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Nitrogen Metabolism — Syllabus

NIOS · Class 12 · Biology

What Nitrogen Metabolism covers in NIOS Class 12 Biology: 4 topics, for the 2026-27 session. Part of the NIOS Class 12 Biology syllabus.

45 questions20 flashcards5 concepts

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4 Topics · NIOS Class 12 Biology · 2026-27

Topics in Nitrogen Metabolism

1

Molecular Nitrogen and the Nitrogen Cycle

  • Nitrogen gas (N₂) makes up 78.03% of the atmosphere by volume.
  • Molecular nitrogen is diatomic and extremely stable due to a triple bond (N≡N), making it highly unreactive under normal atmospheric conditions.
  • Nitrogen has a very low boiling point of -195.8°C, lower than oxygen.
2

Nitrogen Fixation — Biological and Abiological

  • Nitrogen fixation is defined as the conversion of molecular nitrogen (N₂) into nitrogen compounds, especially ammonia (NH₃).
  • Nitrogen fixation is a REDUCTIVE process — it requires reducing conditions and stops in the presence of oxygen.
  • There are two types: Abiological (without living cells) and Biological (with living cells using nitrogenase enzyme).
3

Free-Living and Symbiotic Nitrogen-Fixing Organisms

  • Only a select group of prokaryotes (bacteria and cyanobacteria) can fix nitrogen because they possess nitrogenase.
  • FREE-LIVING organisms fix nitrogen independently without any host: Clostridium (anaerobic, non-photosynthetic), Klebsiella (facultative, non-photosynthetic), Azotobacter (aerobic, non-photosynthetic),
  • SYMBIOTIC nitrogen fixation occurs when nitrogen-fixing microbes associate with a host plant.
4

Nitrate and Ammonia Assimilation by Plants

  • Plants that do not fix nitrogen absorb combined nitrogen (mainly as nitrates — NO₃⁻) from the soil through roots.
  • Nitrate is converted to ammonia in TWO enzymatic steps.
  • STEP 1: Nitrate → Nitrite: Catalyzed by NITRATE REDUCTASE enzyme. This reaction occurs in the CYTOSOL.

Key Concepts

Nitrogen exists in the atmosphere asThe nitrogen cycle is a continuousAbiological nitrogen fixation converts N₂Biological nitrogen fixation is the enzymaticSymbiotic nitrogen fixation occurs when nitrogen

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Frequently Asked Questions

What are the important topics in Nitrogen Metabolism for NIOS Class 12 Biology?
Key topics in Nitrogen Metabolism include Molecular Nitrogen and the Nitrogen Cycle, Nitrogen Fixation — Biological and Abiological, Free-Living and Symbiotic Nitrogen-Fixing Organisms, Nitrate and Ammonia Assimilation by Plants. Study these first, then practise questions on each for the NIOS Class 12 board exam.
How should I revise Nitrogen Metabolism for the NIOS Class 12 board exam?
Learn the core ideas first, then work through the 45 practice questions on Nitrogen Metabolism. Revise definitions regularly and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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