Part 1 — Means of Transport in Plants & Plant–Water Relations (Q1–25)

Part 1 — Means of Transport in Plants & Plant–Water Relations (Q1–25)

Q1.

Which of the following correctly defines diffusion in plants?
A. Bulk movement of water driven by pressure differences
B. Movement of molecules from regions of lower concentration to higher concentration using metabolic energy
C. Passive movement of molecules from region of higher concentration to lower concentration down the concentration gradient ✅
D. Movement of water through living cell membranes via aquaporins only

Explanation:

• A. That is bulk flow, not diffusion.
• B. Movement from low → high using energy is active transport, not diffusion.
• C. (Correct) Diffusion is a passive process in which molecules move from higher to lower concentration until equilibrium is reached. No metabolic energy required.
• D. Movement of water via aquaporins is facilitated water movement (osmosis/water channels), not the general definition of diffusion.

Q2.

Osmosis is best described as:
A. Movement of solute molecules through a membrane to equilibrate concentrations
B. Diffusion of water across a selectively permeable membrane from higher water potential to lower water potential ✅
C. Bulk flow of water in xylem vessels due to transpiration
D. Active uptake of water by root cells using ATP

Explanation:

• A. That describes solute diffusion, not osmosis.
• B. (Correct) Osmosis is the passive movement of water across a semipermeable membrane toward regions of lower water potential (usually higher solute concentration).
• C. Bulk flow (transpiration stream) is different mechanism.
• D. Water uptake is passive (driven by gradients); roots may use energy indirectly to maintain gradients but osmosis itself is passive.

Q3.

Which term represents the component of water potential due to dissolved solutes?
A. Pressure potential (ψp)
B. Solute potential (ψs) ✅
C. Matric potential (ψm)
D. Gravitational potential (ψg)

Explanation:

• A. Pressure potential is due to physical pressure (turgor) and is usually positive in turgid cells.
• B. (Correct) Solute (osmotic) potential is the component of water potential attributable to dissolved solutes; it is negative (reduces total water potential).
• C. Matric potential refers to adsorption of water to surfaces, important in soils.
• D. Gravitational potential relates to height; not the solute component.

Q4.

The water potential (ψ) of pure water at standard atmospheric pressure is:
A. Positive
B. Zero ✅
C. Negative
D. Infinite

Explanation:

• A. Not true for pure water at standard conditions.
• B. (Correct) By convention, the water potential of pure water at standard conditions (reference) is zero. Solutes reduce ψ (make it negative), pressure can increase it (positive).
• C. Negative water potentials occur when solutes are present or under tension.
• D. Not meaningful in this context.

Q5.

If a plant cell is placed in a hypertonic solution, what is likely to happen?
A. Cell becomes turgid due to water influx
B. Plasmolysis occurs as water exits the cell ✅
C. No net movement of water occurs
D. Cell bursts due to increased pressure

Explanation:

• A. Turgidity occurs when placed in hypotonic solution (water enters).
• B. (Correct) In hypertonic (higher external solute) solution, water leaves the cell → protoplast shrinks away from cell wall (plasmolysis).
• C. Net movement occurs because water potential differs.
• D. Burst (lysis) happens in hypotonic if cell wall absent or very weak (animal cells), not in hypertonic.

Q6.

Which pathway for water movement in roots travels through cell walls and intercellular spaces without crossing plasma membranes?
A. Symplast pathway
B. Apoplast pathway ✅
C. Transmembrane pathway
D. Endodermal pathway

Explanation:

• A. Symplast involves movement through cytoplasm via plasmodesmata.
• B. (Correct) Apoplast refers to the cell wall continuum and intercellular spaces — water moves passively without crossing membranes until blocked (e.g., by Casparian strip).
• C. Transmembrane involves repeated crossing of membranes (in and out of cells).
• D. Endodermal pathway is not a standard term for the three; endodermis enforces selective uptake via Casparian strip.

Q7.

