Question
NEET 2022 (paraphrased): Regarding nitrogen fixation, which of the following statements is correct? (a) key enzyme refers to Rhizobium in legume root nodules (b) symbiotic bacteria is characterised by nitrogenase, which is oxygen-sensitive (c) free-living fixers — Azotobacter, Clostridium (d) leghaemoglobin has no role in nitrogen fixation
Solution — Step by Step
This option swaps the definitions. key enzyme is actually nitrogenase, which is oxygen-sensitive, not what (a) claims. So (a) is out.
Similarly, symbiotic bacteria means Rhizobium in legume root nodules. Option (b) is mismatched. Out.
This one reads correctly: free-living fixers — Azotobacter, Clostridium. Match with NCERT wording confirms it.
leghaemoglobin — scavenges O₂ inside nodules to protect nitrogenase — plays a real role in nitrogen fixation, so option (d) is factually wrong.
Correct answer: (c) free-living fixers — Azotobacter, Clostridium.
Why This Works
NEET PYQs on nitrogen fixation rely on definition swapping. The examiner takes two correct facts and pairs them with the wrong terms. If you know each fact cold, the MCQ dissolves in 20 seconds.
Alternative Method
For definition-swap MCQs, don’t read all four options first. Read option (a), decide true/false, then (b), and so on. This prevents confusion between similar-looking options.
NEET reuses the same fact set every 2–3 years with different distractors. Master the four NCERT facts on nitrogen fixation listed here and you’ll handle almost any MCQ on this chapter.
Common Mistake
Students second-guess themselves when they see a familiar option listed as “wrong” in the stem. Trust the elimination — if you know the definition, stick with it.
Keep a one-page sheet of definitions for every NEET chapter. Revise only that sheet in the last 48 hours before the exam.