COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: PLANT PHYSIOLOGY – Solute Transport and Photoassimilate Translocation.
Syllabus Outline
- Uptake, transport and translocation of water, ions, solutes and macromolecules from soil, through cells, across membranes, through xylem and phloem
- Pathways for water uptake by roots – Symplast and transmembrane pathways, Apoplast pathways.
- Role of diffusion, osmosis, osmotic pressure, and water potential in the transport of water and solute in plants.
- Transpiration and its role in plant physiology.
- Effects of environmental parameters on the transport of water and solute.
- Mechanisms of loading and unloading of photo assimilates.
This quiz contains 25 concept-based, most frequently asked MCQs on “PLANT PHYSIOLOGY – Solute Transport and Photoassimilate Translocation”. Each question has a single correct/most appropriate answer.
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1. The bulk flow of water from root to leaf is primarily driven by negative pressure potential (Ψp) in the leaves. Which property of water is most critical for maintaining the continuity of the water column under such tension?
A) Adhesion to hydrophobic lignin walls in xylem
B) High specific heat capacity providing thermal stability
C) Cohesion due to hydrogen bonding between water molecules
D) Capillarity resulting from small vessel diameter
2. Which scenario correctly identifies typical values for the water potential components (Ψs, Ψp) in a healthy turgid parenchyma cell and the surrounding soil?
A) Cell: Ψs negative, Ψp negative; Soil: Ψs zero, Ψp positive
B) Cell: Ψs positive, Ψp positive; Soil: Ψs negative, Ψp near zero
C) Cell: Ψs negative, Ψp positive; Soil: Ψs near zero, Ψp near zero
D) Cell: Ψs zero, Ψp positive; Soil: Ψs negative, Ψp negative
3. In deciduous trees, latewood vessels are narrower than earlywood vessels. What hydraulic advantage do these narrower latewood vessels confer?
A) Faster water flow under low light at night under drought
B) Greater resistance to wall rupture due to low tension at night under drought
C) Reduced friction for more efficient water transport under drought
D) Increased mechanical strength and resistance to cavitation under drought
4. During active transpiration, which sequence correctly represents the relative water potentials from highest to lowest?
A) Stem xylem → Leaf mesophyll → Root cortex → Soil
B) Soil → Root cortex → Stem xylem → Leaf mesophyll
C) Leaf mesophyll → Stem xylem → Root cortex → Soil
D) Root cortex → Leaf mesophyll → Soil → Stem xylem
5. Which conditions most strongly promote cavitation?
A) High root pressure and low humidity
B) Low xylem tension and freezing
C) High xylem tension and freeze–thaw cycles
D) High stomatal conductance and moist soil
6. Which statement correctly describes the role of Plasma Membrane Intrinsic Proteins (PIPs) in roots?
A) Move water from the xylem into the mesophyll against a gradient
B) Function primarily in vacuoles to regulate long-term turgor
C) Generate proton gradients for water uptake during osmotic stress
D) Enhance hydraulic conductivity during osmotic stress
7. Tonoplast Intrinsic Proteins primarily:
A) Enable rapid osmotic water release from the vacuole to the cytoplasm for turgor adjustment
B) Retrieve solutes from the apoplast for turgor adjustment
C) Drive xylem loading against transpiration flow
D) Regulate lignin deposition in cell walls to turgor adjustment
8. Under which condition is root pressure most significant in plant water transport?
A) During midday when transpiration is maximal
B) During the night or early morning when transpiration is minimal
C) Under conditions of high vapour pressure deficit
D) During drought, when xylem tension is highest
9. Which of the following best describes the physiological role of the Casparian strip in endodermal cells?
A) Facilitates passive apoplastic water movement
B) Allows selective ion uptake into the stele by blocking apoplastic flow
C) Stores ions for secondary active transport
D) Acts as a hydrophobic barrier, reducing water loss from the root cortex
10. Water moving from soil to xylem can take different routes. Which sequence correctly describes the transition from apoplastic to symplastic flow?
A) Epidermis → Cortex → Endodermis → Pericycle
B) Cortex → Endodermis → Xylem → Pericycle
C) Root hair → Endodermis → Cortex → Xylem
D) Root hair → Cortex → Endodermis → Pericycle → Xylem
11. How do arbuscular mycorrhizal fungi improve plant water status under drought stress?
A) By reducing root hydraulic conductivity
B) By increasing surface area for water uptake and improving osmotic adjustment
C) By blocking aquaporins to minimise loss
D) By replacing transpiration-driven flow with diffusion
12. Which statement best describes the role of potassium ions in stomatal movement?
A) Accumulation of K+ in guard cells lowers the osmotic potential, opening of stomata.
