Water potential is the measure of the free energy of water. It is the force responsible for movement of water in a system.

• Has the symbol psi \psi
• Measured in bars or megapascals (MPa)

Components

• Pressure potential $\psi _s$
  • Results from exertion of pressure
• Solute potential $\psi _p$
  • Determined by solute concentration

Calculating Water Potential

$$ \psi = \psi _p = \psi _s $$

Standard for measuring $\psi$

• Pure water is the standard.
• Under normal conditions (1 atm), pure water has a water potential of zero.
• $\psi_s$ becomes more negative the more solute it contains
  • The more solute it contains, the more hypertonic it is.

Movement

Water moves from areas of higher water potential to areas of lower water potential (higher concentration of solute).

Calculating Solute Potential

$$ \psi _s = -iCRT $$

where $i = $ number of particles molecule makes in water

where $C = $ molecular concentration

where $R = $ pressure constant

where $T = $ temperature in Kelvin