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Water Potential
WATER POTENTIAL
• Water potential measures water’s tendency to
leave one place in favor of another
Calculating water potential
• Ѱs + Ѱp = Ѱ
• Solute potential
– Adding solute decreases water potential
• Pressure potential
– Increasing pressure increases water potential
Ѱs =-iCRT
• i=ionization constant (number of ions solute
forms in water)
• C=concentration (mol/L)
• R=pressure constant (.0831 liter bars/mol K)
• T=temperature (C+273)
Ѱp
• Plant cells have cell walls
• Turgor pressure pushes out on the cell wall
• Limits expansion
Equilibrium
• Water moves until the water potential outside
the cell is equal to the water potential inside
the cell
Practice problems
• Calculate the water potential of a 1M sucrose
solution at 22° C
• Calculate the water potential of a cell that has
a pressure potential of 4 and a solute
potential of -5
• What would happen to the cell if it were
placed in the 1M sucrose solution?
• The initial molar concentration of the
cytoplasm inside a plant cell is 2M and the cell
is placed in a solution with a concentration of
2.5M.
– Calculate the water potential of the cell and of the
solution
– Predict which direction water will move based on
your calculations
– Predict what will happen to the pressure potential
of the cell
– What will be the water potential of the cell once
the system has reached equilibrium