What is Water Potential?

Water potential

• the force responsible for movement of water in a system

• Has the symbol psi

• Is measured in bars or megapascals

It is a measure of the free energy of water which is less

when it is has to surround solutes.

We are really stuck here, YUK!

Well I told you not to come here!

But, I HAVE to join the party!

Has two components:

• Solute potential (also called osmotic potential) џs which is determined by solute concentration

• Pressure potential џp which results

from exertion of pressure on membranes/walls as water moves in or out; can be positive or negative

The water potential of pure water is given the

value ZERO • Because pure water has the

highest concentration of water molecules, and thus the highest water potential, the water potential of all other solutions must be lower than zero i.e. negative.

Pure water: = 0

Adding solute decreases water

potential!• The more solute there is present

in a solution the more negative it becomes.

• So, solute potential will be a negative number if not pure water.

So hypertonic solutions have negative solute potentials.

water potential = solute potential + pressure potential

(s is pi on your paper)

Water moves from areas of higher water potential to areas of lower water potential (i.e. towards the more negative, concentrated region).

water always "falls" from a high to a low water potential

This will occur until the water potential inside

the cell equals the water potential outside

of the cell.

If this makes no sense whatsoever the key

information to learn is:• The equation given • the water potential of pure

water is zero • water moves from areas of

higher water potential to areas of lower water potential (i.e. towards the more negative region)

A solution in a beaker has sucrose dissolved in water with a solute potential of

-0.7MPa. A flaccid cell is placed in the above beaker with a solute potential of -0.3 bars.

a) What is the pressure potential of the flaccid cell before it was placed in the beaker?

b) What is the water potential of the cell before it was placed in the beaker?

c) What is the water potential in the beaker containing the sucrose?

d) How will the water move?

e) Is the cell hypotonic or hypertonic with respect to the outside initially?

f)If it is hypo/hyper (choose one) tonic – this means that its water potential is

higher/lower (choose one) than the outside.

A solution in a beaker has sucrose dissolved in water with a solute potential of -0.5 bars. A flaccid cell is placed in the above beaker with a solute potential of -0.9 bars.

a) What is the pressure potential of the flaccid cell before it was placed in the beaker?

b) What is the water potential of the cell before it was placed in the beaker?

c) What is the water potential in the beaker containing the sucrose?

d) How will the water move?

e) What is the pressure potential of the plant cell when it is in equilibrium with the sucrose solution outside? Also, what is its final water potential when it is in equilibrium?

f) Is the cell now turgid/flaccid/plasmolysed?

So, what happens when a potato cell in put in pure water?

• Water will move in or out until the wp of the cell will equals the wp surrounding the cell.

• The pressure potential will increase to balance out the solute potential to equal to zero which is the wp of pure water.

• No more net movement of water occurs

So how can we determine the water potential of potato

cells?We place potato cells in different molarities of sucrose. When enough solute is added outside of the potato cells to result in NO more NET movement of water, that is the molarity of the potato cells.

How do you go from molarity of a solution to the solute potential to

figure the wp?• Another equation solute potential = -iCRT I = ionization constant (1 for sucrose) C = molar concentration of sucrose (in this case where no

net gain/loss of water occurs) R = pressure constant (0.0831 liter/bars/mole 0K for

sucrose) T = temperature Kelvin (273 + C)

Units will cancel out to equal bars.

So what is the solute potential of a 0.1 M solution of sucrose at 22 C?

• Solute potential = -iCRT

• i (ionization constant) = 1

• R = 0.0831 (from handbook)

• T = temp K (273 + C of solution)

Ωs = - (1) (0.1) (0.0831) (295)

Ωs = - 2.45 bars

So you will graph the results of the change in weight of the potato cells

in different molarity solutions of sucrose after overnite.

• Where the line crosses the graph at the X axis, representing no gain or loss of water, will be the molarity of the potato cells.

• Then substitute in the equation for solute potential (-iCRT)

So, how do you get the water potential?

Once you determine the solute potential, plug into the equation to determine the water potential. The pressure potential will be zero since water is at equilibrium (no net movement in or out.