Intermediate

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Transport of Water — Osmosis, Absorption & Ascent of Sap

Master Transport of Water in Plants for NEET & Board Exams. Covers diffusion, osmosis, water potential, plasmolysis, imbibition, root absorption, transpiration types, guttation, ascent of sap theories including cohesion-tension theory by Dixon and Joly, and stomatal movement mechanism with solved NEET questions.

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Plants cannot move to find water — yet they manage to transport water from roots deep in the soil all the way to the tips of the tallest trees. This remarkable feat involves a combination of physical forces — osmosis, capillary action, transpiration pull — working together in a precisely coordinated system. Transport in Plants covers how water, minerals, and organic solutes move across membranes, through cells, and over long distances through the vascular system. For NEET, this chapter contributes 3–4 questions every year on osmosis, water potential, plasmolysis, ascent of sap, and transpiration — making it one of the most consistently tested chapters in Plant Physiology.

1. Means of Transport — Overview

Plants transport substances across short distances (cell to cell) and long distances (root to shoot). Different mechanisms operate at each scale:

Mechanism	Type	Energy Required?	Example
Diffusion	Passive	No	O₂, CO₂ exchange in leaves; movement of ions across membranes
Facilitated Diffusion	Passive (carrier/channel proteins)	No	Water through aquaporins; ions through ion channels
Osmosis	Passive (special case of diffusion)	No	Water absorption by roots; stomatal movement
Active Transport	Active (carrier proteins + ATP)	Yes (ATP)	Mineral ion uptake against concentration gradient

Apoplast and Symplast Pathways

Water moves through plant tissues via two routes:

Apoplast pathway: Movement through cell walls and intercellular spaces (outside the plasma membrane). Fast but blocked at the Casparian strip of endodermis.
Symplast pathway: Movement through the cytoplasm connected by plasmodesmata. Slower but continuous through living cells.
Transmembrane pathway: Water moves in and out of cells across membranes repeatedly — slowest pathway.

The Casparian strip (suberin band in endodermal cell walls) forces all apoplastic water to switch to the symplast pathway — this is where selective regulation of mineral uptake occurs.

2. Diffusion

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration — down the concentration gradient — until equilibrium is reached.

Does not require a membrane — occurs in gases, liquids, and through membranes.
Rate depends on: concentration gradient, temperature, molecular size, and the medium.
Gases diffuse much faster than liquids.
In plants: CO₂ and O₂ exchange in leaves occurs by diffusion through stomata.

Facilitated Diffusion

Some molecules (e.g., water, charged ions) cannot cross the lipid bilayer directly — they use transport proteins:

Aquaporins: Channel proteins specific for water — allow rapid osmosis across membranes without energy expenditure.
Ion channels: Allow specific ions to move down their electrochemical gradient.
Carrier proteins: Bind specific molecules and change shape to transport them across.

Facilitated diffusion is still passive (no ATP) but is faster and more selective than simple diffusion.

3. Osmosis and Water Potential

Osmosis is the diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Water Potential (Ψ)

Water potential is the chemical potential of water — it determines the direction of water movement. Represented by the Greek letter psi (Ψ). Key points:

Pure water has the highest water potential = 0 (by convention).
Adding solutes lowers water potential (makes it more negative).
Water always moves from higher (less negative) to lower (more negative) water potential.

$Ψ = Ψ_{s} + Ψ_{p}$

where $Ψ_{s}$ is the solute potential (osmotic potential) and $Ψ_{p}$ is the pressure potential (turgor pressure).

