Water Potential = p + s Water Potential Used to describe the - - PowerPoint PPT Presentation

Water Potential

Ψ = Ψp + Ψs

Water Potential

• Used to describe 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

Water Potential

• Affected by two factors: pressure and the

amount of solute

• Measured in bars

1 bar = approximately 1 atmosphere (unit of pressure)

• Water potential of pure water is 0 bars

Who cares about water potential?

• Plants do!
• Use water potential to transport water to the

leaves for photosynthesis

• Internal water potential of a plant cell is more

negative than pure water – this causes water to move from the soil to the plant roots via

• smosis

Plants

• Lose water (and turgor pressure) via

transpiration through stomata in the leaves

• If the water potential outside the plant cells is

lower than inside the cells, what happens to the plant?

Water Potential Formula

Ψ = Ψp + Ψs

Ψ = Water potential Ψp = Pressure potential Ψs = Solute potential

Solute and Pressure Potential

• Ψs = solute potential: solute potential is

negative because solutes lower the water potential

• Ψp = pressure potential: physical pressure

increases water potential

Osmolarity

• As solute concentration increases, so does
• smolarity

Sample Problem 1

• If a plant cell’s Ψp = 2 bars and its Ψs = - 3.5

bars, what is the resulting Ψ?

Calculate Solute Potential (Ψs)

• Sometimes solute potential must be

calculated first

• Solute potential becomes more negative as

more solute is added

• Ψs = - iCRT

i = ionization constant

C = concentration (Molarity) R = pressure constant T = temperature in Kelvin

Sample Problem 2

• What is the solute potential of a 1.0 M sugar

solution at 22 °C under standard atmospheric conditions? Ψs = - iCRT

Sample Problem 3

• Zucchini cores are measured and determined

to have a sucrose concentration of 0.36M. Calculate the solute potential using the same temperature and atmospheric conditions as the previous question. Ψs = - iCRT

Sample Problem 4

• The zucchini core from Problem 3 is placed in

a beaker of pure water. Will water diffuse into

• r out of the plant cell?