1. Osmosis and Plant Cells
Water Potential
Osmosis and Plant Cells

2. What you need to know!
Plants use the potential energy in water to perform work.
The role of diffusion (osmosis), active transport, and bulk flow in the movement of water and nutrients in plants.
How water potential explains transpiration.

3. Water Potential Ψ Potential Energy of Water
Determines direction of water flow
Determined by two factors: pressure potential (ΨP) and solute potential (ΨS).
The most common unit for water potential problems is bars.
The mathematical equation is:
Ψ = ΨP + ΨS

4. Pressure Potential ΨP
Pressure potential will always be provided for you
Open systems are systems w/out pressure potential
Often cell walls will provide pressure to a cellular example
When not given you may assume it is an open system
ΨP = 0 bars

5. 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)
P can be positive  water in living plant cells is under positive pressure (turgid)

6. Solute Potential ΨS ΨS = -iCRT i = ionization constant
Measure of how easily the solute breaks into ions in water
The value is always given and is normally either 1 or 2
C = molar concentration of solute
This is a measurement of the density of molecules of the solute
Can be determined by graphing data
R = Pressure constant
This will always be the same no matter what: L bars/M K
T = temperature in Kelvin
The temperature will always be given
Sometimes it will be given in Celsius and you will need to convert it into Kelvin: K = °C

7. Solute Potential ΨS Pg 2
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

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

9. Water Potential Ψ
Once you have both pressure potential and solute potential simply add them together:
Ψ = ΨP + ΨS
Make sure to include the units with your answer
Water will always flow from higher water potential to lower water potential

10. Practice:
What is the water potential of a 0.5 M sucrose solution at 20°C in an open system? The ionization constant of sucrose is 1. Round your answer to the tenths place.
Ψ = ΨP + ΨS
Pressure Potential: Open system
ΨP = 0 bars
Solute Potential: ΨS = -iCRT
i = 1, C = .5, R = , T = ( )
ΨS = -(1)(.5)(.0831)(273+20)
Ψs = bars
Ψ = 0 bars bars = bars rounded to bars

11. Practice:
What is the water potential of a plant cell with 0.3 M sucrose solution at 20°C? The cell wall applies 2 bars of pressure on the cellular solution. Round your answer to the tenths place.
Ψ = ΨP + ΨS
Pressure Potential: Cell Wall
ΨP = 2 bars
Solute Potential: ΨS = -iCRT
i = 1, C = .3, R = , T = ( )
ΨS = -(1)(.3)(.0831)(273+20)
Ψs = bars
Ψ = 2 bars bars = bars rounded to -5.3 bars

12. Practice:
Determine the net flow of water if the plant cell from the second practice problem is placed into the sucrose solution from the first practice problem what will be the net flow of water? Justify your answer.
Ψ plant cell = -5.3 bars
Ψ sucrose solution = bars
Remember: Water will always flow from higher water potential to lower water potential
Water will flow out of the cell because the water potential inside the cell is higher than the water potential in the sucrose solution and water always flows from higher to lower potential.