1. Water Potential

2. Water Potential Explanation
The water potential of pure water is zero; aqueous solutions of increasing solute concentration have increasingly negative values. Water tends to move from areas of high water potential (more water, close to 0 value) to areas of low (less water, more solute, neg #’s) water potential. Therefore increasing the solute concentration of a solution, decreases the water potential.
The solute surrounds the water and prevents it from moving around so the potential for the water to move is low.
Osmosis in plants is now described in terms of water potential.

3. Ψ = ΨS + ΨP
(Ψs) If the solute concentration of a solution increases, the potential for the water in that solution to undergo osmosis decreases.
(Ψs) Therefore, the more solute that is added to a solution, the more negative its osmotic (solute) potential gets.
(ΨP). If no physical pressure is applied to a solution, then the solute potential is equal to the water potential.
(ΨP). However, if physical pressure is applied to a solution, then it’s water potential (the potential for the water to move and do work) will be affected.

4. Solute potential formula
Solute potential (Ψs) = –iCRT
i = The number of particles the molecule will make in water; for Sucrose this is given as 1 , for NaCl, it’s 2 since NaCl forms 2 ions, Na+ and Cl-
C = Molar concentration (from your experimental data)
R = Pressure constant = liter bar/mole K
T = Temperature in degrees Kelvin = °C of solution

5. Tonicity & Water Potential
A Hypertonic solution has a lot of solute and therefore a low water potential (negative #)
A Hypotonic solution has very little solute and therefore a high water potential (higher negative number, closer to zero)

6. Practice
1. Calculate the solute potential of a 1.0 M sucrose solution at 25 oC and standard atmospheric conditions.
-24.76
2. Calculate the solute potential of a 0.1 M NaCl solution at 22°C.
-4.9
3. If the concentration of NaCl inside the plant cell is 0.15M, which way will the water flow if the cell is placed in the 0.1 M solution?
Into the cell, there is more NaCl in the cell than out.