Q78 The correct sequence of overall biochemical reaction which expresses the process of denitrification is
| Option | Sequence |
|---|---|
| (A) | NO₃⁻ → NO₂⁻ → NO → N₂O → N₂ |
| (B) | NO₃⁻ → NO₂⁻ → N₂O → NO → N₂ |
| (C) | NO₂⁻ → NO → NO₂⁻ → N₂O → N₂ |
| (D) | NO₃⁻ → NO → NO → NO₂⁻ → N₂ |
The correct answer is option (B): NO₃⁻ → NO₂⁻ → NO → N₂O → N₂. This sequence accurately represents the standard biochemical pathway of denitrification, a key microbial process in the nitrogen cycle.
Question Breakdown
Denitrification converts nitrate (NO₃⁻) to dinitrogen gas (N₂) under anaerobic conditions via facultative bacteria like Pseudomonas and Thiobacillus, using nitrate as an electron acceptor. The process involves four sequential reductions catalyzed by specific enzymes: nitrate reductase (NO₃⁻ to NO₂⁻), nitrite reductase (NO₂⁻ to NO), nitric oxide reductase (2 NO to N₂O), and nitrous oxide reductase (N₂O to N₂).
Option Analysis
- (A) NO₂⁻ → NO₃⁻ → NO → N₂O → N₂: Incorrect; starts with nitrite (NO₂⁻) instead of nitrate (NO₃⁻), and reverses the initial reduction step, which defies thermodynamics as nitrate reduction precedes nitrite.
- (B) NO₃⁻ → NO₂⁻ → NO → N₂O → N₂: Correct; matches the established stepwise reduction pathway observed across microbial studies.
- (C) NO₃⁻ → NO → NO₂⁻ → N₂O → N₂: Incorrect; skips nitrite (NO₂⁻) as a stable intermediate and places it after NO, ignoring the direct enzymatic reduction order.
- (D) NO₃⁻ → NO → NO₂⁻ → N₂O → N₂: Incorrect; same error as (C), with NO before NO₂⁻, which contradicts the sequential enzyme kinetics.
Denitrification biochemical reaction plays a crucial role in the nitrogen cycle, converting nitrate back to atmospheric nitrogen gas through a precise sequence. For CSIR NET aspirants, understanding the correct sequence of denitrification biochemical reaction—NO₃⁻ → NO₂⁻ → NO → N₂O → N₂—is essential for mastering microbial metabolism and environmental biology.
Denitrification Pathway Steps
This anaerobic respiration process occurs in oxygen-limited soils and sediments, driven by denitrifying bacteria. Key steps include:
- Nitrate (NO₃⁻) to nitrite (NO₂⁻) via nitrate reductase.
- Nitrite (NO₂⁻) to nitric oxide (NO) via nitrite reductase.
- Nitric oxide (NO) to nitrous oxide (N₂O) via nitric oxide reductase.
- Nitrous oxide (N₂O) to dinitrogen (N₂) via nitrous oxide reductase.
Intermediates like N₂O contribute to greenhouse effects if incomplete.
Enzymes and Conditions
Each step requires specific enzymes and organic carbon as an electron donor. Low oxygen (<10%) and pH 6-8 optimize the process, common in waterlogged soils relevant to bioremediation.
CSIR NET Exam Relevance
Questions on denitrification biochemical reaction test pathway order and errors in sequences. Option (B) is standard; others fail logical reduction hierarchy. Practice with diagrams reinforces retention for exams.
