COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: PLANT PHYSIOLOGY – Nitrogen Metabolism in Plants.
Syllabus Outline
- Uptake, transport, and reduction to nitrite and ammonium.
- Nitrate reductase and nitrite reductase – structure, cofactors, regulation.
- Ammonium assimilation pathways and regulation of nitrogen uptake and assimilation in plants and microbes.
- Mechanism of nitrogen cycle – Nitrogen fixation, Ammonium Assimilation, Nitrification, Denitrification, Anammox.
- Precursors from central metabolic pathways.
- Biosynthetic families of amino acids (e.g., glutamate, aspartate, serine families).
- Key enzymes and regulatory mechanisms (feedback inhibition, allosteric control).
- Essential vs non-essential amino acids – synthesis pathways in plants and microbes.
- Role of transamination and nitrogen recycling.
This quiz contains 25 concept-based, most frequently asked MCQs on “PLANT PHYSIOLOGY – Nitrogen Metabolism in Plants”. Each question has a single correct/most appropriate answer.
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1. The modular structure of Nitrate Reductase involves a multi-step electron transfer process. In the standard pathway, the correct sequence of electron transfer is:
A) NADH → Mo-cofactor → FAD → Heme
B) NADH → FAD → Heme → Mo-cofactor
C) FAD → NADH → Heme → Mo-cofactor
D) Heme → FAD → NADH → Mo-cofactor
2. What is the primary physiological reason for the compartmentation of Nitrite Reductase (NiR) activity within the chloroplasts/plastids, separate from cytosolic Nitrate Reductase (NR)?
A) To ensure NR is protected from O₂ while NiR is not.
B) To utilise the high concentration of cytosolic NADH for the reduction of NO₂⁻.
C) To facilitate the rapid detoxification of the highly toxic NO₂⁻ and utilise photosynthetic reducing power (Ferredoxin).
D) Because the substrates for NiR (NO₂⁻) are exclusively imported directly into the chloroplast from the environment.