The Casparian strip in roots functions to:
A. Facilitate apoplastic flow into xylem
B. Block apoplastic movement forcing symplastic entry for selective uptake ✅
C. Produce root hairs for increased absorption
D. Store minerals in cortex

Explanation:

• A. It actually blocks apoplastic flow, not facilitate it.
• B. (Correct) The Casparian strip (suberin band in endodermal cell walls) prevents apoplastic passage into the stele, forcing water/solutes to cross plasma membranes — enables selective uptake.
• C. Root hairs are epidermal extensions; Casparian strip is in endodermis.
• D. It is a structural barrier, not a storage tissue.

Q8.

Root pressure is best explained by:
A. Transpiration pull from leaves
B. Active accumulation of solutes in xylem followed by osmotic water uptake causing positive pressure ✅
C. Cohesion and adhesion of water molecules in xylem under tension
D. Capillarity in narrow vessels only

Explanation:

• A. Transpiration pull creates negative pressure (tension), not root pressure.
• B. (Correct) Root pressure arises when roots actively load ions into xylem, lowering xylem water potential; water enters osmotically generating positive hydrostatic pressure that can cause guttation.
• C. Cohesion-tension explains ascent via negative pressure from transpiration, not root pressure.
• D. Capillarity contributes little in tall plants and does not explain positive root pressure.

Q9.

Which statement about water potential components in plant cells is TRUE?
A. Solute potential (ψs) is always positive
B. Pressure potential (ψp) can be positive, negative, or zero ✅
C. Water potential (ψ) is the sum ψs − ψp
D. Pure water has a negative ψ

Explanation:

• A. Solute potential is typically negative (dilutes water potential).
• B. (Correct) Pressure potential varies: positive in turgid cells, zero in flaccid cells, negative under tension (xylem).
• C. Correct formula is ψ = ψs + ψp + other terms (e.g., ψm, ψg), not subtraction as stated.
• D. Pure water at atmospheric conditions is defined as ψ = 0.

Q10.

Imbibition is critical in which process?
A. Diffusion of minerals in soil
B. Absorption of water by dry seeds and swelling ✅
C. Phloem translocation of sucrose
D. Root hair formation

Explanation:

• A. Imbibition is water absorption by hydrophilic substances, not diffusion in soil.
• B. (Correct) Dry seeds/wood swell by imbibition — water is adsorbed to matrix, initiating germination.
• C. Phloem translocation is via bulk flow, not imbibition.
• D. Root hair formation is developmental.

Q11.

Which of the following describes the symplastic route of transport?
A. Movement through cell walls only
B. Movement through cytoplasm interconnected by plasmodesmata ✅
C. Movement via the apoplast until the endodermis
D. Bulk flow through tracheids and vessels

Explanation:

• A. That’s apoplast.
• B. (Correct) Symplast: cytoplasm of adjacent cells connected via plasmodesmata allowing direct cytoplasmic continuity for solute/water movement.
• C. Apoplast path description.
• D. Bulk flow in xylem is different from cellular symplast transport.

Q12.

Which event occurs during plasmolysis?
A. Cell wall collapses completely
B. Plasma membrane pulls away from cell wall as water leaves the cell ✅
C. Protoplast expands and bursts
D. Cell becomes more turgid

Explanation:

• A. Cell wall is rigid and remains; it does not collapse.
• B. (Correct) Loss of water from protoplast in hypertonic medium causes it to shrink away from cell wall (plasmolysis).
• C. Expansion and bursting (lysis) would occur in extreme hypotonic cases in cells without strong walls (animals).
• D. Turgidity is the opposite condition.

Q13.

Which of the following factors will increase transpiration rate?
A. High humidity
B. Low temperature
C. High wind speed ✅
D. Closed stomata

Explanation:

• A. High humidity reduces transpiration by decreasing vapor pressure gradient.
• B. Low temperature reduces vaporization and evaporative demand.
• C. (Correct) Wind removes humid boundary layer, increasing vapor gradient and transpiration.
• D. Closed stomata reduce transpiration.

