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Nitrogen Metabolism

NIOS · Class 12 · Biology

Complete topic list for Nitrogen Metabolism in NIOS Class 12 Biology. Key concepts, sub-topics, and what to focus on for board exams.

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4 Topics · NIOS Class 12 Biology

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

Frequently Asked Questions

What are the important topics in Nitrogen Metabolism for NIOS Class 12 Biology?
Nitrogen Metabolism covers several key topics that are frequently asked in NIOS Class 12 board exams. Focus on the core concepts listed on this page and practise related questions to build confidence.
How to score full marks in Nitrogen Metabolism — NIOS Class 12 Biology?
Start by understanding all key concepts. Practise previous year questions from this chapter. Revise formulas and definitions regularly. Use flashcards for quick revision 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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