💧 Water Relations & Transport Mechanisms
Learn water potential, osmosis, plasmolysis and imbibition for CUET Agriculture. DPD, turgor pressure, wall pressure and water absorption.
Introduction to Transport in Plants
Plants are complex organisms that need to move a variety of substances throughout their body — including water, mineral nutrients, organic food (sugars, amino acids), and growth regulators (hormones). Without an efficient transport system, plants could not sustain growth, photosynthesis, or reproduction.
Transport in plants occurs at two distinct levels:
- Short Distance Transport — This happens via diffusion (random molecular movement) and cytoplasmic streaming (flow of cytoplasm within cells). It does not require specialized vascular tissues like xylem and phloem. This type of transport moves substances across a few cells at most.
- Long Distance Transport — This relies on the plant's vascular system: xylem carries water and dissolved minerals upward, while phloem translocates food (mainly sucrose) from source to sink. Long distance transport involves bulk flow or mass flow, meaning large volumes of fluid move together through conducting vessels.
Direction of Transport
- In herbaceous plants (soft-stemmed plants), transport occurs in all directions — this is called multidirectional transport.
- In rooted plants, the transport of minerals and water is primarily unidirectional — moving from the roots upward through the xylem to the aerial parts (stems, leaves).
Means of Transport
(1) Simple Diffusion
Diffusion is the movement of ions, molecules, and particles from a region of higher concentration to lower concentration until equilibrium is achieved. Think of it like a drop of ink spreading evenly through a glass of water — the ink molecules move from where they are concentrated to areas where they are less concentrated.
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Introduction to Transport in Plants
Plants are complex organisms that need to move a variety of substances throughout their body — including water, mineral nutrients, organic food (sugars, amino acids), and growth regulators (hormones). Without an efficient transport system, plants could not sustain growth, photosynthesis, or reproduction.
Transport in plants occurs at two distinct levels:
- Short Distance Transport — This happens via diffusion (random molecular movement) and cytoplasmic streaming (flow of cytoplasm within cells). It does not require specialized vascular tissues like xylem and phloem. This type of transport moves substances across a few cells at most.
- Long Distance Transport — This relies on the plant's vascular system: xylem carries water and dissolved minerals upward, while phloem translocates food (mainly sucrose) from source to sink. Long distance transport involves bulk flow or mass flow, meaning large volumes of fluid move together through conducting vessels.
Direction of Transport
- In herbaceous plants (soft-stemmed plants), transport occurs in all directions — this is called multidirectional transport.
- In rooted plants, the transport of minerals and water is primarily unidirectional — moving from the roots upward through the xylem to the aerial parts (stems, leaves).
Means of Transport
(1) Simple Diffusion
Diffusion is the movement of ions, molecules, and particles from a region of higher concentration to lower concentration until equilibrium is achieved. Think of it like a drop of ink spreading evenly through a glass of water — the ink molecules move from where they are concentrated to areas where they are less concentrated.
Features of Diffusion:
- It is a passive process (also called "downhill") — it does not require energy expenditure by the cell
- Involves random kinetic motion of molecules — each molecule moves independently
- It is a slow process, especially over long distances
- The driving force is the concentration gradient — the steeper the gradient, the faster the diffusion
- Does not depend on living systems — diffusion occurs even in non-living environments
- It is non-selective — any molecule that can move will diffuse
- Does not require a semipermeable membrane to occur
Factors Affecting Rate of Diffusion:
| Factor | Effect |
|---|---|
| Concentration gradient | Higher gradient → faster diffusion |
| Membrane permeability | More permeable → faster diffusion |
| Temperature | Higher temperature → faster diffusion |
| Pressure | Higher pressure → faster diffusion |
| Density | Rate is inversely proportional to square root of density |
| Particle size | Inversely proportional to size |
Rate of diffusion order by state: Gas > Liquid > Solid — gases diffuse fastest because their molecules are most widely spaced and move most freely.
