid (3-PGA), the first stable product of C3 plants.

🛡️ How to avoid the -1 mark: Commit to memory the major C4 trio tested in NEET: Maize, Sugarcane, and Sorghum

\implies

First product is always OAA (4C).

📖

Core NEET Syllabus Standard

NEET / NCERT Biology · Photosynthesis in Higher Plants Standard Benchmark

Core Scientific Principle Tested:

"In C4 plants, the initial stable product of CO2 fixation is oxaloacetic acid (OAA), a 4-carbon organic acid."

Syllabus Focus: The C4 Pathway (Hatch and Slack Pathway)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

← Previous (Q.105) All Botany Questions Grid Next (Q.107) →

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2021 Botany Question 106 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Sorghum = C4 Plant ⟹ \implies⟹ 4-Carbon OAA (Oxaloacetic Acid)

Step-by-Step Rigorous Workflow & Mathematical Proof

Classification of Sorghum as a C4 Plant

Initial Carboxylation Reaction in Mesophyll Cells

Subsequent Metabolism of OAA

360° Option Elimination Engine Key Feature

NTA Trap Alert: Plant Identity Overlook: Sorghum is C4, Not C3

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

Solve 4 NEET Questions a Day — One per Subject

The 30-Second Exam Hall Shortcut: ⚡ Sorghum = C4 Plant $\implies$ 4-Carbon OAA (Oxaloacetic Acid)