Component	Symbol	Sign	Effect on Water Potential
Solute Potential (Osmotic Potential)	$Ψ_{s}$	Always negative	Lowers water potential; more solutes = more negative $Ψ_{s}$
Pressure Potential (Turgor Pressure)	$Ψ_{p}$	Usually positive (can be negative in xylem)	Raises water potential; wall pressure increases $Ψ_{p}$
Water Potential	$Ψ$	Zero or negative (for solutions)	Direction of water movement — high to low

Osmotic Terms — Turgid, Flaccid and Plasmolysed

Condition	External Solution	Cell State	$Ψ_{p}$	Cell Volume
Turgid	Hypotonic (more dilute)	Water enters; cell swells; wall exerts back pressure	High (positive)	Maximum
Flaccid	Isotonic	No net water movement; cell is limp	Zero	Normal
Incipient Plasmolysis	Hypertonic (begin)	Protoplast just begins to pull away from wall	Zero	Slightly reduced
Plasmolysed	Hypertonic (concentrated)	Protoplast shrinks away from cell wall; cell is plasmolysed	Negative	Reduced

Deplasmolysis

When a plasmolysed cell is placed in a hypotonic solution, water enters and the protoplast returns to its original position — this is called deplasmolysis. It is reversible if the cell has not been damaged. Plasmolysis is used to demonstrate selective permeability of the plasma membrane.

Osmotic Pressure vs Osmotic Potential

Osmotic pressure (OP): The pressure needed to prevent osmosis — always positive; higher concentration = higher OP.
Osmotic potential ( $Ψ_{s}$ ): Always negative; numerically equal to OP but opposite in sign.
$Ψ_{s} = - OP$

4. Imbibition

Imbibition is the absorption of water by solid hydrophilic colloids (called imbibants) causing them to swell. It is a special type of diffusion — the water moves along a water potential gradient but into a solid matrix.

Examples: dry seeds absorbing water and swelling; dry wood swelling when wet; agar plates absorbing water.
Imbibition generates enormous imbibitional pressure — sufficient to break hard seed coats and split rocks.
Requires an affinity between the imbibant and water (e.g., starch, proteins, cellulose are good imbibants).
Imbibition is the first step in seed germination — seeds imbibe water before metabolic activation begins.

5. Absorption of Water by Roots

The primary site of water absorption is the root hair zone — root hairs are extensions of epidermal cells with enormous surface area.

Mechanism of Water Absorption

Root hair cells have a lower water potential (due to dissolved solutes) than the soil water.
Water moves into root hairs by osmosis down the water potential gradient.
Water then moves inward from cell to cell (symplast or apoplast) towards the xylem.
The Casparian strip in the endodermis forces water into the symplast — allowing selective mineral uptake.

Active vs Passive Water Absorption

Type	Driving Force	Condition
Active absorption	Root pressure (osmotic); metabolic energy used for mineral uptake which lowers water potential	Slow transpiration; night; humid conditions
Passive absorption	Transpiration pull — negative pressure (tension) in xylem pulls water up; no ATP used	High transpiration; daytime; dominant mechanism

6. Transpiration

Transpiration is the loss of water vapour from aerial parts of plants (mainly through stomata). It is often called a "necessary evil" — wasteful in terms of water loss, but essential for creating the driving force for water movement.

Types of Transpiration

Type	Site	% of Total Transpiration
Stomatal transpiration	Through stomata	80–90% (dominant)
Cuticular transpiration	Through cuticle of leaves	5–10%
Lenticular transpiration	Through lenticels of stem	1–5% (least)

Significance of Transpiration

Creates the transpiration pull — the main driving force for ascent of sap.
Helps in absorption and upward movement of water and minerals.
Cools the leaf surface by evaporative cooling.
Maintains the shape and structure of cells (turgor pressure).
Concentrates solutes in leaf cells, favouring photosynthesis.

Factors Affecting Transpiration

Temperature: Higher temperature increases transpiration (more evaporation).
Humidity: Higher humidity decreases transpiration (reduces the gradient).
Wind speed: Higher wind speed increases transpiration.
Light: Causes stomata to open → increases transpiration.
Water availability: Low water causes stomatal closure → reduces transpiration.
CO₂ concentration: High CO₂ causes stomatal closure → reduces transpiration.

Guttation

Guttation is the exudation of liquid water (not vapour) from hydathodes at leaf margins — typically in humid conditions at night/early morning when transpiration is low but root pressure is high. The liquid contains dissolved salts (unlike dew, which is pure condensed water).