Q14.

Transpiration pull is generated primarily by:
A. Root pressure
B. Osmotic uptake in guard cells
C. Evaporation of water from mesophyll cell walls creating tension in xylem (cohesion-tension) ✅
D. Active pumping of water by xylem cells

Explanation:

• A. Root pressure contributes in some cases but not main mechanism in tall plants.
• B. Guard cell osmotic changes regulate stomata, not the primary driver of long-distance water movement.
• C. (Correct) Evaporation at leaf surfaces generates negative pressure transmitted through the water column due to cohesion/adhesion — the cohesion-tension theory.
• D. Xylem vessels are dead at maturity and do not actively pump water.

Q15.

Which of the following correctly ranks water movement driving forces in a transpiring plant?
A. Root → stem → leaf (increasing water potential)
B. Soil (higher ψ) → root → xylem (decreasing ψ) → air (lowest ψ) ✅
C. Air → leaf → root (increasing humidity)
D. Leaf → soil → atmosphere

Explanation:

• A. Water moves from higher water potential to lower; plant direction implied is wrong.
• B. (Correct) Water moves from soil (relatively high ψ) → roots → xylem → leaves → atmosphere (lowest ψ due to low water vapor) — a continuum of decreasing ψ.
• C/D. Incorrect directions relative to water potential gradients.

Q16.

Which description best fits a potometer?
A. Measures transpiration rate by recording water uptake as an estimate ✅
B. Measures root pressure directly by exudation volume
C. Measures sap sugar concentration in phloem
D. Measures photosynthetic oxygen evolution only

Explanation:

• A. (Correct) A potometer measures water uptake by a cut shoot; under steady state, water uptake ≈ transpiration rate (assumption: small storage changes).
• B. Root pressure measured by exudation or pressure probe, not standard potometer.
• C/D. Not potometer’s functions.

Q17.

Which of the following is NOT a function of transpiration?
A. Cooling of leaves via evaporative cooling
B. Uptake and transport of mineral ions ✅
C. Creation of transpiration pull for ascent of sap
D. Exchange of gases through stomata

Explanation:

• A. Evaporative cooling is an important function.
• B. (Correct — uptake and transport of minerals is facilitated by mass flow in xylem associated with transpiration, but the function of transpiration itself is not to transport minerals; rather transpiration is mainly water loss. The phrasing asks “NOT a function of transpiration” — mineral uptake is an indirect effect, not a primary function of transpiration.)
• C. True — transpiration pull helps ascent of sap.
• D. Transpiration and stomatal opening are linked to gas exchange.

(Note: Many texts treat mineral transport as facilitated by transpiration-driven mass flow — but strictly speaking, transpiration’s primary functional explanations are cooling and water transport; this question tests that distinction.)

Q18.

Which of these is a true statement about xylem vessels?
A. They are living cells that actively pump water
B. They conduct mainly sugars from leaves to roots
C. They form a continuous column enabling cohesion-tension driven flow ✅
D. They narrow at perforation plates to slow water movement

Explanation:

• A. Xylem vessel elements are dead at maturity and do not actively pump.
• B. Phloem conducts sugars; xylem conducts water and minerals.
• C. (Correct) Xylem vessels/tracheids form continuous water columns; cohesion and adhesion allow tension transmission (transpiration pull).
• D. Perforation plates have openings to allow passage, not to slow flow necessarily.

Q19.

Which statement about apoplastic bypass at the endodermis is correct?
A. Casparian strip allows free apoplastic flow into xylem
B. Ions traveling via apoplast must cross the plasma membrane at endodermis due to Casparian strip ✅
C. Endodermis enhances unregulated mineral uptake
D. Apoplastic flow continues unimpeded to the stele

Explanation:

• A. Casparian strip prevents apoplastic passage, not allow it.
• B. (Correct) Casparian strip blocks apoplastic route; solutes must enter endodermal cell cytoplasm (symplast) to be selectively transported into stele.
• C. Endodermis enables selective uptake, not unregulated.
• D. Incorrect — apoplastic flow is blocked at endodermis.

