Water Potential

Ψ = Ψp + Ψs

Water Potential Video

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what is water potential?

• A measurement of how likely it is that water will move in to or out of a solution.

• Pure water is assigned a potential of 0.

• The more negative the potential, the more likely water will move in to the area

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How do we know which direction water will move?

• TOWARDS THE HIGHER SOLUTE CONCENTRATION (from high water potential to low water potential) 3

Water Potential

– Water potential is determined by solute potential and pressure potential.

Ψ = Ψp + Ψs

– Water moves from regions of high water potential to regions of low water potential.

Ψs = Solute Potential (tonicity)

Ψp = Pressure Potential (external force)

The units for Ψ are pressure units. Typically bars (torr or mmHg)

If a system is at atmospheric pressure, Ψp is 0 (and therefore water potential = solute potential)

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When a cell is in equilibrium with its surrounding solution, the water potentisl of

them are EQUAL!!

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ΨsSolute (osmotic) potential

• Pure water has a solute potential (Ψs) of zero. Solute potential can never be positive.

• Adding more solute is a negative experience; the solute potential becomes negative.

• i = The ionization constant for the solute– for sucrose or glucose, this number is 1 – For NaCl, it would be 2 (Na+, Cl-)

• C = Molar concentration of the solute

• R = Pressure constant = 0.0831 liter bar/mole K

• T = Temperature in Kelvin (273 + °C)

Solute potentialΨs = − iCRT

Water potential

Ex: The initial molar concentration of the cytoplasm inside a cell is 2M and the cell is placed in a solution with a concentration of

2.5M.

• Is the solute potential is more negative inside of the cell or in the solution?

• In which direction will the water move?• Will the cell increase or decrease in mass?• What is the pressure potential of the cell?

Of the solution?• Is the cell hyper, hypo or isotonic to the

solution?10

Practice Problem

• The molar concentration of a sugar solution in an open beaker has been determined to be 0.3M. Calculate the solute potential at 27 degrees. Round your answer to the nearest hundredth.

• The pressure potential of a solution open to the air is zero. Since you know the solute potential of the solution, you can now calculate the water potential.

• What is the water potential for this example? Round your answer to the nearest hundredth.

Practice Problem

• Determine which of the following solutions will gain the most water if placed into a sample of pure water in a piece of dialysis tubing at the temperature indicated:

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Solution Solute Concentration (C)

Temperature

A Sucrose 2M 298K

B NaCl 1M 290K

C Glucose 1M 27 C