Water Potential
Ψ = Ψp + Ψs
Water Potential Video

2

what is water potential?
• The potential energy of water compared to pure water
• Mathematical way to determine the movement of water (osmosis)
• Water always moves from areas of
_______ water potential to areas of
_______ water potential

Water Potential Equation
Ψ = Ψp + Ψs

Pressure Potential
In an open
Container/beaker, the pressure potential is
_________bars
5

Solute Potential
• Osmotic Potential
• Distilled Water (Pure Water) has a solute potential of _____ bars. Why?
6

What happens to the number of
“free” water molecules available as the salt crystal dissolves?
• This explains why solute potential is

How can you determine the ionization constant of a substance?
8

How does increasing the concentration (C) change the magnitude of the solute potential of the solution?
9

Calculate the solute potential of a 0.4M sucrose solution at 25 degrees C
10

If the sucrose solution from the previous example is in an open beaker, what is the pressure potential of the solution?
11

What is the water potential of the solution?
12

If a sample of potato cells with a water potential of -4.9bars is placed in this solution, in which direction will osmosis occur?
WHY?
13

How do we know which direction water will move?
• TOWARDS THE HIGHER SOLUTE
CONCENTRATION (from high water potential to low water potential)

Ψs = Solute Potential (tonicity)
Ψp = Pressure Potential (external force)
The units for Ψ are pressure units. Typically bars (torr or mmHg)
If a system is at atmospheric pressure, Ψp is 0
(and therefore water potential = solute potential)
Ex: A solution in a beaker of water

When a cell is in equilibrium with its surrounding solution, the water potentials of them are EQUAL!!

Ψs
Solute (osmotic) potential
• Pure water has a solute potential (Ψs) of zero.
Solute potential can never be positive.
• Adding more solute is a negative experience; the solute potential becomes negative.

Solute potential
• i = The ionization constant for the solute
– for sucrose or glucose, this number is 1
– For NaCl, it would be 2 (Na+, Cl-)
• C = Molar concentration of the solute
• R = Pressure constant = 0.0831 liter bar/mole K
• T = Temperature in Kelvin (273 + °C)

Water potential

Ex: The initial molar concentration of the cytoplasm inside a cell is 2M and the cell is placed in a solution with a concentration of
2.5M.
• Is the solute potential is more negative inside of the cell or in the solution?
• In which direction will the water move?
• Will the cell increase or decrease in mass?
• What is the pressure potential of the cell?
Of the solution?
• Is the cell hyper, hypo or isotonic to the solution?
20

Practice 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 degrees. Round your answer to the nearest hundredth.

• The pressure potential of a solution open to the air is zero. Since you know the solute potential of the solution, you can now calculate the water potential.
• What is the water potential for this example? Round your answer to the nearest hundredth.

Practice Problem
• Determine which of the following solutions will gain the most water if placed into a sample of pure water in a piece of dialysis tubing at the temperature indicated:
Solution Solute
A
B
C
Sucrose
NaCl
Glucose
Concentration
(C)
2M
1M
1M
Temperature
298K
290K
27 C
23