The primary product of nitrogen fixation is NH₄⁺ (ammonium ion).​

Option Analysis

Nitrogen fixation converts atmospheric N₂ into biologically usable forms via nitrogenase enzymes in bacteria like Rhizobium.​

• (A) N₂: This is the inert starting substrate, not the product; it remains unusable by most organisms until fixed.​

• (B) NH₄⁺: Correct; N₂ reduces to NH₃, which protonates to NH₄⁺ in biological systems for assimilation into amino acids via glutamine synthetase.​

• (C) NO₂⁻: Nitrite forms later via nitrification by bacteria like Nitrosomonas, not fixation.​

• (D) NO₃⁻: Nitrate arises from further nitrite oxidation by Nitrobacter; it’s a downstream product in the nitrogen cycle.​

Nitrogen fixation stands as a cornerstone of the nitrogen cycle, transforming atmospheric N₂ into the primary product of nitrogen fixation: NH₄⁺ (ammonium ion). This process powers plant growth and ecosystems, making it essential for CSIR NET aspirants studying microbial metabolism.​

Nitrogen Fixation Process

Biological nitrogen fixation relies on nitrogenase enzymes in diazotrophs like Rhizobium (symbiotic) or Azotobacter (free-living). The reaction is:
\ceN2+8H++8e−+16ATP−>2NH3+H2+16ADP+16Pi\ceN2+8H++8e−+16ATP−>2NH3+H2+16ADP+16Pi
NH₃ quickly forms NH₄⁺, the primary product ready for glutamine synthesis.​

Key features include:

• FeMo-cofactor (FeMoco) active site for N₂ reduction.​

• High ATP demand (16-24 per N₂).​

• Anaerobic conditions to protect oxygen-sensitive nitrogenase.​

Why NH₄⁺, Not Others?

NH₄⁺ enters the GS-GOGAT pathway for glutamate formation, fueling protein synthesis. NO₂⁻ and NO₃⁻ emerge post-nitrification, while N₂ is the unreactive input.​

CSIR NET Relevance

This MCQ tests nitrogen metabolism basics. Legume nodules exemplify symbiotic fixation, contributing 90% of natural fixation.​