1. Water Potential Measures the tendency of water to leave one place for another
Ψ “psi”
Used by botanist to predict water movement into or out of a cell.

2. Question If we soak potato CELLS in sucrose which way will water move?
Water always moves from a higher water potential to a lower water potential.

3. Look on the Formula Sheet
Ψ = Ψp Ψs
Pressure Potential
Water Potential
Solute Potential
Water always moves to a lower, more negative, water potential.

4. Ψs= Solute Potential Ψs Solutes are always negative
More solutes = more negative
Which way will water move?
What is happening to the pressure potential in cell?

5. Ψp= Pressure Potential
Usually Positive

6. Water Potential in the Potato Cells
Revise your definition: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential to an area of lower water potential
Water always moves to a more negative water potential.

7. Water Potential Can we figure out the water potential of a cell in a solution at equilibrium?
Ψ= Ψp + Ψs
Where there is no % change in mass, the solution in the beaker has the same water potential as the potato cells.
Dynamic Equilibrium

8. Water Potential at Equilibrium
Ψ of Solution in Beaker = Ψ of Potato =
(Ψ= Ψp + Ψs) = (Ψ= Ψp + Ψs)
Potato cells

9. Water Potential Solution Potato Cells (Ψ= Ψp + Ψs) = (Ψ= Ψp + Ψs)
(open beaker)
Ψ=Ψs+(Ψp= 0) = Ψ= Ψp + Ψs
Ψ=Ψs = Ψ= Ψp + Ψs
Ψs = Ψ= Ψp + Ψs
Pure water at atmospheric pressure has a water potential of zero.

10. Solute potential of a solution at equilibrium equals water potential of potato cells.
Ψs = (Ψ= Ψp + Ψs )
Ψs = (Ψ= Ψp + Ψs )
Ψs = Ψ ?
We don’t know individual components of water potential for the potato cells.
We can calculate Ψs and then we will know Ψ.

11. CB Workbook, p.118

14. To Calculate Ψs Ψs = -iCRT (Formula Chart)
i= Ionization constant (sucrose is 1.0 because it does not ionize).
C= Molar Concentration (from line of best fit where the line crosses the x axis)
R= Pressure Constant ( liter bars/mole °K
T= Temperature °K (273 + °C)

15. An example
Ψs = -iCRT
Ψs= -(1.0)(0.36 mole/liter)( liter bar/mole °K)(295 °K)
-8.83 bars
This equals the entire Ψ of the cell.