Q.113 Botany ⭐ 360° Option Elimination Active

NEET 2025 Botany Question 113 Solution

Topic: Biological Classification · Official NTA Booklet Code 45

Question

Which of following organisms cannot fix nitrogen? A. Azotobacter B. Oscillatoria C. Anabaena D. Volvox E. Nostoc Choose the correct answer from the options given below:

✓

Verified Against NTA Final Answer Key

Option (2) : D only

📚 Biological Classification Easy

📋 Given Parameters & Problem Setup

Organisms Listed A. Azotobacter, B. Oscillatoria, C. Anabaena, D. Volvox, E. Nostoc

Target Trait Organisms that CANNOT fix nitrogen

⚡

The 30-Second Exam Hall Shortcut: ⚡ Nitrogen Fixation Exclusivity Fingerprint

⏱️ Save ~20 seconds

Biological N₂ fixation is EXCLUSIVELY prokaryotic (needs the nitrogenase enzyme complex)! Volvox is a eukaryotic green alga, so it CANNOT fix nitrogen!

1 Recall: Only PROKARYOTES possess the nitrogenase enzyme complex capable of reducing N₂ to NH₃.

2 Analyze each organism:

3 - Azotobacter: Free-living aerobic nitrogen-fixing bacterium (Prokaryote). [Fixes N₂]

4 - Oscillatoria: Cyanobacterium capable of N₂ fixation in rice fields (Prokaryote). [Fixes N₂]

5 - Anabaena: Cyanobacterium with heterocysts fixing N₂ (Prokaryote). [Fixes N₂]

6 - Nostoc: Cyanobacterium with heterocysts fixing N₂ (Prokaryote). [Fixes N₂]

7 - Volvox: Colonial freshwater GREEN ALGA (Eukaryote, Chlorophyceae). Lacks nitrogenase! CANNOT fix N₂!

8 Therefore, D only cannot fix nitrogen!

9 Select Option (2) D only!

🛡️

Why You Can 100% Trust This Shortcut: Biological Nitrogen Fixation & Heterocyst Function (NCERT Biology · Biological Classification & Plant Nutrition: Anabaena and Nostoc).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Fundamental Principle of Biological Nitrogen Fixation (BNF)

Reduction of atmospheric dinitrogen ( $N_2$ ) to ammonia ( $NH_3$ ) requires the enzyme nitrogenase:

• The nitrogenase enzyme complex is found exclusively in prokaryotes (bacteria and cyanobacteria).
• No eukaryotic organism (plants, animals, fungi, protists, eukaryotic algae) natively encodes or possesses functional nitrogenase.

Step 2

Classification of the Listed Organisms

Evaluating each candidate organism:

• A. Azotobacter: Free-living, aerobic, heterotrophic eubacterium known as a potent nitrogen fixer in soils.
• B. Oscillatoria: Filamentous cyanobacterium that fixes atmospheric nitrogen and enriches paddy fields.
• C. Anabaena: Filamentous cyanobacterium possessing specialized thick-walled cells called heterocysts for N₂ fixation (symbiont in Azolla, Cycas coralloid roots).
• D. Volvox: A colonial, flagellated eukaryotic green alga belonging to Division Chlorophyta (Plantae). Being a eukaryote, it lacks nitrogenase and cannot fix nitrogen.
• E. Nostoc: Cyanobacterium possessing heterocysts for biological nitrogen fixation in terrestrial and aquatic habitats.

Step 3

Conclusion

Among all five organisms, only Volvox (D) is a eukaryote and completely lacks the ability to fix atmospheric nitrogen.

\text{Cannot fix } N_2 = \text{D (Volvox only)}

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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. Azotobacter is a textbook example of a free-living aerobic nitrogen-fixing bacterium.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking Azotobacter does not fix nitrogen.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Volvox is a eukaryotic green alga and cannot fix nitrogen.

💡

Why this is airtight: None; understands that N₂ fixation is strictly prokaryotic.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Oscillatoria is a cyanobacterium that contributes to nitrogen fixation in paddy fields.

⚠️

Student Fallacy (Why candidates pick this trap): Forgetting that Oscillatoria is a cyanobacterial biofertilizer.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Nostoc possesses heterocysts and is a famous nitrogen-fixing cyanobacterium.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing Nostoc with non-fixing algae.

⚠️

NTA Trap Alert: Confusing Cyanobacteria ("Blue-Green Algae") with True Algae

Cyanobacteria (Oscillatoria, Anabaena, Nostoc) are prokaryotes and fix nitrogen. True algae like Volvox, Chlamydomonas, and Spirogyra are eukaryotes and cannot fix nitrogen.

🛡️ How to avoid the -1 mark: Check whether the organism is a prokaryote (can fix N₂) or a eukaryotic green alga (cannot fix N₂).

📖

Core NEET Syllabus Standard

NEET Biology Core Syllabus · Biological Classification / Mineral Nutrition Standard Benchmark

Core Scientific Principle Tested:

"Only prokaryotes (eubacteria and cyanobacteria) have nitrogenase and can fix atmospheric nitrogen; eukaryotes like Volvox cannot."

Syllabus Focus: Biological Nitrogen Fixation · Nitrogen-Fixing Microbes

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

VOLVOX = Vacation from N₂ fixation (Eukaryote has NO nitrogenase)!

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NEET 2025 Botany Question 113 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Nitrogen Fixation Exclusivity Fingerprint

Step-by-Step Rigorous Workflow & Mathematical Proof

Fundamental Principle of Biological Nitrogen Fixation (BNF)

Classification of the Listed Organisms

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Cyanobacteria ("Blue-Green Algae") with True Algae

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Biological Classification

Symbiotic Mutualistic Association Between Algae and Fungi (Lichens)

Prions as Causative Agents of Bovine Spongiform Encephalopathy (Mad Cow Disease)

Morphological Shapes of Bacteria

Sequential Steps of the Fungi Sexual Reproduction Cycle

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