Water Potential

Water Potential

• the tendency of water to leave one place in favor of another. Water always moves from an area of higher water potential to an area of lower water potential.
• affected by two factors:
• pressure
• the amount of solute.

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Water Potential in Animal Cells

• For example, imagine a red blood cell dropped into distilled water. Water will move into the red blood cell and cause the cell to expand, stretching the flexible membrane. At some point, the pressure of the incoming water will cause the cell to pop, just like an over-filled balloon.

Water Potential in Animal Cells

• Why don't red blood cells pop in the bloodstream?
• Red blood cells don't pop because the blood provides an isotonic environment for the cells.

Water Potential in Plants Cells

• If a plant cell is placed in distilled water, water will enter the cell and the cell contents will expand. However, the elastic cell wall exerts a back pressure, which will limit the net gain of water.

Calculating Water Potential

• Water potential is calculated using the following formula:
• Water potential = pressure potential + solute potential

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Factors that Affect Water Potential

• Pressure potential
• Water will move from the area of high pressure to the area of low pressure
• In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake.
• Solute potential
• Water will move from the area of high solute potential (low solute concentration) to the area of lower solute potential (higher solute concentration)
• The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. 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.

Ψs= -iCRT

• i - Ionization constant
• Greater ionization decreases water potential/increases water movement, OR decrease in ionization increases water potential/decreases water movement.
• C – Concentration
• Increase in concentration decreases water potential/increases water movement, OR Decrease in concentration increases water potential/decreases water movement
• R - Pressure constant
• No change in water potential/movement
• T – Temperature
• Increase in temperature decreases water potential/increases water movement, OR Decrease in temperature increases water potential/decreases water movement.

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Ψs= -iCRT

• i - Ionization constant
• Usually 1 – 2, inclusively
• C – Molar Concentration of Solute
• Increase in concentration decreases water potential/increases water movement, OR Decrease in concentration increases water potential/decreases water movement
• R - Pressure constant (0.0831 liters/mole K)
• No change in water potential/movement
• T – Temperature = in Kelvin (273 + degrees Celsius)
• Increase in temperature decreases water potential/increases water movement, OR Decrease in temperature increases water potential/decreases water movement.

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Practice

• The molar concentration of a sugar solution in an open beaker has been determined to be 0.2 M. Calculate the solute potential at 22 degrees Celsius.
• (1)(.2)(0.0831)(295) = -4.9029 or -5 bars
• What is the overall water potential?
• Water potential = solute potential + pressure potential; in an open beaker, the pressure potential is 0 so the overall water potential is -5 bars.