Part 5 — Nitrogen Metabolism (Q101–125)

Part 5 (Q101–125) of Chapter 12 – Mineral Nutrition, focusing on Nitrogen Metabolism: nitrogen cycle, biological fixation, nodules, nitrogenase, and nitrate assimilation.

Part 5 — Nitrogen Metabolism (Q101–125)

Q101.

Which form of nitrogen is absorbed by most plants?
A. N₂
B. Nitrate (NO₃⁻) ✅
C. Nitric oxide (NO)
D. Amines

Explanation:

• A. N₂ is inert and must be fixed first.
• B. (Correct) Plants mainly absorb nitrogen as nitrate and ammonium.
• C. NO is a signaling molecule, not a nutrient.
• D. Amines are organic N, not directly absorbed.

Q102.

Biological nitrogen fixation converts:
A. NO₃⁻ → N₂
B. N₂ → NH₃ ✅
C. NH₃ → N₂O
D. NH₄⁺ → Nitrate

Explanation:

• A. Denitrification, not fixation.
• B. (Correct) Nitrogenase enzyme in bacteria reduces atmospheric N₂ to ammonia (NH₃).
• C. Side reaction in denitrification.
• D. Nitrification.

Q103.

Nitrogenase enzyme functions only under:
A. High O₂ concentration
B. Anaerobic or low O₂ conditions ✅
C. High temperature and light
D. Inorganic salt-rich medium

Explanation:

• A. O₂ irreversibly inactivates nitrogenase.
• B. (Correct) Nitrogenase works in anaerobic conditions, often protected in nodules by leghemoglobin.
• C/D. Not main requirement.

Q104.

Which cofactor is essential for nitrogenase enzyme?
A. Copper
B. Molybdenum ✅
C. Sodium
D. Chlorine

Explanation:

• A/C/D. Not used in nitrogenase.
• B. (Correct) Nitrogenase requires Fe–Mo cofactor for N₂ reduction.

Q105.

Symbiotic nitrogen-fixing bacteria in legume nodules belong to:
A. Azotobacter
B. Rhizobium ✅
C. Nitrosomonas
D. Clostridium

Explanation:

• A. Azotobacter = free-living nitrogen fixer.
• B. (Correct) Rhizobium infects legume roots → nodules.
• C. Nitrosomonas = nitrifying bacterium.
• D. Clostridium = free-living anaerobic fixer.

Q106.

Nodules of leguminous plants are pink/red due to:
A. Phytochrome
B. Leghemoglobin ✅
C. Hemocyanin
D. Chlorophyll

Explanation:

• A. Phytochrome = light receptor.
• B. (Correct) Leghemoglobin, an O₂-binding pigment, maintains low O₂ for nitrogenase while supplying enough for respiration.
• C. Found in some animals.
• D. Not in nodules.

Q107.

Which step in nitrogen cycle is carried out by Nitrosomonas?
A. Ammonia → Nitrite ✅
B. Nitrite → Nitrate
C. Nitrate → Nitrogen gas
D. Nitrogen gas → Ammonia

Explanation:

• A. (Correct) Nitrosomonas oxidizes NH₃ to NO₂⁻.
• B. Nitrobacter.
• C. Denitrification bacteria (Pseudomonas).
• D. Nitrogen fixation.

Q108.

Nitrate is reduced to ammonium in plants through:
A. Nitrate reductase and nitrite reductase ✅
B. Amylase and invertase
C. Catalase and peroxidase
D. Nitrogenase only

Explanation:

• A. (Correct) NO₃⁻ → NO₂⁻ (nitrate reductase), then NO₂⁻ → NH₄⁺ (nitrite reductase).
• B/C. Not relevant enzymes.
• D. Nitrogenase reduces N₂, not nitrate.

Q109.

Which is the first stable product of nitrogen fixation?
A. Nitrate
B. Ammonia ✅
C. Urea
D. Amino acid

Explanation:

• A. NO₃⁻ comes later in soil cycles.
• B. (Correct) Ammonia is the first stable compound from nitrogenase.
• C/D. Ammonia is incorporated later into amino acids and urea (in animals).

Q110.

Glutamine synthetase enzyme helps in:
A. Fixing N₂
B. Assimilation of ammonium into amino acids ✅
C. Denitrification
D. Protein breakdown

Explanation:

• A. Done by nitrogenase.
• B. (Correct) NH₄⁺ is incorporated into glutamine via glutamine synthetase.
• C/D. Not related.

Q111.

Which process is denitrification?
A. NO₃⁻ → NH₄⁺
B. NO₃⁻ → N₂ ✅
C. N₂ → NH₃
D. NH₃ → NO₂⁻

Explanation:

• A. Assimilation.
• B. (Correct) Denitrification: NO₃⁻ reduced to N₂ gas by Pseudomonas/Thiobacillus.
• C. Nitrogen fixation.
• D. Nitrification.

Q112.

