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The correct sequence of overall biochemical reaction which expresses the process of denitrification is ____

  • A.2NO\(_{3}^{-}\) → 2NO\(_{2}^{-}\) → N\(_{2}\)O → 2NO → N\(_{2}\)
  • B.2NO\(_{3}^{-}\) → 2NO\(_{2}^{-}\) → 2NO → N\(_{2}\)O → N\(_{2}\)
  • C.2NO\(_{3}^{-}\) → 2NO → 2NO\(_{2}^{-}\) → N\(_{2}\)O → N\(_{2}\)
  • D.2NO\(_{3}^{-}\) → N\(_{2}\)O → 2NO → 2NO\(_{2}^{-}\) → N\(_{2}\)

Answer: B

The sequence is 2NO3- → 2NO2- → 2NO → N2O → N2.

Denitrification reduces nitrate stepwise to dinitrogen gas, one enzyme per step:

  1. nitrate reductase: NO3- to NO2-;
  2. nitrite reductase: NO2- to nitric oxide, NO;
  3. nitric oxide reductase: two NO to nitrous oxide, N2O;
  4. nitrous oxide reductase: N2O to N2.

The oxidation state of nitrogen falls steadily, +5, +3, +2, +1, 0, which is a quick check on the order. Option A puts N2O (+1) before NO (+2), option C puts NO before nitrite, and option D jumps from nitrate straight to N2O and then back up to nitrite.

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