Q.64 During biological nitrogen fixation, __________ ATPs are required to fix molecular nitrogen into ammonia.
During biological nitrogen fixation, 16 ATPs are required to fix one N₂ molecule into 2 NH₃. The correct answer is option 4.
Detailed Mechanism
Nitrogenase catalyzes: N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi.
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8 electrons from ferredoxin/flavodoxin reduce N≡N triple bond.
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16 ATP hydrolyzed (2 ATP/electron) power conformational changes in Fe-protein, enabling electron transfer to MoFe-protein.
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H₂ produced as obligatory byproduct (energy cost).
Per NH₃ calculation: 16 ATP ÷ 2 NH₃ = 8 ATP/NH₃, but question specifies “molecular nitrogen into ammonia” (N₂ → 2NH₃), requiring full 16 ATP.
Option Analysis
Option 1: Four ATP
Wrong. Insufficient for nitrogenase; represents ~1/4 electron transfer cost.
Option 2: Eight ATP
Wrong. Covers single NH₃ equivalent but ignores N₂ stoichiometry (produces 2 NH₃).
Option 3: Thirty two ATP
Wrong. Overestimates; some sources mention higher in vivo losses, but textbook standard is 16.
Option 4: Sixteen ATP (Correct)
Standard value: N₂ + 16 Mg-ATP → 2NH₃. Confirmed across microbiology texts.
Biological nitrogen fixation 16 ATP defines nitrogenase energetics in NEET exams. Rhizobium/Azotobacter convert atmospheric N₂ to ammonia using 16 ATP per N₂ fixed—essential for global nitrogen cycle.
Nitrogenase Reaction
N₂ + 8 e⁻ + 8 H⁺ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 Pᵢ
Energy breakdown: 2 ATP per electron × 8 electrons = 16 ATP. Fe-protein cycles deliver electrons to MoFe-protein.
Why 16 ATP?
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Triple N≡N bond energy: ~940 kJ/mol.
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Obligatory H₂ evolution wastes ~25% energy.
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Prevents O₂ inactivation via conformational protection.
NEET key: Question tests complete reaction stoichiometry, not per-NH₃ calculation.
