Plant Physiology – Photosynthesis: Important MCQs

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

Syllabus Outline

1. Mechanism of light reaction – Cyclic and Non-cyclic
2. Light-harvesting complexes
3. Types of photopigments
4. Mechanisms of electron transport
5. Photoprotective mechanisms
6. Mechanism of dark reaction – C3, C2, C4 and CAM
7. CO₂ fixation-C₃, C₄ and CAM pathways.
8. ATP/NADH utilisation to fix CO₂
9. Photosynthesis experiments (e.g. Thomas Engelmann’s experiment, Julius von Sachs’ experiment)
10. Effect of environmental factors on photosynthesis (e.g. Temperature, Concentration of CO₂, O₂ and pH)

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

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1. In green plants, photosynthetic pigments are primarily located on the:

A) Thylakoid membrane

B) Stroma

C) Stromal lamellae

D) Thylakoid lumen

2. Which of the following chlorophyll molecules contains a tetrapyrrole ring structure coordinated with a central magnesium ion?

A) Chlorophyll a

B) Chlorophyll b

C) Chlorophyll c

D) All of the above

3. Which group of plant pigments plays a key role in photoprotection by quenching excess energy and reactive oxygen species generated under high light intensity?

A) Chlorophylls

B) Carotenoids

C) Phycobilins

D) Porphyrins

4. Which electron transport pathway in the light reactions of photosynthesis results in the highest net ATP yield per photon absorbed?

A) Cyclic photophosphorylation

B) Non-cyclic photophosphorylation

C) Both generate equal ATP, but non-cyclic requires additional photons due to PSII involvement

D) No ATP is synthesised during light reactions

5. Which of the following enzymes is the most abundant biological catalyst on Earth?

A) Ribulose-1,5-bisphosphate carboxylase/oxygenase

B) Carbonic anhydrase

C) Lysozyme

D) DNA polymerase

6. How many molecules of ATP are required to fix 6 molecules of CO₂ during the Calvin cycle of C₄ plants?

A) 18

B) 12

C) 30

D) 6

7. In Crassulacean Acid Metabolism (CAM) plants, malate produced at night is temporarily stored in which cellular compartment?

A) Vacuole

B) Peroxisome

C) Cytosol

D) Chloroplast

8. In C₄ photosynthesis, the first stable product formed in the mesophyll cells is:

A) 3-Phosphoglycerate

B) Oxaloacetate

C) Malate

D) Phosphoenolpyruvate

9. Cyclic photophosphorylation is essential for maintaining the ATP/NADPH balance required by the Calvin cycle in C₃ plants. Which statement correctly distinguishes cyclic from non-cyclic photophosphorylation?

A) Cyclic photophosphorylation requires water photolysis and produces both ATP and NADPH.

B) Cyclic photophosphorylation involves only Photosystem I (P700) and produces ATP without net NADP⁺ reduction or O₂ evolution.

C) Non-cyclic photophosphorylation utilises the Cyt b₆f complex, whereas cyclic flow bypasses it.

D) Non-cyclic photophosphorylation yields a higher ATP: NADPH ratio than cyclic flow.

10. Nitrite reduction to ammonium in plants is catalysed by nitrite reductase located in the:

A) Mitochondria

B) Peroxisomes

C) Chloroplasts

D) Vacuole

11. In photorespiration (C₂ cycle), fixed carbon is released as CO₂ exclusively in the:

A) Chloroplast

B) Peroxisome

C) Mitochondrion

D) Cytoplasm

12. Phosphoenolpyruvate carboxylase (PEPCase) in C₄ and CAM plants has a major biochemical advantage over Rubisco because:

A) It has a much higher affinity for CO₂ and lacks oxygenase activity.

B) It is inhibited by O₂, whereas Rubisco is not.

C) It is localised in bundle sheath cells.

