1. Water Potential
Osmosis & Plant cells

2. Plants & water potential
Plants can use the potential energy in water to perform work.
Tomato plant regains turgor pressure – cell pushes against wall due to uptake of water

3. Plants & water potential
The combined effects of
1.) solute concentration
2.) physical pressure (cell wall)
can be measured as Water Potential 
 is measured in kilopascals (kPa) or bars
1 Mpa = 10 atmospheres of pressure or 10 bars

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. Water Potential: an artificial model
Water flows from “hypo” to “hyper”
Or from high  on left to low  on right

6. Pressure Potential – the sum of all pressure on water.
Turgor pressure – force caused by cell membrane pushing against cell wall.
Wall pressure – an equal and opposite force exerted by cell wall.
Other pressures – tension, cohesion, atmospheric, root, etc.
When working problems, use zero for pressure potential in animal cells & open beakers.

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. Ψs Solute (osmotic) potential
Once you know the solute concentration, you can calculate solute potential using the following formula:
Solute potential (ΨS ) = –iCRT

9. Solute potentialΨs = − iCRT
i = The ionization constant
for NaCl this would be 2;
for sucrose or glucose, this number is 1
C = Molar concentration
R = Pressure constant = liter bar/mole K
T = Temperature in degrees Kelvin = °C of solution

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

11. Practice Problem
What is the water potential of a cell with a solute potential of kPa and a pressure potential of 0.43 kPa?

12. Practice Problem
The molar concentration of a sugar solution in an open beaker has been determined to be 0.5M. Calculate the solute potential at 24°C. Round your answer to the nearest hundredth.