60 free MCQs on Transport in Plants with worked answers and explanations. Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.
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Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Q1.
Water is transported in plants through:
Answer: B. Xylem
Why: Xylem transports water and minerals from roots to leaves (upward). Phloem transports sugars (both up and down).
Q2.
The process by which water evaporates from leaf surfaces is:
Answer: A. Transpiration
Why: Transpiration is the evaporation of water from plant surfaces, mainly through stomata on leaves.
Q3.
Stomata open when guard cells:
Answer: B. Are turgid (full of water)
Why: Guard cells become turgid (fill with water) when K+ enters them. Turgor pressure causes stomata to open.
Q4.
Sugars produced in photosynthesis are transported in:
Answer: B. Phloem as sucrose
Why: Sucrose is the main form in which sugars are transported in the phloem from source (leaves) to sink (roots, fruits).
Q5.
The movement of water from a region of higher water potential to lower water potential across a semipermeable membrane is called:
Answer: A. Osmosis
Why: Osmosis is the diffusion of water across a selectively permeable membrane, from a region of higher water potential to one of lower water potential.
Q6.
Root hairs increase the efficiency of water absorption mainly by:
Answer: D. Increasing the surface area in contact with soil moisture
Why: Root hairs are thin, elongated extensions of epidermal cells that greatly increase the surface area available for absorbing water and minerals from soil.
Q7.
The loss of excess water in the form of liquid droplets from leaf margins is called:
Answer: C. Guttation
Why: Guttation is the exudation of water droplets, mostly seen on grass blades, when root pressure forces sap out through small pores called hydathodes.
Q8.
Most of the water absorbed by plant roots is ultimately lost through the process of:
Answer: B. Transpiration
Why: Transpiration, mainly through stomata on leaves, accounts for the loss of nearly 90% of the water absorbed by roots.
Q9.
Minerals absorbed by roots are transported to the rest of the plant mainly through the:
Answer: C. Xylem
Why: Xylem tissue conducts both water and dissolved minerals upward from roots to other parts of the plant.
Q10.
A plant cell placed in a hypertonic solution loses water and its cytoplasm shrinks away from the cell wall, a condition called:
Answer: D. Plasmolysis
Why: Plasmolysis occurs when a plant cell loses water by osmosis in a hypertonic external solution, causing the cell membrane and cytoplasm to shrink away from the rigid cell wall.
Q11.
The plant tissue that transports water and minerals is the:
Answer: A. xylem
Why: Xylem conducts water and dissolved minerals from roots to leaves.
Q12.
The plant tissue that transports food (sugars) is the:
Answer: A. phloem
Why: Phloem carries the sugars made in leaves to the rest of the plant.
Q13.
Water is absorbed by a plant mainly through its:
Answer: A. root hairs
Why: Root hairs greatly increase the surface area for water absorption.
Q14.
The loss of water vapour from the aerial parts of a plant is called:
Answer: A. transpiration
Why: Transpiration is water loss, mostly through the stomata of leaves.
Q15.
Transpiration takes place mainly through the:
Answer: A. stomata
Why: Water vapour escapes chiefly through the stomata on the leaves.
Q16.
The movement of water across a membrane from a dilute to a concentrated solution is:
Answer: A. osmosis
Why: Osmosis is the diffusion of water across a semipermeable membrane.
Q17.
In the xylem, water moves in which overall direction?
Answer: A. roots to leaves
Why: Xylem carries water upward from the roots to the leaves.
Q18.
The transport of food through the phloem is called:
Answer: A. translocation
Why: Translocation is the movement of sugars in the phloem.
Q19.
The dead, tube-like conducting cells of the xylem are the:
Answer: A. vessels and tracheids
Why: Xylem vessels and tracheids are dead cells that conduct water.
Q20.
The living conducting cells of the phloem are the:
Answer: A. sieve tube elements
Why: Sieve tube elements are living cells that conduct food in the phloem.
Q21.
Pressure flow hypothesis explains:
Answer: B. Sugar transport in phloem (source to sink via turgor pressure gradient)
Why: Pressure flow (Munch hypothesis): sugars loaded at source increase osmotic pressure, water enters by osmosis, creating pressure that pushes sap through phloem to sink.
Q22.
Cohesion-tension theory explains water movement in xylem by:
Answer: C. Cohesion between water molecules and tension from transpiration pulling water up
Why: Cohesion-tension theory: water molecules stick together (cohesion) and to xylem walls (adhesion). Transpiration creates tension, pulling a continuous water column up.
Q23.
Companion cells in phloem are connected to sieve tube elements by:
Answer: B. Plasmodesmata (for loading and unloading sugars)
Why: Companion cells are metabolically active and support sieve tube elements via extensive plasmodesmata connections. They load and unload sugars.
Q24.
Which ion channel opening triggers stomatal opening?
Answer: B. H+ ATPase pump exports H+ (protons), creating electrochemical gradient for K+ influx
Why: Stomatal opening: light activates H+ ATPase in guard cells. H+ efflux hyperpolarizes membrane. K+ enters through channels. Water follows by osmosis, increasing turgor.
Q25.
Abscisic acid triggers which response in stomata during drought?
Answer: B. Stomatal closure to reduce water loss
Why: ABA (drought hormone): binds to receptors on guard cells, triggers Ca<sup>2+</sup> signaling, K+ and water efflux, turgor decreases, stomata close.
Q26.
The apoplast pathway of water movement in roots goes through:
Answer: B. Cell walls and intercellular spaces (no membranes crossed)
Why: Apoplast pathway: water moves through cell walls and intercellular spaces. Blocked at Casparian strip in endodermis, forcing water through symplast.
Q27.
Water potential of a solution becomes more negative as the:
Answer: C. Solute concentration in it increases
Why: Adding solutes lowers (makes more negative) the water potential of a solution, since solute particles reduce the free energy of water molecules available to do work.
Q28.
Transpiration pull, central to the ascent of sap, is generated by:
Answer: C. Evaporation from mesophyll cells creating tension that pulls water up
Why: As water evaporates from leaf mesophyll cells during transpiration, it creates a negative pressure (tension) that is transmitted down the continuous water column in the xylem, pulling water upward from the roots.
Q29.
The symplast pathway of water movement across the root differs from the apoplast pathway in that the symplast route:
Answer: A. Moves through cytoplasm of cells joined by plasmodesmata
Why: In the symplast pathway, water moves through the continuous cytoplasm of cells connected by plasmodesmata, crossing at least one membrane, unlike the apoplast pathway, which moves through cell walls and intercellular spaces.
Q30.
Translocation of organic solutes in the phloem can occur in both upward and downward directions because:
Answer: A. Flow direction depends on relative source and sink position, not fixed anatomy
Why: Unlike the unidirectional upward flow in xylem, phloem transport is bidirectional, moving sugars from whichever organ is currently a source to whichever organ is currently a sink, regardless of position.
Q31.
The upward movement of water in the xylem is aided by the pull created during:
Answer: A. transpiration
Why: Transpiration from the leaves creates a pull that draws water up the xylem.
Q32.
The theory that explains water rise in tall trees by cohesion and transpiration pull is the:
Answer: A. cohesion-tension theory
Why: The cohesion-tension theory accounts for the ascent of sap.
Q33.
Guard cells control the opening and closing of the:
Answer: A. stomata
Why: Changes in guard-cell turgor open or close the stomatal pore.
Q34.
The uptake of mineral ions into root cells against a concentration gradient needs:
Answer: A. active transport (energy)
Why: Moving ions against a gradient requires energy (active transport).
Q35.
The water potential of pure water is:
Answer: A. zero
Why: By convention, pure water has a water potential of zero.
Q36.
When a plant cell loses water and its membrane pulls away from the wall, it is said to be:
Answer: A. plasmolysed
Why: Loss of water causes plasmolysis, the shrinking of the protoplast.
Q37.
A plant cell that is fully swollen with water and firm is described as:
Answer: A. turgid
Why: A turgid cell is firm because it is full of water, pressing on its wall.
Q38.
The firmness a plant cell gains when filled with water by osmosis is called:
Answer: A. turgidity
Why: Turgidity gives non-woody plant parts their support and shape.
Q39.
Companion cells are closely associated with the ___ of the phloem:
Answer: A. sieve tubes
Why: Each sieve tube element is supported by an adjacent companion cell.
Q40.
Minerals are absorbed by plant roots mostly in the form of:
Answer: A. ions
Why: Minerals dissolve in soil water and are taken up as ions.
Q41.
Guard cell closure in response to ABA involves:
Answer: B. Ca<sup>2+</sup> as second messenger activating K+ efflux channels and anion channels, causing turgor loss
Why: ABA binds receptors (PYR/PYL), inactivating PP2C phosphatases. This activates SnRK2 kinases, increasing cytosolic Ca<sup>2+</sup>. Ca<sup>2+</sup> activates anion channels (Cl- efflux) and inhibits K+ influx, reducing turgor.
Q42.
The role of 14-3-3 proteins in stomatal regulation is:
Answer: B. Acting as signaling scaffolds that activate H+ ATPase when bound to its phosphorylated C-terminus, promoting stomatal opening
Why: 14-3-3 proteins bind phosphorylated H+ ATPase C-terminus (phosphorylated by protein kinase in response to light/fusicoccin), activating the pump. H+ efflux hyperpolarizes membrane, driving K+ influx and stomatal opening.
