Q60. In legume nodules, nitrogen fixation requires low oxygen despite active respiration. Which mechanism maintains this balance?
[UP PGT Biology, 9 May 2026]
(a) Suppression of respiration lowers oxygen levels
(b) Leghemoglobin buffers oxygen, permitting respiration while protecting nitrogenase
(c) Direct diffusion of nitrogen into mitochondria produces ATP
(d) Storage-compound synthesis consumes all excess oxygen
Answer: (b) Leghemoglobin buffers oxygen, permitting respiration while protecting nitrogenase
Explanation: Leghemoglobin binds oxygen reversibly inside legume root nodules. It maintains a low concentration of free oxygen, protecting oxygen-sensitive nitrogenase, while still permitting oxygen delivery for energy-producing respiration. This coordination enables nitrogen-fixing bacteroids to function effectively.
Q59. Which of the following bacteria is incorrectly paired with its role?
[RRB Group D, 10 August 2026, Shift 3]
(a) Azotobacter — nitrogen fixation in soil
(b) Pseudomonas — nitrification
(c) Nitrobacter — nitrite-to-nitrate conversion
(d) Rhizobium — nitrogen fixation in legumes
Answer: (b) Pseudomonas — nitrification
Explanation: Pseudomonas species are often associated with denitrification rather than the conventional nitrification sequence. Nitrosomonas oxidizes ammonia to nitrite, while Nitrobacter oxidizes nitrite to nitrate. Azotobacter and Rhizobium fix atmospheric nitrogen in free-living and symbiotic settings respectively.
Q58. Which one of these is NOT a prokaryote–angiosperm symbiotic pair capable of fixing atmospheric nitrogen?
[CSIR-UGC NET Life Sciences, 18 December 2025]
(a) Nostoc and Gunnera
(b) Frankia and Casuarina
(c) Bradyrhizobium and Parasponia
(d) Azolla and rice
Answer: (d) Azolla and rice
Explanation: Azolla is an aquatic fern rather than a prokaryotic microorganism, and rice does not form the stated direct symbiosis with it. The nitrogen-fixing partnership involves Azolla and its cyanobacterial partner. The other pairs involve nitrogen-fixing microbes associated with higher plants.
Q57. Azotobacter is beneficial in agriculture because it ________.
[MP Excise Constable, 21 September 2025, Shift 2]
(a) decomposes organic matter
(b) acts as a pesticide
(c) increases phosphorus availability
(d) fixes atmospheric nitrogen
Answer: (d) fixes atmospheric nitrogen
Explanation: Azotobacter is a free-living aerobic soil bacterium capable of biological nitrogen fixation. By transforming atmospheric nitrogen into more accessible nitrogenous compounds, it helps improve soil fertility and can supplement nitrogen nutrition of non-leguminous crops without requiring root nodules.
Q56. Which of the following organisms is NOT involved in nitrogen fixation?
[MP Excise Constable, 11 September 2025, Shift 2]
(a) Azotobacter
(b) Frankia
(c) Nitrosomonas
(d) Rhizobium
Answer: (c) Nitrosomonas
Explanation: Nitrosomonas is a nitrifying bacterium: it oxidizes ammonia to nitrite rather than fixing atmospheric nitrogen. Azotobacter fixes nitrogen independently, Rhizobium does so in legume nodules, and Frankia fixes nitrogen in symbiosis with several non-leguminous woody plants.
Q55. Which of the following is a free-living nitrogen-fixing anaerobic bacterium?
[DSSSB TGT Natural Science, 7 September 2025, Shift 1]
(a) Anabaena
(b) Nostoc
(c) Clostridium
(d) Azotobacter
Answer: (c) Clostridium
Explanation: Certain Clostridium species, notably Clostridium pasteurianum, fix atmospheric nitrogen in oxygen-free conditions. Azotobacter is free-living but aerobic; Anabaena and Nostoc are photosynthetic cyanobacteria. The question specifically requires an anaerobic free-living bacterium rather than simply any nitrogen fixer.
Q54. Which biofertilizer fixes nitrogen in symbiotic association with leguminous plants?
[UPSSSC Agriculture Technical Assistant Mains, 13 July 2025]
(a) Rhizobium
(b) Azolla
(c) Phosphobacteria
(d) Mycorrhiza
(e) None of the above
Answer: (a) Rhizobium
Explanation: Rhizobium enters a mutually beneficial relationship with legume roots and establishes nodules where nitrogen fixation occurs. The plant supplies carbohydrates and shelter, while the bacteria supply fixed nitrogen. Azolla uses cyanobacterial partners; mycorrhizae primarily improve phosphorus and water uptake.
Q53. Under the Fertiliser (Control) Order, what is the minimum viable cell count per millilitre for liquid Rhizobium biofertilizer?
[UPSSSC Agriculture Technical Assistant Mains, 13 July 2025]
(a) 1 × 10⁷ CFU/ml
(b) 1 × 10⁶ CFU/ml
(c) 1 × 10⁹ CFU/ml
(d) 1 × 10⁸ CFU/ml
(e) None of the above
Answer: (d) 1 × 10⁸ CFU/ml
Explanation: The specified minimum for the liquid Rhizobium formulation in this examination is 1 × 10⁸ colony-forming units per millilitre. Viable cell counts help assess inoculant quality because sufficient living microorganisms must survive for effective root colonization and nitrogen fixation.
Q52. Which of these organisms cannot fix atmospheric nitrogen?
A. Azotobacter;
B. Oscillatoria;
C. Anabaena;
D. Volvox;
E. Nostoc.
[NEET UG, 4 May 2025]
(a) A only
(b) D only
(c) B only
(d) E only
Answer: (b) D only
Explanation: Volvox is a colonial green alga and lacks biological nitrogen-fixing machinery. Azotobacter is a nitrogen-fixing bacterium, and cyanobacterial groups such as Anabaena and Nostoc include nitrogen-fixing members. The examination answer identifies Volvox as the non-nitrogen-fixing organism.
Q51. Which characteristic is NOT true for all cyanobacteria?
[HTET PGT Biology 2024 examination, conducted 2025]
(a) They can fix atmospheric nitrogen
(b) They lack a membrane-bound nucleus
(c) They possess chlorophyll a
(d) Their cell walls are not made of cellulose
Answer: (a) They can fix atmospheric nitrogen
Explanation: Cyanobacteria are prokaryotic and contain chlorophyll a, but nitrogen fixation is not universal among them. Only particular taxa possess the appropriate nitrogen-fixing machinery. Genera such as Anabaena and Nostoc include well-known nitrogen-fixing species used in agricultural biofertilization.