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 
 = psi
 is measured in either kilopascals (KPa) or megapascals (MPa)
*1 Mpa = 10 atmospheres of pressure

4. Calculating Water Potential
 = P  S Or
Water = pressure solute
Potential potential potential

5. Solute Potential  S
Solute potential is also called the osmotic potential because solutes affect the direction of osmosis.
 S of any solution at atmospheric pressure is always negative – why?
Answer = less free water molecules to do work

6. Solute Potential  S
Solutes bind water molecules reducing the number of free water molecules  lowers waters ability to do work.

7. P is the physical pressure on a solution.
Pressure Potential P
P is the physical pressure on a solution.
P can be negative  transpiration in the xylem tissue of a plant (water tension). *Think of suction
P can be positive  water in living plant cells is under positive pressure (turgid). *Think of squeezing or applying force

8. Standard for measuring 
Pure water is the standard.
Pure water in an open container has a water potential of zero at one atmosphere (atm) or one bar of pressure.

9. Water Potential: an artificial model
(a) addition of solutes on right side reduces water potential. S = -0.23
Water flows from “hypo” to “hyper” (or from less neg to more neg)
Or from hi  on left to lo  on right

10. Water Potential: an artificial model
(b) adding pressure with plunger  no net flow of water
(c) applying pressure increases water potential solution now has  of +0.07
Water moves right to left

11. Water Potential: an artificial model
(d) negative pressure or tension using plunger decreases water potential on the left.
Water moves from right to left

12. Water relations in plant cells
(b) Flaccid cell in pure water  Water potential is into cell cell becomes turgid

13. Water relations in plant cells
(a) Flaccid cell placed in hypertonic solution Water potential is out of cell  plasmolysis

14. Calculating Solute potential
Need solute concentration
Use the equation
 S = - iCRT
i = # particles molecule makes in water
C = Molar concentration
R = pressure constant liter bar mole oK
T = temperature in degrees Kelvin
= oC

15. Solve for water potential (literal equation)
Knowing solute potential, water potential can be calculated by inserting values into the water potential equation.
 = P  S
In an open container, P = 0

16. Water Potential problems
When calculating ‘Solute Potential’ (-iCRT), the R value will depend on the pressure units given in the problem. If ‘bars’ or ‘atm’ is used, R= L-atm (*bars) / mol 0K. If kPa is used, R= (see below)
UNIVERSAL GAS CONSTANT
R= Latm/molK
R=8.315 dm3kPa/molK

17. A few notes:
R value will depend on the units used for pressure. (see last slide)
The ionization constant will vary, depending on if the solute can ionize in solvent or not. Ex- For NaCl (ionic bond), when dissolved in water it forms 2 ions (Na+ and Cl-), so the value is 2. Ex- For glucose (covalent bond), does not ionize in water, so the value is 1.

18. Let’s Take a class
(self graded) quiz!! (A

19. Practice problem
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?

20. Solution: Ψ s= -iCRT -(1)(0.1M)(0.0831 L bars/mol K)(293 K)=
-2.43 bars Yp=0, so Y= The movement will be into the root cells. Goes from higher (less negative) to lower (more neg).
*M = mol/L
HW- Do the Water Pot. Prac. Prob. sheet

21. Big Idea of Water Potential
So, we can now define osmosis as the movement of water molecules from a region of higher water potential (hypotonic) to a region of lower water potential (hypertonic) through a semi-permeable membrane.
*Hypotonic has less neg value for solute potential; Hypertonic has more neg value

22. Hints & reminders
1. Remember water always moves from [hi] to [lo]. *Hi is less neg value in calculation
2. Water moves from hypo  hypertonic.
3. [Solute] is related to osmotic pressure. Pressure is related to pressure potential.
4. Pressure raises water potential.
5. When working problems, use zero for pressure potential in animal cells & open beakers.
6. 1 bar of pressure = 1 atmosphere