1. Water Potential Made Simple

2. What is Water Potential?
Water potential (psi Ψ) is the potential energy of water per unit volume relative to pure water. It can be any value less than or equal to 0.
Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and capillary action.

3. What are the Units for Water Potential?
We will use bars or megapascals (MPa).
1 MPa = 10 Bars
So 27.3 MPa = _______ Bar
7.88 Bar = _______MPa

4. Formula For Water Potential
Ψ = ΨS + Ψp
Ψ is the water total water potential
ΨS is the solute potential
Ψp is the pressure potential

5. Simple Examples Ψ = -5 bars ΨS = - 2 bars What is the Ψp ?
What is the Ψ if the ΨS = -4.1 bars and the Ψp is 1 bar?
Ψ = bars Ψp = 3.4 bars What is the ΨS ?

6. Let’s Get CRAZY !!!!!

7. Ψp is the Pressure Potential
As plant cells take up water, they tend to swell. The cell wall in plants provides resistance to that swelling. The result is an increased internal pressure in the cell (turgor pressure), which deceases the tendency of the cell to take up more water.
Pressure potential will be greater than or equal to 0. A positive pressure potential will not allow water to enter a cell.
This is the reason why a plant can be surrounded by a HYPOTONIC solution and not explode.
With the help of the cell wall, as water moves in, pressure increases, thereby not allowing any additional water to enter.

8. An Example of 0 Pressure Potential
An open container will have no added pressure, so the pressure potential will be 0.

9. An Example of a Positive Pressure Potential

10. ΨS is the Solute Potential
This is a value calculated using the amount of dissolved solute in a solution. This will be 0 if using distilled water, because there are NO dissolved solutes.
As you dissolve solutes into a solution, the water potential will decrease or become negative. In actuality, you are creating a HYPERTONIC solution. As you know, water will move into a more HYPERTONIC solution Therefor, water will move into a sample with the lower or more negative water potential.

11. If You Add Solute, The Amount Of Water Decreases, Making The Solute Potential Less Than 0, A.K.A a Negative Number

12. Fancy Formula-Don’t Be Scarred!!!!
Ψs = –iCRT
i = Ionization constant or 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 (from your experimental data or it may be given)
R = Pressure constant = liter bar/mole K
T = Temperature in degrees Kelvin = °C of solution

13. It’s All Negative
It is important to remember that the formula start with a – sign, so that means that the solute potential has to be less than 0 as soon as you add just a tiny bit of solute.
This is important because water will begin to move towards the lowest water potential.

14. Let’s Start With
This is the ionization constant. This will be a number that represents how many particles the compound will make when mixed in water.
Most questions will have a sugar, such as glucose or fructose. When these are mixed in water, they will stay together, so the ionization constant is 1.
If you were to use NaCl mixed into water, the Na and Cl separate, making the ionization constant 2.
If you were to use CaSO3 , what is your ionization constant? ______
The higher the ionization constant, the faster the solute potential will become negative.

15. C is the Molar Concentration
This value is the concentration of the solution or cell you are asked about. This number can be a decimal, like .3 M or very concentrated, like 5 M.
The more concentrated, the more negative the solute potential will be.
Remember, you are actually making a more HYPERTONIC solution as you add more solute.

16. R is the Pressure Constant
This will not change and the good news is that it will be given to you on your reference sheet.
The constant is liter ∙ bar/mole ∙ K

17. T is for Temperature
It is important to remember that the temperatures are in Kelvin.
All you need to do is take 273 and add it to the °C given in the question.
20 °C = ________ K
25 °C = ________ K
30 °C = ________ K

18. Let’s Practice

19. 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 C. Round your answer to the nearest hundredth.  

20. You measure the total water potential of a cell and find it to be -0
You measure the total water potential of a cell and find it to be -0.24MPa. If the pressure potential of the same cell is 0.46MPa, what is the solute potential of that cell?

21. If a cell’s ΨP = 3 bars and its ΨS = -4
If a cell’s ΨP = 3 bars and its ΨS = -4.5 bars, what is the resulting Ψ?

22. The cell from question #1 is placed in a beaker of sugar water with ΨS = -4.0 bars. In which direction will the net flow of water be?

23. The original cell from question # 1 is placed in a beaker of sugar water with ΨS = MPa (megapascals). We know that 1 MPa = 10 bars. In which direction will the net flow of water be?

24. The value for Ψ in root tissue was found to be -3. 3 bars
The value for Ψ in root tissue was found to be -3.3 bars. If you take the root tissue and place it in a 0.1 M solution of sucrose at 20°C in an open beaker, what is the Ψ of the solution, and in which direction would the net flow of water be?

25. NaCl dissociates into 2 particles in water: Na+ and Cl-
NaCl dissociates into 2 particles in water: Na+ and Cl-. If the solution in question 4 contained 0.1M NaCl instead of 0.1M sucrose, what is the Ψ of the solution, and in which direction would the net flow of water be?

26. A plant cell with a Ψs of -7
A plant cell with a Ψs of -7.5 bars keeps a constant volume when immersed in an open-beaker solution that has a Ψs of -4 bars. What is the cell’s ΨP?

27. At 20°C, a cell containing 0.6M glucose is in equilibrium with its surrounding solution containing 0.5M glucose in an open container. What is the cell’s ΨP?

28. At 20°C, a cell with ΨP of 3 bars is in equilibrium with the surrounding 0.4M solution of sucrose in an open beaker. What is the molar concentration of sucrose in the cell?