Q20.

Which process explains water ascent to the top of very tall trees (over 100 m)?
A. Root pressure alone
B. Cohesion-tension mechanism (transpirational pull) ✅
C. Active pumping by vessel elements
D. Capillarity alone

Explanation:

• A. Root pressure is insufficient for very tall trees.
• B. (Correct) Cohesion of water molecules and tension generated by transpiration at leaves pull water up continuous xylem columns — the accepted mechanism for tall trees.
• C. Xylem elements are dead and cannot actively pump.
• D. Capillarity helps in small cases but cannot account for ascent in very tall trees.

Q21.

Which of the following reduces water loss by transpiration in xerophytic plants?
A. Thin cuticle with abundant stomata
B. Thick cuticle, sunken stomata, reduced leaf area ✅
C. Large, broad leaves exposed to wind
D. High stomatal density on upper leaf surface

Explanation:

• A. Increases water loss; xerophytes do the opposite.
• B. (Correct) Xerophytes adapt by thick cuticle, sunken stomata, rolled leaves, reduced leaf area, hairs — all reduce transpiration.
• C/D. Increase transpiration, not adaptations for drought.

Q22.

Which force is primarily responsible for short distance movement of water and solutes between cells?
A. Bulk flow in xylem
B. Diffusion and osmosis across membranes ✅
C. Mass flow in phloem only
D. Capillary rise in vessels

Explanation:

• A. Bulk flow acts over long distances (xylem).
• B. (Correct) Short-distance (cell-to-cell) movement occurs via diffusion of solutes and osmosis of water across membranes and through plasmodesmata.
• C. Mass flow in phloem is long-distance and pressure-driven.
• D. Capillarity plays limited role in cell-to-cell transport.

Q23.

Which is true about water potential gradient along a transpiring plant?
A. Highest (least negative) at air, lowest in soil
B. Highest in air, lowest in leaves
C. Highest in soil, lowest in air ✅
D. Uniform from soil to air

Explanation:

• A/B. Incorrect ordering.
• C. (Correct) Water potential is relatively high (less negative) in moist soil, lower in roots/stem/leaves, and most negative in the atmosphere (dry air), driving upward movement.
• D. Not true; gradient is crucial for flow.

Q24.

Which parameter is directly altered when guard cells accumulate K⁺ and malate?
A. Solute potential (ψs) of guard cells decreases making it more negative ✅
B. Pressure potential increases in adjacent mesophyll cells
C. Water potential of atmosphere increases
D. Casparian strip permeability increases

Explanation:

• A. (Correct) Accumulation of K⁺ and malate lowers solute potential (more negative) inside guard cells, causing water to enter by osmosis → guard cell turgidity → stomatal opening.
• B. Mesophyll pressure generally not directly changed by guard cell ion flux.
• C. Atmosphere water potential unrelated to guard cell ions.
• D. Casparian strip unaffected by guard cell ion flux.

Q25.

Which experimental observation would best support the cohesion-tension theory?
A. Guttation observed at leaf margins during night
B. Cutting a transpiring stem under water produces a continuous column of water in xylem and often leads to air entry when cut above water ✅
C. Roots exude sap only when actively pumping minerals
D. Transpiration ceases if leaves are removed

Explanation:

• A. Guttation is due to root pressure and does not alone confirm cohesion-tension (it occurs when transpiration is low).
• B. (Correct) If a transpiring stem is cut under water, a continuous water column is seen; cutting above water often causes air to be sucked in due to tension — this is consistent with xylem sap being under negative pressure (tension) as predicted by cohesion-tension theory.
• C. Root exudation shows root pressure, not cohesion-tension.
• D. Removing leaves reduces transpiration and thus cohesion-tension, but the specific evidence in (B) more directly supports the theory.

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