Q43.
The Casparian strip in root endodermis forces water through:
Answer: B. Symplast (through cell membrane), allowing selective mineral ion transport to xylem
Why: Casparian strip: suberin band in endodermis cell walls blocks apoplast pathway. Water and solutes MUST cross the endodermal plasma membrane (symplast), allowing selective ion transport control.
Q44.
In the cohesion-tension theory, the continuous water column in the xylem does not normally break under tension mainly because of:
Answer: D. Strong cohesive forces between water molecules from hydrogen bonding
Why: Water molecules are held together by hydrogen bonds (cohesion) and adhere to the hydrophilic xylem walls (adhesion), allowing the column to withstand the tension generated by transpirational pull.
Q45.
Root pressure can account for water movement up a stem only to a limited extent because it:
Answer: D. Is a comparatively weak force, most active at night when transpiration is low
Why: Root pressure is generated by active accumulation of ions in root xylem, but it is a relatively weak force and most evident at night/early morning (causing guttation); it cannot explain water rise in tall trees during the day.
Q46.
Loading of sucrose into the phloem sieve tubes at the source mainly involves:
Answer: C. Active co-transport of sucrose with protons, driven by an H+-ATPase gradient
Why: Sucrose is actively loaded into the phloem against its concentration gradient using a proton-sucrose co-transport mechanism, powered by H+-ATPase pumps in the plasma membrane.
Q47.
Mineral ions move from the soil into the xylem of roots mainly by a combination of:
Answer: C. Active uptake by root hair cells, then movement via symplast or apoplast to the xylem
Why: Minerals are actively absorbed (often against a concentration gradient) by root hair cells using energy-dependent transporters, then move radially through the symplast or apoplast (interrupted by the Casparian strip) into the xylem.
Q48.
The 'source to sink' direction of phloem transport can reverse seasonally in a plant because:
Answer: B. Source and sink depend on current sugar export versus import, not on fixed anatomy
Why: A source is any organ that is a net exporter of sugars (e.g., mature leaves), while a sink is a net importer (e.g., roots or developing fruits); the same organ, like a storage root, can switch from sink to source depending on the season.
Q49.
Guttation, the exudation of water droplets from leaf margins, occurs mainly when:
Answer: A. Root pressure exceeds a low transpiration rate, mostly at night or dawn
Why: Guttation happens when root pressure pushes excess xylem sap out through specialised pores (hydathodes) at leaf margins, most noticeable at night/early morning when transpiration is minimal and humidity is high.
Q50.
Imbibition, as seen in a dry seed absorbing water, is distinct from osmosis mainly because it:
Answer: A. Is adsorption of water by hydrophilic colloids like cell wall and storage proteins
Why: Imbibition is the diffusion of water into solids (like dry seeds) that causes them to swell, driven by adsorption onto hydrophilic surfaces, and does not require a semi-permeable membrane the way osmosis does.
Q51.
The chief force responsible for the ascent of sap in very tall trees is:
Answer: A. transpiration pull
Why: Transpiration pull, aided by water cohesion, mainly lifts sap in tall trees.
Q52.
Root pressure is best seen as ___ from the margins or tips of leaves:
Answer: A. guttation
Why: Guttation is the exudation of water droplets driven by root pressure.
Q53.
The cohesion between water molecules that keeps the water column intact is due to:
Answer: A. hydrogen bonding
Why: Hydrogen bonds hold water molecules together, giving cohesion.
Q54.
The movement of sugar in the phloem is driven by a difference in ___ between source and sink:
Answer: A. pressure
Why: The pressure-flow mechanism moves sugar from high-pressure source to low-pressure sink.
Q55.
Stomata generally close when a plant experiences:
Answer: A. a water shortage
Why: Under water stress, guard cells lose turgor and the stomata close to save water.
Q56.
The Casparian strip of the root endodermis controls the ___ taken by water and ions:
Answer: A. pathway
Why: The waterproof Casparian strip forces water and ions through the cell membranes, regulating their pathway.
Q57.
Imbibition is the absorption of water by ___ substances such as dry seeds:
Answer: A. solid hydrophilic ones
Why: Imbibition is water uptake by hydrophilic solids like seeds and wood.
Q58.
The pressure-flow (mass flow) hypothesis explains transport in the:
Answer: A. phloem
Why: The pressure-flow hypothesis describes how sugars move in the phloem.
Q59.
Besides cooling the plant, transpiration also assists in the ___ of minerals:
Answer: A. upward transport
Why: The transpiration stream carries absorbed minerals up the plant.
Q60.
Water enters root hairs by osmosis because the soil solution is ___ compared with the cell sap:
Answer: A. more dilute
Why: The dilute (hypotonic) soil solution has a higher water potential, so water moves into the root cells.