Water potential
Water Potential, Ψ* , describes the likelihood water
will diffuse in or out of a cell. Based on two factors:
Ψ = Ψs+ Ψp
• 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
*greek letter psi
Solute Potential, Ψs
(aka solute conc.)
cell
environs
Ψ = Ψs+ Ψp
• 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
Pressure Potential, Ψp
Ψ = Ψs+ Ψp
• if pressure in cell increases, so does Ψ
• living cells usually have positive Ψp
(turgid, not flaccid)
- pressure
0 pressure
+ pressure
Sum of solute, pressure potentials yields water potential
No solute, no pressure
Net diffusion into cell
No net diffusion
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
To quantify Ψ find solute potential:
temperature
Ψs = -iCRT
K = C + 273
ionization factor
sucrose does not
ionize in water
i = 1.0
pressure constant
.0831 L bars/moles K
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 = -24.51bars
Ψ = -24.51bars + Ψp
Since water at atmospheric pressure has a pressure
potential of zero, water pot. = solute pot. (+ 0)
Ψ = -24.51bars