7. Ascent of Sap — Movement of Water in Xylem

The upward movement of water and dissolved minerals through the xylem is called ascent of sap. In tall trees (up to 120 m), this requires tremendous force. Several theories have been proposed:

Theories of Ascent of Sap

Theory	Proposed by	Mechanism	Status
Root Pressure Theory	Priestley	Osmotic pressure builds in roots pushing water up	Insufficient alone — only pushes ~2–3 bar; cannot explain tall trees
Capillary Theory	—	Surface tension in narrow xylem vessels pulls water up	Insufficient — capillarity can only raise water ~1 m in xylem diameter
Cohesion-Tension-Transpiration Pull Theory	Dixon & Joly (1894)	Transpiration creates tension (negative pressure) in xylem; cohesion of water molecules transmits pull from leaves to roots	Most widely accepted

Cohesion-Tension Theory — Details

Proposed by Dixon and Joly (1894) — also called the Transpiration Pull theory. It is based on three properties of water:

Transpiration pull: Water evaporates from mesophyll cells through stomata → water potential in mesophyll cells drops → water is drawn from xylem into mesophyll → negative pressure (tension) is created in xylem.
Cohesion of water: Water molecules are strongly attracted to each other by hydrogen bonds. This cohesion keeps the water column intact even under tension.
Adhesion: Water molecules adhere to the hydrophilic walls of xylem vessels — helps resist breaking of the water column.

The combination creates an unbroken water column from roots to leaves. The tension in xylem can reach −15 to −80 bar — sufficient to lift water in the tallest trees. This is passive transport — no metabolic energy is required.

Root Pressure

Root pressure is a positive pressure that develops in the root xylem due to active mineral uptake lowering the water potential of xylem. Water moves in by osmosis creating pressure. Evidence:

Bleeding: When a stem is cut, xylem fluid oozes out due to root pressure.
Guttation: Exudation of water droplets from hydathodes at night when transpiration is low.
Root pressure alone cannot account for tall trees — it supplements the transpiration pull.

8. Opening and Closing of Stomata

Stomata are pores in the leaf epidermis surrounded by two kidney-shaped (or dumbbell-shaped in grasses) guard cells. Their opening and closing regulates gas exchange and transpiration.

Mechanism of Stomatal Opening

In light, guard cells photosynthesise — CO₂ is used up, pH rises.
Starch is converted to malate and glucose — osmotic concentration increases.
K⁺ ions are actively pumped into guard cells — water potential decreases.
Water enters guard cells by osmosis → guard cells become turgid → stomata open.
The inner wall of guard cells is thicker and inextensible — so turgidity causes the cells to bow outward, opening the pore.

Mechanism of Stomatal Closing

In darkness or water stress: K⁺ ions move out of guard cells.
Osmotic concentration decreases → water moves out by osmosis.
Guard cells become flaccid → stomata close.
ABA (Abscisic Acid) triggers stomatal closure under water stress — acts as the stress signal.

Condition	Stomatal Response	Mechanism
Light	Open	K⁺ influx, malate accumulation, guard cells turgid
Darkness	Close	K⁺ efflux, guard cells flaccid
Water deficit (drought)	Close	ABA release triggers K⁺ efflux
High CO₂	Close	CO₂ accumulation inhibits K⁺ uptake
Low CO₂	Open	Promotes K⁺ influx into guard cells

Water Potential Problems — Step-by-Step Method

NEET frequently asks which direction water will move between two cells or solutions. Follow this approach every time:

Step	Action
1	Calculate water potential: $Ψ = Ψ_{s} + Ψ_{p}$ for each cell/solution.
2	Remember: $Ψ_{s}$ is always negative (solutes lower water potential). $Ψ_{p}$ is positive in turgid cells, zero in flaccid cells, negative in xylem.
3	Water moves from higher (less negative) $Ψ$ to lower (more negative) $Ψ$ .
4	At equilibrium: $Ψ$ of both cells/solutions is equal. For a fully turgid cell: $Ψ = 0$ (meaning $Ψ_{s} + Ψ_{p} = 0$ , i.e., $Ψ_{p} = - Ψ_{s} = OP$ ).
5	DPD (Deficit Pressure / Suction Pressure) = OP − TP = $- Ψ_{s} - Ψ_{p} = - Ψ$ . DPD is the "sucking power" of a cell for water. A fully turgid cell has DPD = 0.

