Group I-II Matches

Nitrobacter (P) catalyzes nitrite (NO₂⁻) oxidation to nitrate (NO₃⁻) during nitrification in the nitrogen cycle.

Nitrite reductase (Q) reduces nitrite (NO₂⁻) to ammonium (NH₄⁺) in nitrogen assimilation pathways.

Nitrogenase (R) fixes atmospheric N₂ to 2NH₃ in symbiotic or free-living bacteria.

Nitrate reductase (S) reduces nitrate (NO₃⁻) to nitrite (NO₂⁻) as the first step in plant nitrate assimilation.

Option Analysis

Option (A) P-4, Q-3, R-2, S-1

Incorrect. P-4 mismatches Nitrobacter with N₂ fixation; Nitrobacter performs NO₂⁻ → NO₃⁻. Q-3 mismatches nitrite reductase; it does NO₂⁻ → NH₄⁺, not nitrification. R-2 correct, S-1 wrong as nitrate reductase does not oxidize nitrite.

Option (B) P-4, Q-3, R-1, S-2

Incorrect. P-4 and Q-3 mismatch as above. R-1 wrong; nitrogenase fixes N₂, not nitrate reduction. S-2 mismatches; nitrate reductase reduces, does not fix N₂.

Option (C) P-1, Q-2, R-4, S-3

Correct. All matches align: Nitrobacter (1: NO₂⁻ → NO₃⁻), nitrite reductase (2: NO₂⁻ → NH₄⁺), nitrogenase (4: N₂ → 2NH₃, note Group II lists 2NH₃), nitrate reductase (3: NO₃⁻ → NO₂⁻).

Option (D) P-3, Q-4, R-2, S-1

Incorrect. P-3 mismatches oxidation with assimilation reduction. Q-4 wrong for N₂ fixation. R-2 correct, but S-1 mismatches nitrate reductase function.

Key Processes

Nitrogen fixation converts N₂ to 2NH₃ via nitrogenase in diazotrophs.

Assimilation reduces NO₃⁻ to NO₂⁻ (nitrate reductase) then NO₂⁻ to NH₄⁺ (nitrite reductase) in plants.

Nitrification by Nitrobacter oxidizes NO₂⁻ to NO₃⁻.

CSIR NET Relevance

Nitrogen fixation and assimilation matching questions test core CSIR NET concepts in the nitrogen cycle, linking microbes like Nitrobacter and enzymes such as nitrogenase to specific reactions.

These matches appear in exams to differentiate fixation (N₂ → NH₃), assimilation (NO₃⁻/NO₂⁻ → NH₄⁺), and nitrification. Option C correctly pairs P-1 (Nitrobacter-NO₂⁻→NO₃⁻), Q-2 (reductase-NO₂⁻→NH₄⁺), R-4 (N₂→2NH₃), S-3 (NO₃⁻→NO₂⁻).