Which organism is a non-symbiotic nitrogen fixer?
A. Rhizobium
B. Azotobacter ✅
C. Nostoc
D. Frankia

Explanation:

• A. Symbiotic with legumes.
• B. (Correct) Azotobacter is a free-living aerobic N₂ fixer.
• C/D. Nostoc and Frankia are symbiotic.

Q113.

Which cyanobacterium is often found fixing nitrogen symbiotically in paddy fields?
A. Nostoc
B. Anabaena ✅
C. Chlorella
D. Volvox

Explanation:

• A. Nostoc can also fix but less associated with rice paddies.
• B. (Correct) Anabaena (often in association with Azolla fern) fixes nitrogen in rice fields.
• C/D. Green algae, not N₂-fixers.

Q114.

Frankia is a nitrogen-fixing bacterium associated with:
A. Rice
B. Non-leguminous plants like Alnus ✅
C. Wheat
D. Barley

Explanation:

• A/C/D. Cereals do not host Frankia.
• B. (Correct) Frankia fixes nitrogen in root nodules of non-legumes (actinorhizal plants) such as Alnus, Casuarina.

Q115.

Ammonia formed during fixation is assimilated into amino acids through:
A. GS-GOGAT pathway ✅
B. Calvin cycle
C. Krebs cycle
D. Glycolysis

Explanation:

• A. (Correct) GS (glutamine synthetase) and GOGAT (glutamate synthase) pathways assimilate NH₄⁺ into glutamine and glutamate.
• B-D. Not directly involved in N assimilation.

Q116.

Which of the following is NOT involved in nitrogen fixation?
A. Nitrogenase
B. Leghemoglobin
C. Nitrate reductase ✅
D. ATP

Explanation:

• A/B/D. Essential for fixation.
• C. (Correct) Nitrate reductase reduces nitrate to nitrite in assimilation, not N₂ fixation.

Q117.

Which gas inhibits nitrogenase activity strongly?
A. Oxygen ✅
B. Carbon dioxide
C. Nitric oxide
D. Methane

Explanation:

• A. (Correct) O₂ irreversibly inactivates nitrogenase unless protected by leghemoglobin.
• B-D. Not strong inhibitors.

Q118.

The nitrogenase enzyme requires:
A. 4 ATP per N₂ reduced
B. 16 ATP per N₂ reduced ✅
C. No ATP
D. Only NADPH

Explanation:

• A. Too low.
• B. (Correct) Reduction of N₂ to 2 NH₃ requires 16 ATP.
• C/D. ATP is essential, not optional.

Q119.

Which plant pigment buffers oxygen concentration in root nodules?
A. Hemoglobin (animal type)
B. Leghemoglobin ✅
C. Cytochrome b
D. Chlorophyll

Explanation:

• A. Hemoglobin is in animals.
• B. (Correct) Leghemoglobin in nodules maintains low free O₂, essential for nitrogenase.
• C/D. Not O₂ buffers.

Q120.

Which step of nitrogen cycle is carried out by Nitrobacter?
A. NH₃ → NO₂⁻
B. NO₂⁻ → NO₃⁻ ✅
C. NO₃⁻ → NH₄⁺
D. N₂ → NH₃

Explanation:

• A. Nitrosomonas.
• B. (Correct) Nitrobacter oxidizes nitrite to nitrate.
• C. Assimilation.
• D. Fixation.

Q121.

The host plant supplies what to Rhizobium in nodules?
A. Oxygen
B. Carbohydrates ✅
C. Ammonia
D. Nitrate

Explanation:

• A. O₂ is minimized, not supplied in high amounts.
• B. (Correct) Plant provides carbohydrates as energy source for Rhizobium.
• C/D. These are N forms, not supplied by host.

Q122.

The NH₄⁺ produced in nodules is incorporated into:
A. DNA
B. Amino acids like glutamate ✅
C. Nitrate
D. Fatty acids

Explanation:

• A/D. Not direct assimilation products.
• B. (Correct) Ammonium is rapidly assimilated into glutamate, glutamine, asparagine.
• C. Nitrate is oxidized form, not assimilation product.

Q123.

Which pathway assimilates ammonium through reductive amination?
A. GS-GOGAT pathway ✅
B. Krebs cycle
C. Urea cycle
D. Glycolysis

Explanation:

• A. (Correct) Reductive amination occurs via GS-GOGAT.
• B-D. Not involved in NH₄⁺ assimilation.

Q124.

Which of the following is a denitrifying bacterium?
A. Nitrosomonas
B. Pseudomonas ✅
C. Azotobacter
D. Nostoc

Explanation:

• A. Nitrifying bacterium.
• B. (Correct) Pseudomonas converts nitrates to N₂ gas (denitrification).
• C/D. Free-living nitrogen fixers.

Q125.

Which form of nitrogen is toxic at high levels inside plants and must be quickly assimilated?
A. NO₃⁻
B. NH₄⁺ ✅
C. N₂
D. Urea

Explanation:

• A. Nitrate is less toxic, stored in vacuoles.
• B. (Correct) Ammonium is highly toxic, so plants rapidly incorporate it into amino acids.
• C. N₂ is inert.
• D. Urea is not typically produced in plants.

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