Water potential is a measure of the tendency of water to move from one area to another. It is determined by two factors: solute concentration and pressure. Pure water has a water potential of zero, while solutions have negative water potentials due to the lower concentration of water molecules. Water potential can be calculated from the sum of solute (osmotic) potential, due to solute concentration, and pressure potential, such as the pressure in plant cells from the rigid cell wall. Water will move from areas of high water potential to low water potential.

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2. Water potential is a concept
that helps to describe the
tendency of water to move
from one area to another,
particularly into or out of cells.
oWater molecules move randomly.
oWhen water is enclosed by a membrane some of
the moving water molecules will hit the membrane,
exerting pressure on it.
oThis pressure is known as water potential.

3. It is measured in units of pressure.
The unit used will be bars. Can be
measured in MPa (megapascals) or
kPa (kilopascals).
Pure water has a water potential of
zero.
A solution will have a lower
concentration of water molecules so
it will have a negative water
potential.

4. Water Potential
• We look at water movement in terms
of water potential. (ψ psi)
• Two factors:
– Solute concentration and pressure
• Pure water ψ =0
• The addition of solute lowers the water
potential. (negative number)
Water potential determines the rate
and direction of osmosis.

5. Pressure potential is important in plant cells
because they are surrounded by a cell wall
which, is strong and rigid.
When water enters a plant cell, its volume
increases and the living part of the cell
presses on the cell wall.
The cell wall gives very little and so
pressure starts to build up inside the cell.
This has the tendency to stop more water
entering the cell and also stops the cell from
bursting.
When a plant cell is fully inflated with
water, it is called turgid.
(Pressure potential is called turgor pressure in plants)
ψp
ψp
ψp
ψp

6. Water potential (ψ) =
pressure potential (ψp ) + solute (osmotic) potential (ψs)
Pressure potential (ψp):
In a plant cell, pressure exerted by the
rigid cell wall that limits further water
uptake
Solute potential (ψs): The effect of solute concentration. Pure
water at atmospheric pressure has a
solute potential of zero. As solute is
added, the value for solute potential
becomes more negative. This causes
water potential to decrease also.
*As solute is added, the water
potential of a solution drops, and
water will tend to move into the
solution.

7. Water potential (ψ) =
pressure potential (ψp )
+ solute potential (ψs)
(osmotic)
This is an open
container, so the ψp = 0
This makes the ψ = ψs
The ψs =-0.23, so ψ is
-0.23 MPa, and water
moves into the
solution.
Water moves from a place of high water potential to a place of low water potential.

10. Can a solution with a molarity of 0.2 be in equilibrium
with a solution with a molarity of 0.4?
YES!
Two solutions will be at equilibrium when the water
potential is the same in both solutions. This does not
mean that their solute concentrations must be the
same, because in plant cells the pressure exerted by
the rigid cell wall is a significant factor in determining
the net movement of water.

11. Solute (osmotic) potential (ψs )= –iCRT
i = The number of particles the molecule will make in water; for
NaCl this would be 2; for sucrose or glucose, this number is 1
C = Molar concentration
R = Pressure constant = 0.0831 liter bar/mole K
T = Temperature in degrees Kelvin (273 + °C) of solution
Example Problem:
The molar concentration of a sugar solution in an open beaker
has been determined to be 0.3M. Calculate the solute potential
at 27°C degrees. Round your answer to the nearest hundredth.
What is the water potential?
Answer: -7.48
Solute potential = -iCRT
= -(1) (0.3 mole/1) (0.0831 liter bar/mole K) (300
K)
= -7.48 bar
Water potential = -7.48 + 0, so water potential = -7.48