Applications of Diffusion:
- Essential for gas exchange in plants (CO₂ in, O₂ out during photosynthesis)
- Movement of substances from one cell part to another
- Cell-to-cell movement of molecules
- Loss of water vapor during transpiration (evaporation from intercellular spaces into the atmosphere)
(2) Facilitated Diffusion
Sometimes molecules need help crossing cell membranes. Facilitated diffusion is the movement of substances across the membrane that depends on the membrane lipid composition. Substances that dissolve easily in lipids pass through the membrane quickly (simple diffusion). However, substances that are water-soluble and cannot easily pass through the lipid bilayer are assisted by special transmembrane proteins.
Site: Transmembrane proteins (also called tunnel proteins) embedded in the cell membrane.
Features:
- It is a passive, downhill process — no ATP energy is needed
- Driving force is the concentration gradient
- Depends on a living system (because proteins are required)
- Movement occurs from higher to lower concentration only — never against the gradient
- When all protein transporters are occupied and working at maximum capacity, saturation occurs — adding more substrate will not increase the rate
- It is sensitive to changes in protein shape (conformational changes)
- It is specific — each protein selectively allows only certain molecules to pass
Key Facts:
- Channel proteins do not establish a concentration gradient — the gradient must already exist for facilitated diffusion to work
- Some channel proteins are always open; others are controlled (gated) — they open or close in response to signals
- Porins: Large channel proteins found in outer membranes of mitochondria and some organisms; allow passage of molecules up to a certain size
- Aquaporins: Specialized water channel proteins in plants; found as different types in various membranes, greatly increasing the rate of water transport
Types of Passive Transport:
| Type | Description |
|---|---|
| Symport | Two molecules move in the same direction across the membrane |
| Antiport | Two molecules move in opposite directions across the membrane |
| Uniport | A single molecule moves across the membrane independently |
TIP
Remember the transport types with this memory aid: Symport = Same direction, Antiport = Against (opposite) direction, Uniport = Uno (one molecule alone).
(3) Active Transport
Active transport is the movement of substances against the concentration gradient (from low to high concentration) using energy in the form of ATP. This is performed by protein pumps embedded in cell membranes. Active transport is essential when cells need to accumulate specific ions or molecules to concentrations higher than the surrounding environment.
Features:
- It is an uphill process — requires energy (ATP)
- Driving force is ATP (not the concentration gradient)
- Depends on a living system (protein pumps must be functional)
- When all transporters are engaged, saturation occurs
- Involves conformational changes in pump proteins as they move substances
- It is specific and selective — each pump handles particular molecules
Comparison of Transport Processes
| Property | Simple Diffusion | Facilitated Diffusion | Active Transport |
|---|---|---|---|
| Membrane protein required | No | Yes | Yes |
| Energy (ATP) required | No | No | Yes |
| Selectivity | No | Yes | Yes |
| Uphill transport | No | No | Yes |
| Saturation | No | Yes | Yes |
| Hormone regulation | No | Yes | Yes |
| Inhibitor effect | No | Yes | Yes |
IMPORTANT
For exams, remember: Simple diffusion needs nothing special. Facilitated diffusion needs proteins but no ATP. Active transport needs both proteins AND ATP.
Plant Water Relations
Water is essential for virtually all physiological activities in plants. Here are some key facts about water in plants:
- Watermelon contains 92% water
- Most herbaceous plants contain 85-90% water (meaning only 10-15% is dry matter weight)
- Different plant parts have different water content — woody parts have less water, while soft/meristematic parts (actively dividing tissues) have more
- Seeds appear dry but still contain some water; they cannot survive if completely desiccated
- A mature corn plant absorbs about 3 liters of water per day, while the plant retains only about 5 gallons over its entire lifetime — most absorbed water is lost through transpiration!
Membrane Permeability
The ability of a membrane to allow substances to pass through it varies. Understanding membrane types is crucial for grasping how water and solutes move in plants:
| Membrane Type | Description | Example |
|---|---|---|
| Permeable | Allows both solute and solvent to pass freely | Primary cell wall |
| Semi-permeable | Allows solvent but not solute to pass | Cellophane, parchment paper, ferricyanide membrane |
| Selectively permeable | Allows solvent and some selected solutes to pass | Living cell membrane (plasma membrane, tonoplast) |
| Impermeable | Allows neither solute nor solvent to pass | Cutin-lined membranes, plastic, polythene |
NOTE
The living cell membrane is selectively permeable — not semi-permeable. This is an important distinction for exam questions. Semi-permeable membranes are non-living (like cellophane), while selectively permeable membranes are living and can actively choose which solutes to allow.
