Water Potential
Calculating Water Potential (Van’t Hoff Equation):Water potential (Y) = pressure potential (YP) + solute potential (YS)
Y = YP + YS
Pressure potential (YP): / In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake.
Solute potential (YS):
[ ] = ¯ YS = ¯ Y
Water at atmospheric pressure:
YP = 0
Y = 0 + YS
Y = YS
Water in an open container:
YP = 0, YS = 0,
Y = 0 / Solute potential is sometimes referred to as osmotic potential or Yp
The effect of solute concentration on water potential:
Pure water at atmospheric pressure has a solute potential of zero because there are no solutes and the pressure in the container is zero. For this reason, solute potential is always zero because as solute is added, the value for solute potential becomes more negative because the solution has less water than the same volume of pure water. This causes water potential to decrease also. In sum, as solute is added, the water potential of a solution drops, and water will tend to move into the solution.
When a solution is enclosed by a rigid cell wall, the movement of water into the cell will exert pressure on the cell wall. This increase in pressure within the cell will raise the water potential.
Calculating Solute Potential:
Solute potential (YS) = - ionization constant (i) x Molar concentration (C) x Pressure constant (R) x Temperature oK (T)
YS = -iCRT
i = ionization constant: (sucrose 1.0)
C = Molar concentration:
R = Pressure constant: (0.0831)
T = Temperature oK:
(oC + 273) / The ionization constant is a standard value called the Van’t Hoff Factor and varies for each solute. Sucrose = 1, Sodium Chloride = 2
Molar concentration is the concentration of the solute. (Sometimes called m)
The pressure constant is the ideal gas constant = 0.0831
Temperature in degrees Kelvin is calculated by adding 273 to the degrees celsius.
Tonicity: a measure of the osmotic pressure gradient (as defined by the water potential of the two solutions) of two solutions separated by a semi-permeable membrane.
Hypotonic:
(diluted solution, less solute, more water, less negative yP)
Hypertonic:
(concentrated solution, more solute, less water, more negative yP)
Isotonic:
(equal solute potentials and water potentials, equal water) / When two solutions are isotonic the solute potentials will be equal, and there will be no net movement of water.
When different, the solution that is hypotonic will have higher solute potential (less negative yS) whereas the solution that is hypertonic will have lower solute potential (more negative yS). Difference in solute potentials will cause water molecules to move from a hypotonic solution to a hypertonic solution.
Visual Examples:
Water potential:
Tonicity: