Atmospheric nitrogen (N₂) is abundant (78% of air) but biologically inert — the triple bond (N≡N) requires enormous energy to break. Biological nitrogen fixation (BNF) is the enzyme-driven reduction of N₂ to ammonia (NH₃) by certain prokaryotes, making nitrogen available for amino acid and nucleotide synthesis.
The enzyme: Nitrogenase — found exclusively in prokaryotes called diazotrophs. It is irreversibly inactivated by oxygen. The overall reaction:
N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi
Key organisms:
The Rhizobium–legume symbiosis:
High-frequency NEET traps in this topic:
(Reference: NCERT Class 11 Biology Chapter 12, page 237)
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is the key enzyme responsible for biological nitrogen fixation?
Which of the following statements about nitrogenase is correct?
*Rhizobium* fixes nitrogen in association with legumes. In its free-living state, *Rhizobium* is:
Leg-haemoglobin in root nodules of legumes functions as:
The overall equation for biological nitrogen fixation shows:
In cyanobacteria such as *Anabaena*, nitrogen fixation occurs in specialized cells called heterocysts. Which feature of heterocysts is essential for protecting nitrogenase?
A student claims that *Azotobacter* cannot fix nitrogen because it is aerobic and nitrogenase is oxygen-sensitive. What is the flaw in this reasoning?
In a Rhizobium–legume symbiosis, identify the correct combination of facts:
Given
A NEET question states: "Which of the following is a free-living nitrogen-fixing aerobic bacterium?" Options: (a) *Rhizobium* (b) *Azotobacter* (c) *Clostridium* (d) *Anabaena*
Required
Identify the organism that satisfies ALL three criteria: free-living AND nitrogen-fixing AND aerobic.
Concept
Biological nitrogen fixation occurs in both free-living and symbiotic prokaryotes. The question requires distinguishing organisms by (1) lifestyle (free-living vs symbiotic), (2) N₂-fixing ability, and (3) oxygen relationship (aerobic vs anaerobic).
Classification
- *Rhizobium*: symbiotic (fixes N₂ only in nodules) — fails "free-living" criterion. - *Azotobacter*: free-living, aerobic, nitrogen-fixing — satisfies all three. - *Clostridium*: free-living, nitrogen-fixing, but ANAEROBIC — fails "aerobic" criterion. - *Anabaena*: free-living, nitrogen-fixing cyanobacterium — photosynthetic, often considered aerobic, but N₂ fixation occurs in anaerobic heterocysts. The classic textbook answer for "free-living aerobic N₂ fixer" is *Azotobacter*.
Reasoning
*Azotobacter* is the textbook example of a free-living aerobic diazotroph. It uses respiratory protection (highest known bacterial respiration rate) to maintain low intracellular O₂ while remaining aerobic overall.
Answer selection
(b) *Azotobacter*
Final answer
*Azotobacter* — free-living, aerobic, N₂-fixing bacterium.
Common trap
Selecting *Rhizobium* because it is the most well-known N₂ fixer associated with legumes. But free-living *Rhizobium* does NOT fix nitrogen. Also, selecting *Clostridium* (which is free-living and N₂-fixing but anaerobic) by missing the "aerobic" qualifier.
Similar NEET-style question
"Which of the following organisms fixes atmospheric nitrogen in anaerobic free-living conditions?" (a) *Azotobacter* (b) *Rhizobium* (c) *Clostridium* (d) *Frankia* Answer: (c) *Clostridium* — free-living, anaerobic N₂ fixer. *Azotobacter* is aerobic; *Rhizobium* is symbiotic; *Frankia* is symbiotic (actinorhizal root nodules). ---
In the NEET syllabus; removed from current NCERT.
Rhizobium (legumes — root nodules), Frankia (Casuarina), Azotobacter (free-living aerobic), Cyanobacteria (Anabaena, Nostoc). Nitrogenase: Mo-Fe protein; reduces N2 → NH3; requires anaerobic conditions; leghaemoglobin scavenges O2 in nodules.
-- NCERT Class 11 Biology, Ch. 12, p. 237These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
C3: 3-PGA (3-carbon) is first stable product. C4: oxaloacetate (4-carbon) is first stable product.
Question asks first stable product of CO2 fixation in C3/C4 plant.
Pathway name = carbons in first product. C3 → 3C; C4 → 4C.
Category: Negative Marking
Light reactions: water splits at PS II (P680) FIRST, electrons flow PS II → PS I (P700). PS II numbered AFTER PS I in discovery, but reaction order is PS II → PS I.
Question on Z-scheme order, water photolysis, electron source.
Discovery order ≠ reaction order. Photolysis at PS II; PS II reduces PS I.
Root cause: concept gap
Mnemonic: C3 → 3-PGA (3-carbon); C4 → OAA (4-carbon). Hatch-Slack pathway is C4.
Root cause: concept gap
Glycolysis is in CYTOPLASM (cytosol). Krebs cycle in mitochondrial matrix; ETC in inner membrane.
Root cause: term confusion
RuBisCO = ribulose-1,5-bisphosphate CARBOXYLASE/OXYGENASE. Carboxylase activity drives Calvin; oxygenase causes photorespiration.
38 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Which of the following statements about RuBisCO is true?
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
The reaction centre in PS II has an absorption maxima at
Which micronutrient is required for splitting of water molecule during photosynthesis?
Which hormone promotes internode/petiole elongation in deep water rice?
Which of the following combinations is required for chemiosmosis?
Which one of the following plants does not show plasticity?
What amount of energy is released from glucose during lactic acid fermentation?
The first stable product of CO fixation in sorghum is 2
Which of the following statements is correct ? cambial ring
flÎÁh ∑§Ë ¬˝Á∑˝§ÿÊ •Áœ∑§Ã◊ Á∑§‚ ŒÊÒ⁄UÊŸ „Ê ÃË „Ò? 114. The process of growth is maximum during :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
photosynthesis pathway product confusion
C3 and C4 pathways share the Calvin cycle; students who cannot anchor the first stable product in each pathway invert the carbon counts: 3-PGA (3-carbon) is the C3 first stable product, OAA (4-carbon) is the C4 first stable product. RuBisCO has dual carboxylase and oxygenase activity; students who recall only one role select incorrect statements about its function, including denying that it causes photorespiration.
ps i ps ii assignment reversal
PS I and PS II are numbered in order of discovery, not order of action in the Z-scheme. PS II acts first (water photolysis, O2 release, P680 reaction centre at 680 nm); PS I acts second (P700 reaction centre at 700 nm, NADPH production). Students who use Roman numeral order assign water photolysis to PS I and attribute the 700 nm absorption maximum to PS II.
respiration compartment swap
Glycolysis feeds the mitochondrial Krebs cycle, creating a mental link: students place glycolysis in the mitochondrial matrix. The anchor -- glycolysis occurs in the cytosol because it predates mitochondria and operates in anaerobes -- is lost under time pressure.
rubisco function confusion
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) has dual activity: carboxylase drives CO2 fixation in the Calvin cycle; oxygenase drives photorespiration. Questions give multiple statements about RuBisCO and ask which set is correct; students who have not anchored the dual-function name omit or deny the oxygenase role, selecting option sets that describe it as carboxylase only.
assertion reason unchecked reason
A/R questions in plant physiology present a true Assertion (e.g., ATP is used at two steps in glycolysis) alongside a Reason that sounds mechanistically plausible but may be factually imprecise or not the correct explanation. Students accept the Reason without independently checking whether its specific biochemical claim is accurate.
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