Water Potential (Ψ): Definition, Factors, Formula, and How It Works in Plants

 

Water Potential (Ψ): Definition, Factors, and Calculation

What is Water Potential?

Water potential (Ψ) is a measure of the potential energy of water in a system compared to pure water, and it determines the direction in which water will move.
In simple terms, it reflects water’s ability to do work—that is, to move from one place to another and displace other molecules.

The highest water potential possible, under standard atmospheric pressure, is 0, which is the water potential of pure distilled water. This is because pure water has the maximum ability to move and displace other substances.

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Effect of Solute on Water Potential

When solute is added to pure water (without applying external pressure), its water potential decreases.

A lower water potential means water is less likely to move. This is because the solute reduces the chances of forming a concentration gradient that favors water movement out of that solution.

For example, if solute is added to the left side of a semipermeable membrane, the water potential there becomes more negative. Water will then move from the side with higher water potential (right) to the side with more negative water potential (left).

Rule:
💧 Water always moves towards a more negative water potential.

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Water Potential Formula

The total water potential is given by:

$$\Psi = \Psi_S + \Psi_P$$

Where:

• ΨS = Solute (osmotic) potential

• ΨP = Pressure potential

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1. Solute Potential (ΨS)

• Directly related to solute concentration.

• As solute concentration increases, ΨS becomes more negative.

• Pure water has a ΨS of 0.

• If no pressure is applied, water potential equals solute potential.

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2. Pressure Potential (ΨP)

• Refers to physical pressure applied to a solution.

• In plant cells, this is often turgor pressure exerted by the cell wall.

• Positive pressure increases water potential, while negative pressure decreases it.

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Example in Plant Cells

When a plant cell is placed in pure water:

1. Water enters the cell because pure water has a higher Ψ than the cell’s cytoplasm.

2. The central vacuole fills, pushing against the cell wall.

3. This pressure (ΨP) prevents bursting and eventually balances the osmotic potential.

4. At equilibrium, Ψ inside the cell equals Ψ outside, so no net water movement occurs.

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Calculating Solute Potential

The formula for solute potential is:

$$\Psi_S = -iCRT$$

Where:

• i = Ionization constant (number of particles the solute dissociates into)

• C = Molar concentration (mol/L)

• R = Pressure constant (0.0831 liter·bar/mol·K)

• T = Temperature in Kelvin (°C + 273)

Example:
If you know the solute concentration, temperature, and ionization constant, you can calculate ΨS directly using this equation.

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Key Points to Remember

• Pure water has Ψ = 0.

• Adding solute makes Ψ more negative.

• Increasing pressure makes Ψ more positive.

• Water moves from higher to lower water potential.