D) It has both carboxylase and oxygenase activity, which helps in photorespiration in case of a low ratio of CO₂ and O₂

13. In linear electron flow, the immediate electron donor to oxidised P700⁺ of PSI is:

A) Plastoquinone

B) Plastocyanin

C) Ferredoxin

D) Pheophytin

14. Photosynthetic electron transport is thermodynamically uphill (ΔG > 0) and driven by:

A) ATP consumption by NADP⁺ reductase

B) Proton gradient formation and ATP synthesis

C) Light-induced excitation of P680 and P700 to P680* and P700*

D) Reduction of water and oxidation of NADP⁺

15. Pyruvate orthophosphate dikinase, required for phosphoenolpyruvate regeneration in C₄ mesophyll cells, is localised in the:

A) Bundle sheath mitochondria

B) Bundle sheath chloroplasts

C) Mesophyll mitochondria

D) Mesophyll cytoplasm

16. During the nighttime phase of CAM metabolism:

A) Stomata closed, Rubisco active

B) Stomata open, PEPCase active, malate stored in the vacuole

C) Stomata open, Rubisco active, starch synthesised

D) Stomata closed, PEPCase inactive

17. Non-photochemical quenching, mediated by zeaxanthin, primarily functions to:

A) Increase NADP⁺ reduction

B) Dissipate excess excitation energy as heat

C) Promote light-harvesting complex transfer to PSI

D) Stabilise P680

18. Kranz anatomy in C₄ plants involves:

A) Uniform enzyme distribution

B) Daytime stomatal closure

C) Rubisco localised in the mesophyll

D) Two cell types: mesophyll and bundle sheath

19. Elevated oxygen concentration in C₃ plants reduces net CO₂ assimilation because:

A) O₂ acts as PSI electron acceptor

B) O₂ competes with CO₂ for Rubisco

C) O₂ induces stomatal closure

D) O₂ inhibits ATP synthase

20. The splitting of water during photosynthesis occurs at which site?

A) Photosystem I

B) Photosystem II

C) Cytochrome b6f complex

D) Stroma

21. In C₄ photosynthesis, the first stable product formed in the mesophyll cells is:

A) 3-Phosphoglycerate (3-PGA)

B) Malate

C) Oxaloacetate (OAA)

D) Phosphoenolpyruvate (PEP)

22. Plastocyanin is a mobile electron carrier located in the thylakoid lumen. Its metal cofactor and oxidation states are:

A) Iron; Fe²⁺/Fe³⁺

B) Copper; Cu⁺/Cu²⁺

C) Manganese; Mn²⁺/Mn³⁺

D) Zinc; Zn⁺/Zn²⁺

23. Which of the following statements correctly differentiates chlorin from porphyrin?

I – Chlorin is more chemically stable than porphyrin.

II – Chlorin has one reduced double bond in one of its pyrrole rings.

III – Chlorin exhibits longer wavelength absorption than porphyrin.

IV – Chlorin contains three pyrrole rings and one pyrroline ring, whereas porphyrin has four pyrrole rings.

A) I only

B) I and II

C) I and III

D) II, III and IV

24. In the Z-scheme of photosynthesis (non-cyclic), electron carriers are arranged according to their midpoint redox potentials. Charge separation in PSII forms P680⁺ (strong oxidant) and Pheophytin⁻ (strong reductant). Which sequence correctly orders the components from most negative (strongest reductant) to most positive (strongest oxidant)?

(i) Pheophytin/Pheophytin⁻

(ii) P680/P680*

(iii) Mn₄CaO₅/Mn₄CaO₅⁺

(iv) Plastoquinone/Plastoquinol (PQ/PQH₂)

A) (ii) → (i) → (iv) → (iii)

B) (i) → (ii) → (iv) → (iii)

C) (i) → (iv) → (iii) → (ii)

D) (ii) → (i) → (iii) → (iv)

25. Assertion (A): Photorespiration is energetically wasteful because it consumes ATP and NADPH while releasing fixed CO₂.

Reason (R): The C₂ cycle detoxifies 2-phosphoglycolate and recovers 3-PGA.

A) Both true, and R explains A

B) Both true, but R is not the correct explanation of A

C) A true, R false

D) A false, R 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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