3. Nitrate (NO₃⁻) uptake into plant cells is primarily mediated by:
A) Passive diffusion across the plasma membrane
B) Symport transporters coupled to the Na⁺ electrochemical gradient
C) Antiport transporters exchanging NO₃⁻ for H⁺
D) Symport systems coupled to the H⁺ electrochemical gradient
4. The enzyme Nitrogenase, found in symbiotic nitrogen-fixing bacteria, is highly sensitive to:
A) CO₂
B) Oxygen
C) Ammonium ions
D) Light intensity
5. In the nitrogenase complex, which cofactor is responsible for the reduction of dinitrogen (N₂) to ammonia (NH₃)?
A) Molybdenum-iron (MoFe) cofactor
B) Iron-sulfur (Fe-S) cluster
C) FAD cofactor
D) Heme cofactor
6. Which molecule serves as the direct electron donor to the Fe protein in the nitrogenase complex?
A) NADH
B) Reduced Ferredoxin
C) ATP
D) Cytochrome b₆f
7. In legume root nodules, the function of Leghemoglobin is to:
A) Supply ATP to the bacteroids
B) Facilitate oxygen transport
C) Act as a nitrogen carrier protein
D) Regulate nitrate uptake from soil
8. Which of the following reactions correctly represents the overall process of biological nitrogen fixation?
A) N₂ + 3O₂ + 6e⁻ + 12ATP → 2NO₃⁻ + H₂ + 12ADP + 12Pi
B) N₂ + 6H⁺ + 6e⁻ + 12ATP → 2NH₃ + H₂ + 12ADP + 12Pi
C) N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
D) N₂ + H₂O → 2NH₄⁺ + O₂
9. Rhizobium bacteria enter legume roots primarily through:
A) Root cap cells
B) Cortical plasmodesmata
C) Root hair curling
D) Apoplastic flow through the xylem
10. The Fe protein component of nitrogenase serves primarily to:
A) Bind and activate molecular nitrogen
B) Transfer electrons to the MoFe protein using ATP hydrolysis
C) Regenerate reduced ferredoxin
D) Store oxygen for bacteroids
11. Which of the following processes contributes to the loss of fixed nitrogen from agricultural soils?
A) Denitrification
B) Nitrification
C) Mineralisation
D) Annamox
12. During nitrification, ammonia is first oxidised to nitrite by:
A) Nitrosomonas
B) Nitrobacter
C) Azotobacter
D) Rhizobium
13. Denitrification under anaerobic conditions results in the production of:
A) NO₂⁻ and NO₃⁻
B) NH₄⁺ and urea
C) NO, N₂O, and N₂ gases
D) NO₂⁻ only
14. The balance between nitrate assimilation and photorespiration in leaves is strongly influenced by:
A) Light intensity and carbon availability
B) Soil phosphorus content
C) Organic nitrogen and O2
D) Potassium ion concentration
15. Which of the following processes releases molecular nitrogen (N₂) back into the atmosphere?
A) Nitrification
B) Denitrification
C) Nitrogen fixation
D) Assimilation
16. The process of mineralisation converts:
A) Organic nitrogen to ammonium
B) Ammonium to nitrate
C) Nitrate to nitrogen gas
D) Nitrogen gas to ammonia
17. Which intermediate is common to both photorespiratory and nitrogen assimilation pathways?
A) Glycine
B) Glutamate
C) α-Ketoglutarate
D) Serine
18. In plants, the photorespiratory release of CO₂ and NH₄⁺ occurs in which organelle?
A) Peroxisome
B) Mitochondrion
C) Chloroplast
D) Cytosol
19. Which of the following statements is true regarding the coordination between nitrate reduction and photosynthetic carbon metabolism?
A) Nitrate reduction is independent of photosynthesis.
B) Both processes compete for reducing equivalents in the chloroplast.
C) Nitrate reduction in leaves is coupled with light-driven NADPH production and sugar availability.
D) Nitrate reduction proceeds only in the dark when photosynthesis is inactive.
20. In higher plants, the diurnal regulation of Nitrate Reductase activity is controlled primarily by:
A) Light-dependent transcription and post-translational phosphorylation
B) Circadian oscillations in soil nitrate
C) Changes in cellular potassium concentration
D) Leaf turgor pressure variations
21. Which enzyme catalyses the conversion of nitrite (NO₂⁻) to nitric oxide (NO) during abiotic stress in plants?
A) Nitrate Reductase
B) Nitrite Reductase
C) Nitric Oxide Synthase
D) Glutamate Dehydrogenase
22. Nitrogen deficiency typically causes which visual symptom in older leaves?
A) Chlorosis
B) Anthocyanin accumulation
C) Marginal necrosis
D) Curling and thickening
23. During nitrogen assimilation, the major source of carbon skeletons for amino acid biosynthesis is derived from:
A) Calvin cycle intermediates such as 3-PGA
B) Glycolytic intermediates such as pyruvate and α-ketoglutarate
C) Photorespiratory intermediates
D) Fatty acid degradation products
24. A plant is treated with an inhibitor that specifically blocks the NADH dehydrogenase activity required for the first step of nitrate assimilation. Which immediate physiological consequences are most likely to be observed?
I – Accumulation of NO₃⁻ in the root and leaf tissues.
II – Sharp decrease in NO₂⁻ levels in the cytosol.
III – Increased activity of GS2 in the chloroplast to compensate for lost N assimilation.
IV – Decreased NADH pool size in the cytosol.
A) I, II and IV
B) I and II
C) II, III and IV
D) I, II, III and IV
25. Nitrite Reductase (NiR) catalyses the six-electron reduction of NO₂⁻ to NH₄⁺. Consider the requirements for this reaction in plant leaves.
I – NiR utilises Ferredoxin (Fdred), primarily derived from photosynthetic electron transport, as its electron source.
II – The entire NiR reaction occurs in the cytosol, generating NH₄⁺, which is immediately assimilated.
III – NiR activity is strongly dependent on light intensity because it relies on Photosystem I activity.
IV – Inhibition of Photosystem II (PSII) activity by a specific herbicide would diminish NiR activity more significantly than NR activity.
A) I, II and III
B) I and III
C) I, III and IV
D) II, III and IV
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References
- Hopkins, W. G., & Hüner, N. P. A. (2008). Introduction to Plant Physiology, John Wiley & Sons. 4th Edition.
- Pandey, S. N., & Sinha, B. K. (2015). Plant Physiology, Vikas Publishing House. 4th Edition.
- Jain, V. K. (2018). Fundamentals of Plant Physiology, S. Chand Publishing. 20th Edition.
- Verma, Mohit & Verma, S. K. (2022). A Textbook of Plant Physiology, Biochemistry and Biotechnology, S. Chand Publishing. 13th Edition.
- Lincoln Taiz, Ian Max Møller, Angus Murphy, and Eduardo Zeiger (2022). Plant Physiology and Development, Oxford University Press, 7th Edition.
- Nelson, David L. & Cox, Michael M. (2021). Lehninger Principles of Biochemistry, W. H. Freeman, 8th Edition.
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