Key Relationships to Memorise

Fully turgid cell: $Ψ_{p} = Ψ_{s}$ (in magnitude), $Ψ = 0$ , DPD = 0
Fully plasmolysed cell: $Ψ_{p} = 0$ , $Ψ = Ψ_{s}$ , DPD = OP
Flaccid cell (incipient plasmolysis): $Ψ_{p} = 0$
Pure water: $Ψ = 0$ (highest possible water potential)

NEET Power Tips — Avoid These Common Mistakes

Water moves from high to low water potential — NOT from high to low osmotic pressure (OP). OP is the magnitude; water potential includes both solute and pressure components. Always use water potential ( $Ψ$ ) for direction of flow.
Transpiration pull (cohesion-tension theory) is the most accepted theory for ascent of sap — proposed by Dixon and Joly (1894). Root pressure alone cannot explain water rise in tall trees — it contributes only at night when transpiration is low.
Guttation liquid contains solutes (dissolved salts, amino acids) — unlike dew, which is pure condensed water. This distinction is directly tested in NEET.
Stomatal transpiration accounts for 80–90% of total water loss — cuticular is 5–10% and lenticular is least (1–5%). Many students incorrectly choose cuticular as dominant.
K⁺ (potassium ions) are the key to stomatal movement — K⁺ influx opens stomata; K⁺ efflux closes them. ABA triggers K⁺ efflux during water stress. This ion is explicitly named in NCERT.
Casparian strip is made of suberin (not lignin or cellulose) — it is present in endodermal cell walls and forces apoplastic water into symplast pathway. This is a very commonly tested NEET structural detail.
Imbibition is not osmosis — osmosis requires a selectively permeable membrane; imbibition occurs in solid colloids without a membrane. Seeds germinate via imbibition first, then osmosis begins.

Practice Questions (NEET Level)

Q1: The water potential of pure water at standard temperature and pressure is:

A) +1 bar
B) -1 bar
C) Zero
D) Infinity

Answer: C) Zero.

Explanation: By convention, the water potential of pure water at standard temperature and pressure is defined as exactly zero. Adding any solute lowers the water potential below zero (making it a negative value). Applying positive pressure raises the water potential above zero. Therefore, under standard conditions, pure water represents the maximum possible baseline water potential.

Q2: The solute potential of a cell is -8 bars and the pressure potential is +3 bars. The water potential of the cell is:

A) +11 bars
B) -11 bars
C) -5 bars
D) +5 bars

Answer: C) -5 bars.

Explanation:

Water potential ( $Ψ$ ) is the sum of solute potential ( $Ψ_{s}$ ) and pressure potential ( $Ψ_{p}$ ):
$Ψ = Ψ_{s} + Ψ_{p}$
$Ψ = - 8 + 3 = - 5 bars$ .

The negative sign indicates that water will tend to move into this cell if it is placed in pure water or a solution with a higher water potential.

Q3: Which of the following is the most accepted theory for ascent of sap in tall trees?

A) Root pressure theory
B) Capillary theory
C) Cohesion-tension-transpiration pull theory
D) Vital force theory

Answer: C) Cohesion-tension-transpiration pull theory.

Explanation: The cohesion-tension theory, proposed by Dixon and Joly (1894), is the most widely accepted explanation for the ascent of sap. Transpiration from the leaves creates a negative pressure (tension) in the xylem. The cohesion of water molecules (due to hydrogen bonding) keeps the water column intact, transmitting this upward pull all the way down to the roots. Root pressure alone is insufficient to push water up tall trees.