Osmosis
The term osmosis was introduced by Nollet. Osmosis is the movement of water molecules through a semipermeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration).
Key Facts:
- Osmosis is a special type of diffusion — specifically, the diffusion of water across a membrane
- The driving force is the difference in water potential (osmotic potential)
- Net direction depends on both pressure and concentration gradients
- Water molecules move from higher chemical potential (dilute solution) to lower chemical potential (concentrated solution) side
- Requires two chambers separated by a semipermeable membrane
Osmotic Pressure
The concept of osmotic pressure was introduced by O. Pfeffer.
Osmotic pressure (OP) is the pressure generated in an active (hypertonic) solution. It is the minimum pressure needed to stop water from entering the solution through a semipermeable membrane. It is denoted by the Greek letter pi (π).
Van't Hoff Equation:
π = iCRT
Where:
- i = ionization constant (number of particles the solute dissociates into)
- C = concentration (molarity)
- R = gas constant (0.082 L atm/mol K)
- T = absolute temperature (Kelvin)
Example: At 0°C, osmotic pressure of 1 mol glucose solution = 1 × 0.082 × 273 = 22.4 atm
Key Facts about Osmotic Pressure (Click to expand)
- OP functions as a measure of solute concentration — the higher the solute concentration, the higher the OP (which resists water diffusion into the solution)
- Numerically, OP equals solute potential but with opposite sign: ψ_s = −π
- Osmotic pressure is electrolyte-dependent — electrolytes dissociate into ions, increasing the effective particle count and thus OP
- Different plants have different OP values
- OP order: Xerophytes < Mesophytes < Halophytes < Mangroves (mangroves have the highest OP because they must extract water from salty environments)
- Highest OP measured: 202.5 atm in Atriplex confertifolia (a shrub from family Chenopodiaceae)
- Pure water OP = 0 (zero)
- Measured using an osmometer
- In organisms, OP is measured by cryoscopic method (freezing point depression — solutions freeze at lower temperatures than pure water)
Types of Solutions
| Solution Type | Description |
|---|---|
| Hypertonic | Higher solute concentration than the cell — water moves out of cell |
| Hypotonic | Lower solute concentration than the cell — water moves into the cell |
| Isotonic | Same solute concentration as the cell — no net water flow (equilibrium) |
Turgor Pressure (TP)
When a cell is placed in a hypotonic solution, water enters the cell by osmosis. As water fills the central vacuole, the protoplast (living contents of the cell) expands and pushes outward against the cell wall. This outward-pushing hydrostatic pressure is called turgor pressure.
- The counter-pressure exerted by the cell wall pushing back inward is called wall pressure
- At equilibrium: TP = WP (turgor pressure equals wall pressure)
Key Facts:
- Turgor pressure is a centrifugal force (pushes outward from the center of the cell)
- Maximum TP is found in a fully turgid cell — where TP equals the OP of the cell
- For a plasmolyzed cell, TP = 0 (and can even become negative in severely flaccid cells)
- If a free-floating cell has no solute inside, TP = 0
- A flaccid cell is one where TP = 0 — the cell is limp and wilted
Significance of Turgor Pressure:
- Maintains cell shape and rigidity — this is why plants wilt when they lose water
- Essential for cell elongation and growth — growing cells need turgor to expand
Plasmolysis
When a cell is placed in a hypertonic solution, water moves out of the cell. The protoplast shrinks and pulls away from the cell wall. This process is called plasmolysis.
Key Facts:
- During plasmolysis, the cell membrane separates from the cell wall
- Water moves out because the external solution has higher OP (lower water potential) than the cell sap
- Plasmolysis occurs only in cells that have completed their maturation (have a central vacuole)
- The process is generally reversible — endosmosis can reverse it
Stages of Plasmolysis:
- Incipient plasmolysis — plasmolysis just begins; protoplast starts separating from the wall at the corners
- Evident plasmolysis — plasmolysis is clearly visible; protoplast has significantly shrunk away from the wall
Deplasmolysis: When a plasmolyzed cell is placed back in a hypotonic solution, water re-enters by osmosis and the protoplast swells back to normal. This reversal is called deplasmolysis.
Applications of Plasmolysis:
- To determine whether a cell is living or dead (plasmolysis occurs only in living cells — dead cells cannot plasmolyze because their membranes are non-functional)
- In food preservation (salt/sugar create hypertonic environment, drawing water out of microbes)
- To destroy weeds (applying salt to soil)
- To measure osmotic pressure by observing the concentration at which plasmolysis begins
Diffusion Pressure Deficit (DPD) / Suction Pressure (SP)
The reduction in diffusion pressure of a solution compared to pure solvent is called DPD (Diffusion Pressure Deficit), also known as Suction Pressure (SP). DPD represents the cell's "thirst" for water — the higher the DPD, the more strongly the cell absorbs water.
Mathematical Expression:
DPD = OP − TP
| Cell State | DPD Value |
|---|---|
| Free solution (no container) | DPD = OP (since TP = 0) |
| Normal cell (partially turgid) | DPD = OP − TP (OP > TP) |
| Fully turgid cell | DPD = 0 (since OP = TP, the cell cannot absorb more water) |
| Flaccid cell | DPD = OP (since TP = 0) |
| Plasmolyzed cell | DPD = OP + TP (TP is negative, so it adds to the deficit) |
- DPD is directly proportional to solute concentration
- Water always moves from an area of lower DPD to higher DPD
- Pure water has DPD = 0 (the lowest possible value)
TIP
Think of DPD as a cell's "water hunger." A fully turgid cell has zero hunger (DPD = 0) because it's full. A plasmolyzed cell is extremely hungry for water (high DPD).
Water Potential (ψ_w)
Water potential is the free energy of water per unit volume. It is denoted by ψ_w (Greek letter psi) and measured in units of pressure (Pa, atm, bar). Water potential tells us in which direction water will move — water always flows from higher water potential to lower water potential.
Water Potential Equation:
ψ_w = ψ_s + ψ_p
Where:
- ψ_s = Solute potential (osmotic potential) — always negative (adding solute decreases water potential)
- ψ_p = Pressure potential (turgor pressure) — usually positive in turgid cells
Key Relationships:
| Condition | ψ_w Value |
|---|---|
| Pure water | ψ_w = 0 (maximum, taken as reference) |
| Solution (solute added) | ψ_w is negative (decreases from zero) |
| Pure water + external pressure | ψ_w becomes positive |
Conclusions:
- All solutions have negative water potential (lower than pure water)
- ψ_s is always negative — adding solute always decreases water potential
- When pure water or a solution is placed under pressure greater than atmospheric, the water potential increases (ψ_p is positive)
- Water flows from higher water potential to lower water potential — this is the fundamental rule governing water movement in plants
IMPORTANT
For exam purposes: Pure water ψ_w = 0 (the highest possible). All solutions have negative ψ_w. Water moves from high ψ_w to low ψ_w — just like water flows downhill.
Imbibition
Imbibition is a special type of diffusion where dry solid (colloid) substances absorb water by adsorption (water molecules cling to the surface and interior of the solid material).
Results of Imbibition:
- The imbibant (absorbing substance) increases in volume — it swells
- Imbibition pressure is generated — this can be surprisingly powerful
Prerequisites for Imbibition:
- A potential difference in water between the adsorbent and the medium (the adsorbent must be drier)
- Affinity between the adsorbent and the liquid (the adsorbent must be attracted to the liquid — for example, cellulose attracts water but repels oil)
Classical Examples: Absorption of water by dry seeds and dry wood
Applications:
- Imbibition force was used by prehistoric humans to break large rocks and boulders — they would insert dry wooden wedges into cracks and pour water on them
- Seedlings emerge from soil by the force of imbibition (the swelling seed pushes through the soil)
Imbibition Capacity Order:
Agar-agar > Pectin > Protein > Starch > Cellulose
Key Facts:
- Imbibition causes swelling in seeds — this is why seeds and wooden doors swell in the rainy season
- Different substances have different imbibition capacities depending on their chemical composition
- Seasonal swelling of wooden structures (doors, window frames) is due to imbibition of atmospheric moisture
Transpiration
Transpiration is the loss of water in the form of vapor from the aerial parts (stems, branches, and leaves) of plants. It occurs mainly through stomata on leaf surfaces, but also through the cuticle and lenticels. Transpiration is accompanied by carbon dioxide and oxygen exchange through the same openings.
