Respiration and Photorespiration: Important MCQs

COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: PLANT PHYSIOLOGY – Respiration and Photorespiration.

Syllabus Outline

1. Glycolysis, Citric acid cycle (Krebs Cycle) and Electron Transport Chain
2. Plant mitochondrial electron transport and ATP synthesis.
3. Alternative oxidase pathway
4. Photorespiratory pathway.
5. Conditions for photorespiration.

This quiz contains 25 concept-based, most frequently asked MCQs on “PLANT PHYSIOLOGY – Respiration and Photorespiration”. Each question has a single correct/most appropriate answer.

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1. In plant glycolysis, Pyrophosphate-dependent Phosphofructokinase is crucial for metabolic flexibility. The primary allosteric regulator that activates this enzyme is:

A) ATP

B) Citrate

C) Fructose 2,6-bisphosphate

D) Pyruvate

2. During anaerobic conditions (fermentation), the primary purpose of converting Pyruvate to Ethanol or Lactate is to:

A) Produce additional ATP by substrate-level phosphorylation.

B) Regenerate NAD⁺ required for continued glycolysis.

C) Directly produce energy equivalents required by the cell.

D) Prevent the accumulation of toxic pyruvate.

3. Which enzyme complex in the TCA cycle utilises NAD⁺ as a co-substrate and releases a molecule of CO₂ and NADH?

A) Pyruvate Dehydrogenase Complex

B) Succinate Dehydrogenase

C) Isocitrate Dehydrogenase

D) Fumarase

4. The conversion of two Glycine molecules into one Serine molecule occurs in the mitochondrion during photorespiration. The resulting Serine must be transported back to which organelle to complete the carbon recovery cycle?

A) Chloroplast

B) Peroxisome

C) Cytosol

D) Nucleus

5. Which of the following TCA cycle enzymes is embedded in the inner mitochondrial membrane and also functions as a component of the Electron Transport Chain (ETC)?

A) Isocitrate Dehydrogenase

B) Alpha-Ketoglutarate Dehydrogenase

C) Succinate Dehydrogenase

D) Malate Dehydrogenase

6. The conversion of Glycerate back to 3-Phosphoglycerate (3-PGA) to re-enter the Calvin cycle requires energy input. This phosphorylation step takes place in the:

A) Mitochondrion, utilising ATP generated by oxidative phosphorylation.

B) Peroxisome, utilising ATP derived from surrounding reactions.

C) Chloroplast, utilising ATP generated during the light reactions of photosynthesis.

D) Cytosol, utilising ATP generated during glycolysis.

7. What is the net yield of ATP directly produced via substrate-level phosphorylation from the complete aerobic oxidation of one molecule of glucose?

A) 2 ATP

B) 4 ATP

C) 36 ATP

D) 38 ATP

8. Uncouplers such as Dinitrophenol inhibit ATP synthesis but allow the electron transport chain to run at high speed. This occurs because Dinitrophenol:

A) Binds directly to Complex IV, preventing oxygen reduction.

B) Displaces the required Pi substrate from ATP synthase.

C) Increases the permeability of the inner mitochondrial membrane to protons.

D) Blocks the electron transfer from Complex I to ubiquinone.

9. The proton motive force across the inner mitochondrial membrane is composed of two components: the chemical potential and the electrical potential. Which of the following scenarios maximises the contribution of electrical potential?

A) High external pH and high internal pH.

B) High internal concentration of negatively charged ions.

C) High membrane permeability to H⁺

D) Low internal pH and high internal negative charge.

10. Malonate is a competitive inhibitor of Succinate Dehydrogenase (Complex II). What is the consequence of adding Malonate to mitochondria respiring on succinate?

A) Increased FADH₂ oxidation and increased O₂

B) Accumulation of fumarate and decreased ubiquinol reduction.

C) Accumulation of succinate and decreased O₂

D) Inhibition of the Pyruvate Dehydrogenase Complex.

11. The F₁ component of the ATP synthase complex is located in the mitochondrial matrix. Its primary function is to:

A) Translocate protons across the membrane.

B) Provide the channel for Pi entry.

C) Catalyse the synthesis of ATP from ADP and Pi.

D) Stabilise the F₀ proton channel structure.

12. Cytochrome c, a small protein of the electron transport chain, moves freely in the intermembrane space. What is its role?

A) Transfers electrons from Complex I to Complex III.

B) Transfers electrons from Complex II to Complex IV.

C) Transfers electrons from Complex III to Complex IV.

D) Acts as the final electron acceptor in the chain.

13. Cyanide is a potent inhibitor of cellular respiration because it binds to and inhibits:

A) Complex I

B) Complex II

C) Complex III

D) Complex IV

14. Which of the following is an inhibitor commonly used in respiratory studies that selectively blocks the activity of the Alternative Oxidase pathway?