B) Loss of K+ from guard cells increases turgor pressure, closing stomata.
C) K+ efflux promotes water entry into guard cells, opening stomata.
D) K+ accumulation increases cytosolic pH, closing stomata.
13. During drought stress, abscisic acid causes stomatal closure primarily by:
A) Activating proton pumps in guard cell membranes
B) Stimulating efflux of K+ and Cl− ions from guard cells
C) Promoting osmotic water uptake in guard cells
D) Increasing ATP hydrolysis in chloroplasts
14. In the pressure flow hypothesis, what drives the movement of sucrose from source to sink?
A) Concentration gradient of CO2
B) Hydrostatic pressure gradient
C) Active transport of sucrose into sieve elements at the sink
D) Transpiration pulls from the leaf
15. Apoplastic phloem loading involves:
A) Passive diffusion of sucrose into companion cells
B) Use of Ca2+ channels for sugar transport
C) Direct plasmodesmatal connection between mesophyll and phloem
D) Proton-coupled active uptake of sucrose into sieve element–companion cell complexes
16. Which of the following best describes the function of companion cells in the phloem?
A) Provide mechanical support to sieve elements
B) Regulate metabolic loading and unloading of solutes in sieve elements
C) Transport water into the xylem
D) Store starch for long-term energy
17. The rate of phloem transport in plants is typically in the range of:
A) 0.01 to 0.1 cm/h
B) 0.5 to 1 cm/h
C) 10 to 100 cm/h
D) 100 to 1000 cm/h
18. Which of the following best defines a “source–sink transition”?
A) Conversion of a young leaf from importer to exporter of photosynthates as it matures
B) Switching of a storage organ from starch synthesis to degradation
C) Seasonal change from vegetative to reproductive growth
D) Directional reversal of xylem water flow
19. What is the primary function of callose in sieve tube elements?
A) Strengthens the cell wall during secondary growth
B) Temporarily seals sieve plates to regulate flow during injury or stress
C) Enhances the loading of sucrose under high pressure
D) Regulates hormonal signalling to regulate flow during injury or stress
20. In sink tissues such as roots or tubers, phloem unloading occurs mainly through:
A) Active apoplastic transport involving proton symporters
B) Passive symplastic diffusion through plasmodesmata
C) Vesicular exocytosis
D) Facilitated diffusion via aquaporins
21. Which of the following best describes the physiological role of reactive oxygen species under drought conditions?
A) Act as toxic by-products only
B) Induce antioxidant defence mechanisms
C) Inhibit antioxidant enzymes to promote abscisic acid synthesis
D) Helps to close stomata through osmotic dehydration
22. Plants utilising the apoplastic loading mechanism often possess specialised companion cells characterised by numerous wall ingrowths that dramatically increase the plasma membrane surface area. These cells are known as:
A) Intermediary cells
B) Parenchyma cells
C) Transfer cells
D) Bundle sheath cells
23. Which of the following components are essential prerequisites for successful phloem loading via the polymer trapping mechanism?
I – Highly branched and numerous plasmodesmata connecting mesophyll, bundle sheath, and companion cells.
II – The presence of specialised enzymes such as raffinose synthetase localised in the companion cells.
III – Transfer cells equipped with wall ingrowths to facilitate apoplastic release.
A) I only
B) I and II only
C) II and III only
D) I, II, and III
24. In a potato tissue experiment, cores were immersed in sucrose solutions of increasing molarity (0.1–0.6 M). The equilibrium point (no net change in mass) is reached when:
I – The pressure potential (Ψp) of the cell equals the water potential of the external solution.
II – The solute potential (Ψs) of the external solution equals the solute potential of the cell.
III – The water potential of the tissue equals that of the external solution.
A) I only
B) I and II only
C) III only
D) I and III only
25. Assertion (A): The ascent of water through xylem is a passive process driven by pressure gradients caused by transpiration.
Reason (R): The rate of water ascent is ultimately regulated by stomatal aperture, which determines leaf tension.
A) Both A and R are true, and R correctly explains A.
B) Both A and R are true, but R does not explain A.
C) A is true, but R is false.
D) A is false, but R is true.
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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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