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

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)  

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.

Units will cancel out to equal bars. 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.