Types of Transpiration
| Type | Description | Percentage |
|---|---|---|
| Stomatal | Through stomata on leaves | 50-90% of total |
| Cuticular | Through cuticle (waxy layer on epidermis) | 9-9.9% |
| Lenticular | Through lenticels on bark of stems and fruits | 0.1-1% |
NOTE
Total transpiration = stomatal + cuticular + lenticular transpiration. Stomatal transpiration is by far the most significant, which is why stomatal regulation is so important for plant water management.
Structure of Stomata
- Stomata consist of two guard cells surrounding a pore (stomatal opening)
- Guard cells are kidney-shaped (crescent) in dicots and dumbbell-shaped in monocots (grasses)
| Feature | Guard Cells | Subsidiary Cells |
|---|---|---|
| Shape in dicots | Kidney/crescent | Barrel-shaped |
| Chloroplasts present | Yes (radial arrangement) | No |
| Cytoplasm | Present | Present |
| Outer wall | Thin and elastic | Equal thickness |
| Inner wall | Thick and inelastic | Equal thickness |
| Cellulose microfibrils | Radially arranged | Absent |
| PEPcase | Present | Absent |
Mechanism of Stomatal Opening and Closing
The opening and closing of stomata is controlled by changes in turgor pressure of the guard cells:
Opening:
- Guard cells become turgid when K⁺ ions and water enter → the thin outer walls bulge outward, pulling the thick inner walls apart → stomata open
Closing:
- Guard cells become flaccid when K⁺ and water leave → the guard cells lose turgor and collapse together → stomata close
K⁺ Ion Transport Theory (Levitt, 1974) — the most widely accepted mechanism for stomatal movement. K⁺ ions are actively pumped into guard cells (opening) or released from guard cells (closing).
ABA (Abscisic acid) Theory:
- ABA causes K⁺ to leave guard cells, replaced by H⁺ ions
- This makes guard cells flaccid, closing stomata
- ABA is the stress hormone — it accumulates during drought, causing stomatal closure to conserve water
Factors Affecting Transpiration Rate
(A) External Factors:
| Factor | Effect |
|---|---|
| Atmospheric humidity | Inversely proportional — high humidity reduces transpiration because the air is already saturated with moisture |
| Temperature | Up to a limit, higher temperature → higher transpiration (warmth increases evaporation) |
| Light | Stimulates stomatal opening → increases transpiration |
| Wind velocity | Moderate wind increases transpiration by removing humid air; very high wind may decrease it by forcing stomatal closure |
| Atmospheric pressure | Inversely proportional — low pressure makes it easier for water vapor to escape |
(B) Internal (Plant) Factors:
| Factor | Effect |
|---|---|
| Number of stomata | More stomata → more transpiration |
| Distribution of stomata | Dicots have more stomata on the lower surface; monocots have equal on both surfaces |
| Percentage of open stomata | More open stomata → more transpiration |
| Leaf anatomy (mesophyll) | More palisade tissue → more transpiration |
| Cuticle thickness | Thicker cuticle → less transpiration (cuticle acts as a waterproof barrier) |
| Water status of plant | Water stress → ABA release → stomatal closure → less transpiration |
| Canopy structure | Dense canopy → less wind flow → less transpiration |
| Root-shoot ratio | Proportional to transpiration rate |
Anti-transpirants
Anti-transpirants are substances that reduce the rate of transpiration. They are important for dry farming and reducing water loss in arid regions.
| Anti-transpirant | Mechanism |
|---|---|
| Phenyl mercuric acetate (PMA) | Closes stomata |
| Aspirin (acetyl salicylic acid) | Closes stomata |
| ABA | Closes stomata |
| Silicon emulsion, Silicon oil | Coats leaf surface (physical barrier) |
| CO₂ | Reduces stomatal opening |
| Low-viscosity wax/oil | Coats leaf surface (physical barrier) |
Transpiration Ratio
Transpiration Ratio = Moles of H₂O transpired / Moles of CO₂ fixed
| Plant Type | Transpiration Ratio | Efficiency |
|---|---|---|
| C₄ plants | 200-350 | Low ratio = more efficient |
| C₃ plants | 500-1000 | High ratio = less efficient |
| CAM plants | 50-100 | Lowest ratio = most efficient |
TIP
CAM plants (like cactus) are the most water-efficient because they open stomata only at night. C₄ plants are more efficient than C₃ plants because they minimize photorespiration.
Important Instruments
| Instrument | Use |
|---|---|
| Potometer | Measures rate of transpiration (actually measures water uptake, which is nearly equal to transpiration) |
| Porometer | Measures stomatal aperture (how wide stomata are open) |
| Cobalt chloride paper test | Compares transpiration rate from two leaf surfaces (the paper turns from blue to pink when it absorbs moisture) |
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Short distance transport | Via diffusion and cytoplasmic streaming; no vascular tissue needed |
| Long distance transport | Via xylem (water/minerals upward) and phloem (food source→sink); involves bulk/mass flow |
| Simple diffusion | Passive, no energy, higher → lower concentration; rate: Gas > Liquid > Solid |
| Factors affecting diffusion | Concentration gradient, membrane permeability, temperature, pressure, density (inversely), particle size (inversely) |
| Facilitated diffusion | Passive but needs transmembrane proteins; no ATP; shows saturation and specificity |
| Symport / Antiport / Uniport | Same direction / Opposite directions / Single molecule transport |
| Aquaporins | Specialized water channel proteins in plant membranes |
| Active transport | Against concentration gradient; requires ATP and protein pumps; shows saturation |
| Watermelon water content | 92%; herbaceous plants 85-90% |
| Corn plant daily water absorption | 3 liters/day |
| Membrane types | Permeable (cell wall), Semi-permeable (cellophane), Selectively permeable (living cell membrane), Impermeable (cutin) |
| Osmosis | Coined by Nollet; water movement through semipermeable membrane from lower to higher solute concentration |
| Osmotic pressure (OP) | Coined by O. Pfeffer; formula: π = iCRT; R = 0.082 L atm/mol K |
| OP of 1M glucose at 0°C | 22.4 atm |
| OP order | Xerophytes < Mesophytes < Halophytes < Mangroves (highest) |
| Highest OP in plants | 202.5 atm in Atriplex confertifolia (Chenopodiaceae) |
| Hypertonic / Hypotonic / Isotonic | Water moves out / Water moves in / No net flow |
| Turgor pressure (TP) | Outward hydrostatic pressure; at equilibrium TP = WP; fully turgid cell: TP = OP; flaccid cell: TP = 0 |
| Plasmolysis | Cell in hypertonic solution → protoplast shrinks; stages: Incipient → Evident; only in living mature cells |
| Deplasmolysis | Reversal by placing plasmolyzed cell in hypotonic solution |
| DPD / Suction Pressure | DPD = OP − TP; fully turgid: DPD = 0; flaccid: DPD = OP; water moves from lower to higher DPD |
| Water potential (ψ_w) | ψ_w = ψ_s + ψ_p; pure water = 0 (maximum); all solutions are negative; water flows high → low ψ_w |
| Imbibition | Dry solid absorbs water by adsorption; capacity: Agar > Pectin > Protein > Starch > Cellulose |
| Transpiration types | Stomatal (50-90%), Cuticular (9-9.9%), Lenticular (0.1-1%) |
| Guard cells | Kidney-shaped (dicots), Dumbbell-shaped (monocots); contain chloroplasts |
| Stomatal opening mechanism | K⁺ Ion Transport Theory (Levitt, 1974); K⁺ enters → turgid → opens; K⁺ leaves → flaccid → closes |
| ABA and stomata | ABA causes K⁺ to leave guard cells → stomatal closure (stress hormone response) |
| Anti-transpirants | PMA, Aspirin, ABA (close stomata); Silicon oil, wax (coat surface); CO₂ (reduces opening) |
| Transpiration ratio | CAM: 50-100 (most efficient), C₄: 200-350, C₃: 500-1000 (least efficient) |
| Potometer | Measures transpiration rate |
| Porometer | Measures stomatal aperture |
| Cobalt chloride paper | Compares transpiration from upper vs lower leaf surfaces (blue → pink) |
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