Part 6 — Advanced Nitrogen Cycle & Nitrate Assimilation (Q126–150)

Part 6 (Q126–150) of Chapter 12 – Mineral Nutrition, focusing on Advanced Nitrogen Cycle Concepts, Nitrate Assimilation, and Experimental/Clinical Correlations — all NEET-relevant.

Part 6 — Advanced Nitrogen Cycle & Nitrate Assimilation (Q126–150)

Q126.

Which process adds biologically available nitrogen to ecosystems?
A. Nitrification
B. Nitrogen fixation ✅
C. Denitrification
D. Ammonification

Explanation:

• A. Converts NH₃ → NO₂⁻/NO₃⁻, but doesn’t add new N.
• B. (Correct) Nitrogen fixation converts atmospheric N₂ to NH₃.
• C. Removes N from soil as N₂ gas.
• D. Recycles organic N → NH₃.

Q127.

Nitrate reductase enzyme is located in:
A. Chloroplast
B. Cytosol ✅
C. Mitochondria
D. Nucleus

Explanation:

• A. Nitrite reductase is in chloroplast.
• B. (Correct) Nitrate reductase is cytosolic, reduces NO₃⁻ → NO₂⁻.
• C/D. Not involved.

Q128.

Nitrite reductase enzyme requires reducing power from:
A. Ferredoxin in chloroplasts ✅
B. Cytochrome oxidase
C. NADH in mitochondria
D. ATP synthase

Explanation:

• A. (Correct) Nitrite reductase (in chloroplast) reduces NO₂⁻ to NH₄⁺ using ferredoxin.
• B/C/D. Not correct.

Q129.

Which of the following is not a step in nitrate assimilation?
A. NO₃⁻ → NO₂⁻
B. NO₂⁻ → NH₄⁺
C. NH₄⁺ → amino acids
D. NH₃ → N₂ gas ✅

Explanation:

• A-C. Steps in assimilation.
• D. (Correct) NH₃ → N₂ is denitrification, not assimilation.

Q130.

Nitrate reductase requires which cofactor?
A. Molybdenum ✅
B. Iron only
C. Copper
D. Chlorophyll

Explanation:

• A. (Correct) Mo is essential cofactor of nitrate reductase.
• B. Iron is part of electron transport but not main cofactor.
• C/D. Not involved.

Q131.

In nitrogen assimilation, glutamate is produced primarily by:
A. GS-GOGAT cycle ✅
B. Calvin cycle
C. Glycolysis
D. Urea cycle

Explanation:

• A. (Correct) GS-GOGAT cycle incorporates NH₄⁺ into glutamate and glutamine.
• B-D. Unrelated cycles.

Q132.

Which process links carbon and nitrogen metabolism in plants?
A. Calvin cycle only
B. GS-GOGAT cycle ✅
C. Glycolysis only
D. Photorespiration

Explanation:

• A/C/D. Not direct link.
• B. (Correct) GS-GOGAT uses carbon skeletons (α-ketoglutarate) and assimilates nitrogen → amino acids.

Q133.

Which is the main amino acid donor for biosynthesis of other amino acids?
A. Glycine
B. Glutamate ✅
C. Proline
D. Cysteine

Explanation:

• A/C/D. Not major donors.
• B. (Correct) Glutamate is the key amino group donor via transamination.

Q134.

Which enzyme catalyzes transfer of amino group between keto acids?
A. Transaminase ✅
B. Nitrogenase
C. Nitrate reductase
D. Catalase

Explanation:

• A. (Correct) Transaminases transfer amino groups from glutamate to other keto acids.
• B/C/D. Not involved.

Q135.

Which process in nitrogen cycle is carried out by Pseudomonas?
A. Denitrification ✅
B. Nitrification
C. Nitrogen fixation
D. Ammonification

Explanation:

• A. (Correct) Pseudomonas converts nitrate → N₂.
• B-D. Carried out by other microbes.

Q136.

Industrial nitrogen fixation (Haber–Bosch process) reduces N₂ to NH₃ using:
A. Sunlight and chlorophyll
B. High temperature and pressure with catalyst ✅
C. Ferredoxin
D. Nitrate reductase

Explanation:

• A/C/D. Biological fixation pathways.
• B. (Correct) Haber–Bosch: N₂ + 3H₂ → 2NH₃, with Fe catalyst, high pressure and temperature.

Q137.

Why is nitrate assimilation energy-demanding?
A. Nitrate uptake needs no energy
B. Nitrate reduction requires ATP and reducing equivalents ✅
C. NO₃⁻ diffuses passively into cells
D. It is a photochemical reaction only

Explanation:

• A/C/D. Misconceptions.
• B. (Correct) Both nitrate reductase and nitrite reductase need reducing power and ATP.

