D2.3 Water Potential

D2.3 Water Potential

Overview

Water potential (Ψ) is a measure of the potential energy of water in a system compared to pure water at atmospheric pressure and room temperature. It determines the direction of water movement — water always moves from regions of higher water potential to regions of lower water potential.

Understanding water potential is essential for explaining osmosis, plant water relations, and how cells respond to different environments.

What is Water Potential?

Definition

Water potential is the tendency of water to move from one area to another. It is measured in units of pressure, typically kilopascals (kPa) or megapascals (MPa).

Reference Point

• Pure water at atmospheric pressure has a water potential of zero (Ψ = 0)

• Adding solutes decreases water potential (makes it more negative)

• Applying pressure increases water potential (makes it more positive/less negative)

The Water Potential Equation

$\Psi = \Psi_s + \Psi_p$

Where:

• Ψ (psi) = Total water potential

• Ψₛ = Solute potential (osmotic potential)

• Ψₚ = Pressure potential

Components of Water Potential

Solute Potential (Ψₛ)

Also called osmotic potential

Definition: The effect of dissolved solutes on water potential.

Key points:

• Solutes lower water potential

• Always zero or negative (Ψₛ ≤ 0)

• Pure water has Ψₛ = 0

• More solutes = more negative Ψₛ

• Measured relative to pure water

Why solutes lower water potential:

• Solute molecules interact with water molecules

• Reduces the free energy of water

• Water molecules are less able to move

• Fewer water molecules available for osmosis

Factors affecting Ψₛ:

Calculating solute potential:

$\Psi_s = -iCRT$

Where:

• i = Ionisation constant (number of particles per molecule; e.g., NaCl = 2)

• C = Molar concentration (mol/L)

• R = Pressure constant (8.314 kPa·L·mol⁻¹·K⁻¹)

• T = Temperature (Kelvin)

Pressure Potential (Ψₚ)

Also called turgor pressure (in plant cells)

Definition: The physical pressure exerted on water.

Key points:

• Can be positive, zero, or negative

• In turgid plant cells: positive (cell wall pushing back against contents)

• In flaccid cells: approximately zero

• In xylem under tension: can be negative

• Atmospheric pressure is the reference (Ψₚ = 0 at atmospheric pressure)

In plant cells:

• Water enters by osmosis → cell swells

• Cell wall resists expansion → exerts pressure on contents

• This pressure is turgor pressure (positive Ψₚ)

• Turgor pressure increases water potential, reducing further water uptake

In animal cells:

• No rigid cell wall

• Cannot generate significant positive pressure potential

• Ψₚ ≈ 0 in most animal cells

Water Movement and Osmosis

The Principle

Water moves from higher water potential to lower water potential

This is a passive process — it does not require energy input.

$\text{Water moves: High } \Psi \rightarrow \text{ Low } \Psi$

Osmosis Defined

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

Key features:

• Passive process (no ATP required)

• Requires a selectively permeable membrane

• Water moves down its water potential gradient

• Solutes cannot cross the membrane (or cross slowly)

Terminology for Solutions

Water movement:

• From hypotonic (high Ψ) to hypertonic (low Ψ)

• Until equilibrium reached (isotonic, or Ψ equalised)

Water Potential in Plant Cells

Plant Cell Structure Relevant to Water Relations

States of Plant Cells

1. Turgid Cell

Condition: Cell placed in hypotonic solution (higher Ψ outside)

Process:

1. Water enters by osmosis

2. Vacuole swells

3. Cytoplasm pushes against cell wall

4. Cell wall resists expansion → creates turgor pressure (positive Ψₚ)

5. Turgor pressure increases Ψ inside cell

6. Water entry slows and stops when Ψ inside = Ψ outside

At full turgor: $\Psi = \Psi_s + \Psi_p = 0$

The positive Ψₚ exactly balances the negative Ψₛ

Example:

• Ψₛ = −800 kPa

• Ψₚ = +800 kPa

• Ψ = −800 + 800 = 0 kPa

Significance:

• Provides support and rigidity to non-woody plants

• Maintains cell shape

• Drives cell expansion during growth

• Keeps leaves and stems upright

2. Flaccid Cell

Condition: Cell in isotonic solution OR beginning to lose water

Process:

1. No net water movement (isotonic) OR water leaving cell

2. Vacuole shrinks slightly

3. Cell contents no longer push firmly against wall

4. Turgor pressure is low or zero (Ψₚ ≈ 0)

State: $\Psi = \Psi_s + 0 = \Psi_s$

Water potential equals solute potential (negative)

Appearance:

• Cell is soft, not firm

• Plant wilts (if many cells flaccid)

3. Plasmolysed Cell

Condition: Cell placed in hypertonic solution (lower Ψ outside)

Process:

1. Water leaves cell by osmosis

2. Vacuole shrinks significantly

3. Cytoplasm pulls away from cell wall

4. Plasma membrane separates from cell wall

5. Space between membrane and wall fills with external solution

Stages of plasmolysis:

At plasmolysis:

• Ψₚ = 0 (no pressure against wall)

• Ψ = Ψₛ (water potential equals solute potential)

• Cell is at its minimum volume without membrane damage

Plasmolysis is usually reversible if cell is returned to hypotonic solution before permanent damage occurs.

Turgid Cell:          Flaccid Cell:         Plasmolysed Cell:
┌──────────────┐      ┌──────────────┐      ┌──────────────┐
│ ┌──────────┐ │      │ ┌──────────┐ │      │              │
│ │          │ │      │ │          │ │      │   ┌──────┐   │
│ │  Vacuole │ │      │ │  Vacuole │ │      │   │      │   │
│ │          │ │      │ │          │ │      │   │      │   │
│ └──────────┘ │      │ └──────────┘ │      │   └──────┘   │
└──────────────┘      └──────────────┘      └──────────────┘
 Cell wall pressed     Wall not pressed      Membrane pulled
 by contents           by contents           away from wall
 Ψₚ = high (+)         Ψₚ ≈ 0                Ψₚ = 0

Summary: Plant Cell States

Water Potential in Animal Cells

Animal cells lack a cell wall, so they respond differently to osmotic stress.

States of Animal Cells

In Hypotonic Solution

Process:

1. Water enters by osmosis

2. Cell swells

3. No cell wall to resist expansion

4. Cell may burst (lysis/cytolysis)

For red blood cells: Called haemolysis

In Isotonic Solution

Process:

1. No net water movement

2. Cell maintains normal shape and volume

3. Normal physiological state

Example: Blood plasma is isotonic to red blood cells (~0.9% NaCl)

In Hypertonic Solution

Process:

1. Water leaves by osmosis

2. Cell shrinks

3. Cell becomes crenated (wrinkled/shrivelled)

For red blood cells: Called crenation

Hypotonic:          Isotonic:           Hypertonic:
    ╱───╲              ○                  ○╱╲○
   │     │         ╱───────╲              ∨∨
    ╲___╱         │         │            ╱∧∧╲
                   ╲_______╱              ○╲╱○
  (Swollen/         (Normal)            (Crenated/
   Lysed)                                shrivelled)

Osmoregulation in Animals

Animals must regulate water and solute balance to maintain isotonic conditions:

Water Movement in Plants

Pathway of Water Through a Plant

Water moves from soil → root → stem → leaf → atmosphere

1. Water Uptake by Roots

Water potential gradient: \Psi_{\text{soil}} > \Psi_{\text{root hair}} > \Psi_{\text{cortex}} > \Psi_{\text{endodermis}} > \Psi_{\text{xylem}}

Root hair cells:

• Have lower Ψ than soil water

• Water enters by osmosis

• Large surface area for absorption

Pathways across root:

Casparian strip:

• Waxy band in endodermis cell walls

• Blocks apoplast pathway

• Forces water through cell membranes (symplast)

• Allows selective uptake of minerals

• Prevents backflow of water

2. Transport in Xylem

Cohesion-Tension Theory:

Water potential in xylem:

• Often very negative (−0.5 to −3.0 MPa)

• Tension pulls water upward

• Ψₚ is negative in xylem vessels

3. Water Loss from Leaves

Transpiration:

1. Water evaporates from cell walls of mesophyll cells

2. Water vapour diffuses through air spaces

3. Exits through stomata

4. Creates water potential gradient that pulls more water up

Stomatal control:

• Guard cells regulate stomata opening

• Open: gas exchange and transpiration occur

• Closed: reduces water loss but limits CO₂ uptake

Water Potential Gradient in a Plant

                        Atmosphere
                        Ψ = −100 MPa (very low)
                             ↑
                        ┌────────┐
                        │  Leaf  │ Ψ = −1.5 MPa
                        └────────┘
                             ↑
                        ┌────────┐
                        │  Stem  │ Ψ = −0.8 MPa
                        │ (Xylem)│
                        └────────┘
                             ↑
                        ┌────────┐
                        │  Root  │ Ψ = −0.3 MPa
                        └────────┘
                             ↑
                        ┌────────┐
                        │  Soil  │ Ψ = −0.1 MPa
                        └────────┘

        Water flows DOWN the water potential gradient
        (from higher/less negative to lower/more negative)

Calculating Water Potential

Basic Calculations

Using Ψ = Ψₛ + Ψₚ

Example 1: Fully turgid cell

• Ψₛ = −1200 kPa

• Ψₚ = +1200 kPa

• Ψ = −1200 + 1200 = 0 kPa

Example 2: Flaccid cell

• Ψₛ = −800 kPa

• Ψₚ = 0 kPa

• Ψ = −800 + 0 = −800 kPa

Example 3: Partially turgid cell

• Ψₛ = −1000 kPa

• Ψₚ = +400 kPa

• Ψ = −1000 + 400 = −600 kPa

Determining Direction of Water Movement

Compare water potentials:

• Water moves from higher Ψ to lower Ψ

• "Higher" means less negative OR more positive

• "Lower" means more negative OR less positive

Example:

• Cell A: Ψ = −400 kPa

• Cell B: Ψ = −700 kPa

• Water moves from A to B (−400 is higher than −700)

Incipient Plasmolysis Calculations

At incipient plasmolysis:

• Ψₚ = 0 (no turgor pressure)

• Ψ = Ψₛ (water potential equals solute potential)

• External solution Ψ = Cell Ψ (at equilibrium)

This allows determination of cell Ψₛ: If a cell is at incipient plasmolysis in 0.4 M sucrose solution with Ψ = −1000 kPa:

• Cell Ψₛ = −1000 kPa (since Ψₚ = 0)

Experimental Determination of Water Potential

Method 1: Incipient Plasmolysis

For plant cells (e.g., onion epidermis):

1. Prepare solutions of different concentrations (e.g., 0.1M to 0.6M sucrose)

2. Place tissue samples in each solution

3. Observe cells under microscope after equilibration

4. Determine concentration at which 50% of cells are plasmolysed

5. This is the incipient plasmolysis point

6. At this point: External Ψ = Cell Ψₛ

Calculation:

• Use tables or formula to find Ψ of the solution at incipient plasmolysis

• This equals the solute potential of the cell

Method 2: Change in Mass/Length

For plant tissues (e.g., potato cylinders):

1. Cut identical cylinders of tissue

2. Measure initial mass and/or length

3. Place in solutions of different concentrations

4. Leave for equilibration (30–60 minutes)

5. Blot dry and measure final mass/length

6. Calculate percentage change

$\text{Percentage change} = \frac{\text{Final} - \text{Initial}}{\text{Initial}} \times 100$

Analysis:

• Plot percentage change vs. solution concentration

• Find concentration where change = 0 (equilibrium)

• At this point: External Ψ = Tissue Ψ

Interpretation:

• Positive change (gain): Tissue was in hypotonic solution

• Negative change (loss): Tissue was in hypertonic solution

• Zero change: Tissue was in isotonic solution

% Change
in mass
    │
 +20├───●
    │     ●
 +10├───────●
    │         ●
   0├───────────●─────── ← Isotonic point
    │             ●        (Tissue Ψ)
 -10├───────────────●
    │                 ●
 -20├─────────────────────
    └─────────────────────→
      0.1  0.2  0.3  0.4  0.5
         Sucrose concentration (M)

Method 3: Chardakov Method

Using density changes:

1. Prepare solutions of known concentrations

2. Add small amount of methylene blue to each

3. Cut tissue pieces and place in each solution

4. Leave for equilibration

5. Remove tissue

6. Using a pipette, add drop of coloured solution to a tube of uncoloured solution of same concentration

7. Observe whether drop rises, sinks, or stays in place

Interpretation:

• Drop sinks: Tissue absorbed water; solution became more concentrated/dense

• Drop rises: Tissue lost water; solution became more dilute/less dense

• Drop stays: No net water movement; isotonic

Water Potential and Osmoregulation

Freshwater vs Saltwater Organisms

Freshwater Fish

Challenge: Body fluids are hypertonic to surrounding water

• Water constantly enters by osmosis (through gills, skin)

• Salts tend to diffuse out

Osmoregulatory mechanisms:

1. Do not drink water

2. Produce large volumes of dilute urine

3. Actively absorb salts through gills (chloride cells)

4. Obtain salts from food

Saltwater Fish

Challenge: Body fluids are hypotonic to surrounding water

• Water constantly lost by osmosis

• Salts tend to diffuse in

Osmoregulatory mechanisms:

1. Drink large amounts of seawater

2. Absorb water in gut

3. Actively secrete salts through gills (chloride cells)

4. Produce small volumes of concentrated urine

5. Excrete excess salt through specialised cells

Anadromous Fish (e.g., Salmon)

• Migrate between freshwater and saltwater

• Must switch osmoregulatory mechanisms

• Physiological changes during transition

Practical Applications

Agriculture

Irrigation:

• Soil water potential must be higher than root water potential

• Saline soils have low (very negative) water potential

• Crops cannot absorb water from saline soils

• Leads to wilting and crop failure

Fertiliser application:

• Excessive fertiliser lowers soil water potential

• Can cause "fertiliser burn" (plasmolysis of root cells)

• Must be applied at appropriate concentrations

Food Preservation

Salting and sugaring:

• High salt/sugar concentration creates hypertonic environment

• Microbial cells lose water (plasmolysis)

• Inhibits growth of bacteria, fungi

• Examples: salted fish, jam, honey

Medicine

Intravenous (IV) solutions:

• Must be isotonic to blood (~0.9% NaCl or 5% glucose)

• Hypotonic solutions cause haemolysis

• Hypertonic solutions cause crenation

Horticulture

Understanding wilting:

• Soil water potential too low (drought, salinity)

• Cannot maintain turgor pressure

• Loss of support → wilting

Common Exam Questions

Typical Question Types

1. Define water potential and its components (4 marks)

   • Water potential (Ψ): tendency of water to move; measured in kPa

   • Ψ = Ψₛ + Ψₚ

   • Solute potential (Ψₛ): effect of solutes; always negative or zero

   • Pressure potential (Ψₚ): effect of physical pressure; can be positive, zero, or negative

2. Explain how water moves into plant root cells (4 marks)

   • Soil water has higher water potential than root cells

   • Water moves from high Ψ to low Ψ

   • Water enters root hair cells by osmosis

   • Creates water potential gradient across cortex to xylem

3. Describe what happens to a plant cell in hypertonic solution (5 marks)

   • Cell has higher water potential than solution

   • Water leaves cell by osmosis

   • Vacuole shrinks

   • Cytoplasm pulls away from cell wall

   • Plasmolysis occurs

   • Ψₚ = 0 at incipient plasmolysis

4. Compare the response of plant and animal cells to hypotonic solutions (4 marks)

   • Both: water enters by osmosis

   • Plant: cell swells; wall resists; becomes turgid; Ψₚ increases

   • Animal: cell swells; no wall; may lyse (burst)

   • Plant cells survive due to cell wall; animal cells may be destroyed

5. Calculate water potential given values (3 marks)

   • Apply formula: Ψ = Ψₛ + Ψₚ

   • Show working with units

   • Give final answer with correct sign and units

6. Describe an experiment to determine water potential of plant tissue (5 marks)

   • Cut identical tissue pieces

   • Place in solutions of different concentrations

   • Leave to equilibrate

   • Measure change in mass/length

   • Plot percentage change vs. concentration

   • Find concentration where change = 0 (isotonic point)

   • This gives tissue water potential

Key Terminology Glossary

Summary Comparison Tables

Water Potential Components

Cell States Summary

Plant vs Animal Cell Responses

Freshwater vs Saltwater Fish