Water Potential
Defines water potential and its components, solute potential and pressure potential, in plant cells.
What is Water Potential?
The potential energy of water per unit volume relative to pure water; it determines the direction of water movement.
Key formula / rule: Water Potential
Key points
- Define water potential and its components.
- Explain the factors affecting water potential.
- Relate water potential to the movement of water in plants.
- Calculate water potential given solute and pressure potentials.
Common exam trap
Confusing water potential with water concentration.
Definitions
- Term
Water Potential (Ψw)
- Meaning
The potential energy of water per unit volume relative to pure water; it determines the direction of water movement.
- Term
Solute Potential (Ψs)
- Meaning
The potential energy component due to the presence of dissolved solutes, which lowers the free energy of water. It is always negative or zero.
- Term
Pressure Potential (Ψp)
- Meaning
The potential energy component due to mechanical pressure, typically positive in plant cells (turgor pressure) and negative in xylem.
- Term
Turgor Pressure
- Meaning
The outward pressure exerted by the protoplast against the cell wall, a component of pressure potential.
Learning objectives
Define water potential and its components.
Explain the factors affecting water potential.
Relate water potential to the movement of water in plants.
Calculate water potential given solute and pressure potentials.
Formulae
- Name
Water Potential
- Note
Ψw is water potential, Ψs is solute potential, Ψp is pressure potential.
- Expression
Ψw = Ψs + Ψp
- Name
Solute Potential (Ideal Solution)
- Note
i = ionization constant, C = molar concentration, R = ideal gas constant, T = absolute temperature. This formula is often used conceptually or for specific calculations.
- Expression
Ψs = -iCRT
Prerequisites
Basic understanding of osmosis and diffusion.
Knowledge of cell structure (cell wall, plasma membrane, vacuole).
Concept of concentration gradients.
Common mistakes
Confusing water potential with water concentration.
Forgetting that solute potential is negative.
Assuming pressure potential is always zero in plant cells.
Not understanding the direction of water movement based on potential differences.
Keywords
Water Potential
Solute Potential
Pressure Potential
Osmosis
Turgor Pressure
Plant Physiology
Water Transport
Free Energy
Practice preview
What is the symbol for water potential?…
easy
Which of the following statements about water potential is INCORRECT?…
medium
If a plant cell is placed in a hypertonic solution, what will happen to its water potential?…
medium