A) Rotenone

B) Antimycin A

C) Cyanide

D) Salicylhydroxamic acid

15. The physiological advantage conferred by the Alternative Oxidase pathway, particularly under stress conditions such as high light intensity, is primarily related to:

A) Maximising ATP yield per carbon oxidised.

B) Increased generation of reactive oxygen species to trigger defence responses.

C) Dissipating excess reducing power and minimising reactive oxygen species production by keeping the ubiquinone pool oxidised.

D) Coupling the electron transport directly to the oxidation of nitrite to nitrate.

16. The primary electron donor to the Alternative Oxidase is:

A) Reduced ubiquinone

B) Cytochrome c

C) NADH in the matrix

D) Complex IV

17. Photorespiration is initiated by the oxygenase activity of RuBisCO, leading to the formation of 2-phosphoglycolate. This 2-phosphoglycolate must be detoxified and recycled via a pathway spanning three organelles. These organelles, in order of initial carbon entry (from 2-phosphoglycolate conversion) are:

A) Chloroplast → Mitochondrion → Peroxisome

B) Peroxisome → Mitochondrion → Chloroplast

C) Chloroplast → Peroxisome → Mitochondrion

D) Peroxisome → Chloroplast → Mitochondrion

18. What is the key carbon-recovering reaction that occurs in the mitochondrion during the photorespiratory cycle?

A) Conversion of Glycine to Serine, with CO₂

B) Conversion of Glyoxylate to Glycine.

C) Conversion of Serine to Hydroxypyruvate.

D) Conversion of Glycerate to 3-Phosphoglycerate.

19. In addition to high temperature, reduced water availability (drought) can indirectly enhance photorespiration by:

A) Directly changing the pK_a of RuBisCO active sites.

B) Causing stomatal closure, which lowers the internal leaf CO₂ concentration relative to O₂.

C) Inhibiting the activity of mitochondrial NADH dehydrogenase, causing redox imbalance.

D) Activating the C4 pathway to compensate for carbon loss.

20. During the photorespiratory cycle, the conversion of Glycolate to Glyoxylate is catalysed by Glycolate Oxidase. This reaction is unique because it directly consumes which molecule?

A) NADH

B) O₂

C) ATP

D) CO₂

21. Which pair of intermediates is transported out of the mitochondrion and back into the peroxisome to continue the photorespiratory cycle?

A) Glycine and Serine

B) CO₂ and NH₃

C) Serine and NH₃

D) Serine and NADH

22. If the mitochondrial transporter responsible for exporting Serine back to the peroxisome were inhibited, which photorespiratory intermediate would be expected to accumulate rapidly inside the mitochondrion?

A) Glycolate

B) Glycine

C) 3-Phosphoglycerate

D) Glycerate

23. Which shuttle system is used in plants to transfer electrons from cytoplasmic NADH to the mitochondria?

A) Malate-Aspartate Shuttle

B) Glycerol-3-Phosphate Shuttle

C) Both Shuttle system depending upon the energy requirement

D) None of the above

24. Which components of the Classical Electron Transport Chain in plant mitochondria are responsible for establishing the proton gradient?

I – Complex I (NADH Dehydrogenase)

II – Complex II (Succinate Dehydrogenase)

III – Complex III (bc1 complex)

IV – Complex IV (Cytochrome c Oxidase)

A) I and II Only

B) I, III, and IV Only

C) II, III, and IV Only

D) I, II, III, and IV

25. Assertion (A): High ambient temperatures significantly increase the rate of photorespiration in C3 plants.

Reason (R): High temperatures decrease the ratio of RuBisCO oxygenase activity relative to its carboxylase activity, favouring CO₂ fixation.

A) Both (A) and (R) are true, and (R) is the correct explanation for (A).

B) Both (A) and (R) are true, but (R) is NOT the correct explanation for (A).

C) (A) is true, but (R) is false.

D) (A) is false, but (R) is true.

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References

1. Hopkins, W. G., & Hüner, N. P. A. (2008). Introduction to Plant Physiology, John Wiley & Sons. 4th Edition.
2. Pandey, S. N., & Sinha, B. K. (2015). Plant Physiology, Vikas Publishing House. 4th Edition.
3. Jain, V. K. (2018). Fundamentals of Plant Physiology, S. Chand Publishing. 20th Edition.
4. Verma, Mohit & Verma, S. K. (2022). A Textbook of Plant Physiology, Biochemistry and Biotechnology, S. Chand Publishing. 13th Edition.
5. Lincoln Taiz, Ian Max Møller, Angus Murphy, and Eduardo Zeiger (2022). Plant Physiology and Development, Oxford University Press, 7^th Edition.
6. Nelson, David L. & Cox, Michael M. (2021). Lehninger Principles of Biochemistry, W. H. Freeman, 8^th Edition.

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