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Water Potential

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Sarah Blechacz

on 16 September 2013

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Transcript of Water Potential

Water Potential!
predicts which way water diffuses through
abbreviated by greek letter psi
free energy per mole of water
calculated from 2 major components...
1) the solute potential

2) the pressure potential
water potential= pressure potential + solute potential
Added pressure causes the water potential of a solution to increase!

It is measured by pressure
potential (ψρ).

As water enters a cell with a cell wall, pressure will develop inside and will push on the walls of the cell.
Addition of solute causes water potential of a solute to decrease.

The effect of solute on water potential is the solute potential, also called osmotic potential, (ψs).

The more solute, the greater the solute potential, and the lower the water potential.
Water potential in a nutshell:
Water Potential
*water moves from an area of higher water potential to lower water potential
*ψ of pure water in an open beaker = 0

*An increase in positive pressure raises the pressure potential and water potential
*Addition of solute to water lowers solute potential and decreases water potential
The solute potential (ψS) = – iCRT
i = the ionization constant
C = the molar concentration
R = the pressure constant (R = 0.0831 liter bars/mole-K)
T = the temperature in K (273 + °C)
Solute Potential
Turgor Pressure:
Pressure that counteracts the diffusion of water into the cell
: Over time, enough positive turgor pressure builds up to oppose the more negative solute potential of the cell. Eventually, the water potential of the cell equals the water potential of the pure water outside the cell (ψ of cell = ψ of pure water = 0). At this point, a dynamic equilibrium is reached and net water movement ceases
A 0.15 M solution of sucrose at atmospheric pressure (ψP = 0) and 25°C has an osmotic potential of -3.7 bars and a water potential of -3.7 bars. A bar is a metric measure of pressure and is the same as 1 atmosphere at sea level. A 0.15 M NaCl solution contains 2 ions, Na+ and Cl-; therefore i = 2 and the water potential = -7.4 bars.
You will be doing a water potential lab this week- these notes may come in hand when you have to design your experiments.
Refer back to the examples provided if you get confused!

If solute is added to the water surrounding the plant cell, the water potential of the solution surrounding the cell decreases. If enough solute is added, the water potential outside the cell is equal to the water potential inside the cell, and there will be no net movement of water. However, the solute concentrations inside and outside the cell are not equal, because the water potential inside the cell results from the combination of both the turgor pressure (ψP) and the solute pressure (ψS).
When solutes are added...
...water will leave the cell
(moving from an area of higher water potential to an area of lower water potential.)
The water loss causes the cell to lose turgor.
A continued loss of water will cause the cell membrane to shrink away from the cell wall, and the cell will
more solute is added to the water surrounding the cell,
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