1. Water Potential
Ψ = Ψp + Ψs
Water Potential Video

2. what is water potential?
A measurement of how likely it is that water will move in to or out of a solution.
Pure water is assigned a potential of 0.
The more negative the potential, the more likely water will move in to the area 2

3. How do we know which direction water will move?
TOWARDS THE HIGHER SOLUTE CONCENTRATION (from high water potential to low water potential) 3

4. Water Potential Ψ = Ψp + Ψs
Water potential is determined by solute potential and pressure potential.
Ψ = Ψp + Ψs
Water moves from regions of high water potential to regions of low water potential.

5. 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)

6. When a cell is in equilibrium with its surrounding solution, the water potentisl of them are EQUAL!!6

7. 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.

8. Solute potentialΨs = − iCRT
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 = liter bar/mole K
T = Temperature in Kelvin (273 + °C)

9. Water potential Figure: 32.1c Caption:
(c) If wall pressure is very high, it can counteract the tendency of water to move via osmosis as in the example given here.

10. In which direction will the water move?
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?

11. 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.

12. The pressure potential of a solution open to the air is zero
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.

13. 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
Concentration (C)
Temperature A
Sucrose 2M
298K B
NaCl 1M
290K C
Glucose
27 C