Water Potential
Calculating Water Potential (Van’t Hoff Equation):
Water potential () = pressure potential (P) + solute potential (S)
 = P + S
Pressure potential (P):
Solute potential (S):
[ ] =  S =  
Water at atmospheric pressure:
P = 0
 = 0 + S
 = S
Water in an open container:
P = 0, S = 0,
 = 0
In a plant cell, pressure exerted by the rigid cell wall that limits further water
uptake.
Solute potential is sometimes referred to as osmotic potential or 
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 (S) = - ionization constant (i) x Molar concentration (C) x Pressure constant (R) x Temperature oK (T)
S = -iCRT
i = ionization constant: (sucrose 1.0)
C = Molar concentration:
R = Pressure constant: (0.0831)
o
T = Temperature K:
(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 P)
When two solutions are isotonic the solute potentials will
be equal, and there will be no net movement of water.
Hypertonic:
(concentrated solution, more solute, less water, more
negative P)
When different, the solution that is hypotonic will have
higher solute potential (less negative S) whereas the
solution that is hypertonic will have lower solute
potential (more negative S). Difference in solute
potentials will cause water molecules to move from a
hypotonic solution to a hypertonic solution.
Isotonic:
(equal solute potentials and water potentials, equal water)
Visual Examples:
Water potential:
Tonicity:
Hypertonic Solution or Hypertonicicty
When the osmotic pressure of the solution outside the blood cells in higher
than the osmotic pressure inside the red blood cells, the solution is
hypertonic. The water inside the blood cells exits the cells in an attempt to
equalize the osmotic pressure, causing the cells to shrink or crenate.
Isotonic Solution or Isotonicity
When the osmotic pressure outside the red blood cells is the same as the
pressure inside the cells, the solution is isotonic with respect to the
cytoplasm. This is the usual condition of red blood cells in plasma.
Hypotonic Solution or Hypotonicity
When the solution outside of the red blood cells has a lower osmotic pressure
than the cytoplasm of the red blood cells, the solution is hypotonic with
respect to the cells. The cells take in water in an attempt to equalize the
osmotic pressure, causing them to swell and potentially burst.