Q138.

Nitrate reductase is an inducible enzyme, meaning:
A. It is always active
B. It is synthesized only when nitrate is present ✅
C. It is produced constitutively
D. It is light-independent

Explanation:

• A/C. Constitutive enzymes differ.
• B. (Correct) Nitrate reductase is induced by substrate nitrate.
• D. Light influences ferredoxin for nitrite reductase, not this.

Q139.

Which deficiency reduces nitrate assimilation directly?
A. Iron and molybdenum ✅
B. Calcium and potassium
C. Sulphur and magnesium
D. Copper and boron

Explanation:

• A. (Correct) Fe and Mo are cofactors for nitrate/nitrite reductases.
• B-D. Affect other functions.

Q140.

Which nitrogen cycle step replenishes soil fertility naturally?
A. Denitrification
B. Nitrification
C. Biological nitrogen fixation ✅
D. Mineralization

Explanation:

• A. Removes nitrogen.
• B. Converts forms but does not add new N.
• C. (Correct) Biological fixation introduces new usable nitrogen.
• D. Mineralization recycles N, not replenishment.

Q141.

Nitrite accumulation in plants is toxic because:
A. It interferes with photosynthesis ✅
B. It forms ATP
C. It strengthens cell wall
D. It enhances protein synthesis

Explanation:

• A. (Correct) NO₂⁻ is toxic and interferes with metabolic processes if not rapidly reduced.
• B-D. Not true.

Q142.

Which bacteria oxidize nitrite (NO₂⁻) to nitrate (NO₃⁻)?
A. Nitrosomonas
B. Nitrobacter ✅
C. Rhizobium
D. Pseudomonas

Explanation:

• A. NH₃ → NO₂⁻.
• B. (Correct) Nitrobacter oxidizes nitrite to nitrate.
• C. N₂ fixation.
• D. Denitrification.

Q143.

Which condition favors denitrification?
A. Aerated soils
B. Waterlogged and anaerobic soils ✅
C. Dry sandy soil
D. Neutral pH soils

Explanation:

• A. Inhibits denitrification.
• B. (Correct) Low O₂ in waterlogged soils favors denitrifying bacteria.
• C/D. Not favorable.

Q144.

In legumes, infection thread formation is the first step in:
A. Root hair curling and Rhizobium entry ✅
B. Nitrogenase activation
C. Leghemoglobin synthesis
D. Nitrate assimilation

Explanation:

• A. (Correct) Rhizobium enters through root hairs via infection thread.
• B-D. Occur later.

Q145.

Nod factor secreted by Rhizobium triggers:
A. Root hair curling ✅
B. ATP hydrolysis
C. Leghemoglobin synthesis
D. Denitrification

Explanation:

• A. (Correct) Nod factors induce root hair curling, allowing Rhizobium entry.
• B-D. Not main effects.

Q146.

Leghemoglobin resembles animal hemoglobin in:
A. Oxygen binding function ✅
B. Location in blood
C. Storage of nitrate
D. Being an enzyme

Explanation:

• A. (Correct) Both bind O₂ for regulation.
• B. Plants don’t have blood.
• C/D. Not roles of leghemoglobin.

Q147.

Which is a symbiotic nitrogen fixer in non-leguminous plants?
A. Azotobacter
B. Frankia ✅
C. Nitrosomonas
D. Anabaena

Explanation:

• A. Free-living.
• B. (Correct) Frankia associates with Alnus, Casuarina.
• C. Nitrifier.
• D. Cyanobacterium in ferns, not woody plants.

Q148.

Nitrate assimilation occurs mainly in:
A. Leaves ✅
B. Roots only
C. Flowers
D. Seeds

Explanation:

• A. (Correct) Assimilation requires light/ferredoxin → mainly in leaves.
• B. Roots can reduce NO₃⁻ partially but major assimilation in leaves.
• C/D. Not major sites.

Q149.

Ammonification is the process of:
A. Conversion of NH₄⁺ → NO₂⁻
B. Conversion of organic nitrogen → NH₄⁺ ✅
C. N₂ → NH₃
D. NO₃⁻ → N₂

Explanation:

• A. Nitrification.
• B. (Correct) Decomposition of organic matter releases ammonium.
• C. Fixation.
• D. Denitrification.

Q150.

Which group of organisms play a central role in every step of the nitrogen cycle?
A. Fungi
B. Bacteria ✅
C. Algae
D. Protozoa

Explanation:

• A/C/D. Limited roles.
• B. (Correct) Bacteria mediate fixation, nitrification, denitrification, ammonification.

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