1. Water potential Ψ = Ψs+ Ψp
Water Potential, Ψ* , describes the likelihood water will diffuse in or out of a cell. Based on two factors:
• solute concentration gradient
(aka solute potential)
• pressure in the cell (aka pressure potential)
• water moves from the sol’n with higher water
potential to that with lower water potential
Ψ = Ψs+ Ψp
*greek letter psi

2. Ψ = Ψs+ Ψp Solute Potential, Ψs (aka solute conc.) • hypertonic
Ψs in environs > in cell
cell loses water via osmosis
• hypotonic
Ψs in environs < in cell
Turgor pressure, bail, or burst
• Here, cell solute potential has
increased, thus cell water
concentration decreased
• Ψ of pure water = 0, so Ψs is always
negative
Solute Potential, Ψs
(aka solute conc.)
Ψ = Ψs+ Ψp
cell
environs

3. Pressure Potential, Ψp Ψ = Ψs+ Ψp
• if pressure in cell increases, so does Ψ
• living cells usually have positive Ψp
(turgid, not flaccid)
- pressure pressure pressure

4. Sum of solute, pressure potentials yields water potential
No solute, no pressure Net diffusion into cell No net diffusion

5. Again:
(aka solute potential)
At 75, no pressure, but great solute gradient = net diffusion into cell
Negative water potential
At 100, pressure potential prevents continued solute diffusion despite solute potential = no net diffusion
Zero water potential

6. To quantify Ψ find solute potential: Ψs = -iCRT
temperature
K = C + 273
pressure constant
.0831 L bars/moles K
ionization factor
sucrose does not
ionize in water
i = 1.0
molar concentration
moles/L
Ex: solute potential of a 1.0M
sucrose sol’n at 22C at :
std atmospheric pressure
Ψs = -(1)(1.0)(.0831)(295)
Ψs = bars
Ψ = bars + Ψp
Ψ = bars
Since water at atmospheric pressure has a pressure
potential of zero, water pot. = solute pot. (+ 0)