30 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.
Take the timed Transport in Plants quiz →Below are 30 practice questions on Transport in Plants, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Transport in Plants notes.
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.
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.
Q12.
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.
Q13.
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.
Q14.
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.
Q15.
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 Ca2+ signaling, K+ and water efflux, turgor decreases, stomata close.
Q16.
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.
Q17.
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.
Q18.
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.
Q19.
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.
Q20.
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.
Q21.
Guard cell closure in response to ABA involves:
Answer: B. Ca2+ 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 Ca2+. Ca2+ activates anion channels (Cl- efflux) and inhibits K+ influx, reducing turgor.
Q22.
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.
Q23.
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.
Q24.
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.
Q25.
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.
Q26.
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.
Q27.
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.
Q28.
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.
Q29.
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.
Q30.
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.