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