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

Water Potential Ψ = Ψ p + Ψ s

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 osmosis

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 = Ψ p + Ψ s Ψ = Water potential Ψ p = Pressure potential Ψ s = Solute potential

Solute and Pressure Potential • Ψ s = solute potential : solute potential is negative because solutes lower the water potential • Ψ p = pressure potential : physical pressure increases water potential

Osmolarity • As solute concentration increases, so does osmolarity

Sample Problem 1 • If a plant cell’s Ψ p = 2 bars and its Ψ s = - 3.5 bars, what is the resulting Ψ ?

Calculate Solute Potential ( Ψ s ) • Sometimes solute potential must be calculated first • Solute potential becomes more negative as more solute is added • Ψ s = - 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? Ψ s = - 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. Ψ s = - iCRT

Sample Problem 4 • The zucchini core from Problem 3 is placed in a beaker of pure water. Will water diffuse into or out of the plant cell?