Q4: Guttation differs from transpiration in that:

A) Guttation involves water vapour; transpiration involves liquid water
B) Guttation occurs through hydathodes and involves liquid water with solutes; transpiration is mainly water vapour through stomata
C) Guttation occurs during the day; transpiration occurs at night
D) Guttation is driven by transpiration pull; transpiration is driven by root pressure

Answer: B) Guttation occurs through hydathodes and involves liquid water with solutes; transpiration is mainly water vapour through stomata.

Explanation: Guttation is the exudation of liquid water (which contains dissolved minerals and amino acids) through specialized pores called hydathodes at the leaf margins. It is driven by root pressure and usually occurs at night or early morning when transpiration is low. Transpiration, on the other hand, is the loss of pure water vapour mainly through stomata and is driven by transpirational pull.

Q5: The Casparian strip in the endodermis is made of:

A) Cellulose
B) Lignin
C) Suberin
D) Pectin

Answer: C) Suberin.

Explanation: The Casparian strip is a band of suberin—a waxy, highly water-impermeable polymer—deposited in the radial and transverse walls of endodermal cells in roots. It blocks the apoplastic pathway (movement through cell walls), forcing all water and dissolved minerals to cross the selectively permeable plasma membrane into the symplast before they can enter the vascular cylinder (stele).

Q6: A plant cell is placed in a solution. The cell's solute potential is -10 bars and the solution's water potential is -8 bars. What will happen?

A) Water will move from the cell into the solution
B) Water will move from the solution into the cell
C) No net water movement
D) The cell will undergo plasmolysis immediately

Answer: B) Water will move from the solution into the cell.

Explanation:

Assuming the cell is initially flaccid (pressure potential = 0), the cell's total water potential is equal to its solute potential: -10 bars.

The surrounding solution has a water potential of -8 bars.

Water always moves passively from a region of higher (less negative) water potential to a region of lower (more negative) water potential. Because -8 is greater than -10, water will move from the solution INTO the cell.

Q7: Which of the following correctly describes the role of potassium ions in stomatal movement?

A) K⁺ influx into guard cells causes stomatal closing
B) K⁺ efflux from guard cells causes stomatal opening
C) K⁺ influx into guard cells increases their osmotic concentration, causing water entry and stomatal opening
D) K⁺ has no role in stomatal movement; CO₂ is the primary regulator

Answer: C) K⁺ influx into guard cells increases their osmotic concentration, causing water entry and stomatal opening.

Explanation: During the day, K⁺ ions are actively pumped into the guard cells from the surrounding subsidiary epidermal cells. This accumulation of ions increases the osmotic concentration (lowering the water potential) inside the guard cells. As a result, water enters the guard cells via osmosis, making them turgid. Because the inner walls of guard cells are thick and inelastic, this turgidity causes them to bow outward, thereby opening the stomatal pore. Conversely, the efflux of K⁺ leads to water loss, flaccidity, and stomatal closure.

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DifficultyIntermediate

Est. Read Time28 minutes

CategoryTransport In Plants

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Transport of Water — Osmosis, Absorption & Ascent of Sap

1. Means of Transport — Overview

Apoplast and Symplast Pathways

2. Diffusion

Facilitated Diffusion

3. Osmosis and Water Potential

Water Potential (Ψ)

Osmotic Terms — Turgid, Flaccid and Plasmolysed

Deplasmolysis

Osmotic Pressure vs Osmotic Potential

4. Imbibition

5. Absorption of Water by Roots

Mechanism of Water Absorption

Active vs Passive Water Absorption

6. Transpiration

Types of Transpiration

Significance of Transpiration

Factors Affecting Transpiration

Guttation

7. Ascent of Sap — Movement of Water in Xylem

Theories of Ascent of Sap

Cohesion-Tension Theory — Details

Root Pressure

8. Opening and Closing of Stomata

Mechanism of Stomatal Opening

Mechanism of Stomatal Closing

Water Potential Problems — Step-by-Step Method

Key Relationships to Memorise

Practice Questions (NEET Level)

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Practice Questions

Topic Information

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