AP Biology Unit 2 Water Potential

Water Potential (WP)

• Potential (stored) energy of water

• Symbol Ψ (psi)

Water Potential is measured in what 2 pressure units?

• SI unit —> MPa (megapascals)

• Metric Unit —> bar (1 MPa = 10 bars)

Ψ of Pure Water

0 MPa (0 bar)

• Open container; normal atmospheric pressure & temp

Water moves from areas of _______ to ______

HIGH water Potential to LOW water potential

WP is calculated using a formula ________

Ψ = Ψ_s + Ψ_p

• Ψ = total water potential

• Ψ_s = solute potential

• Ψ_p = pressure potential

Solute Potential (Ψ_s)

• The effect of solutes on WP (and thus osmosis)

• Ψ_s = negative ( < 0 MPa)

• More solute —> More negative WP —> More water movement

• Adding solute decreases WP

Formula to calculate Solute Potential

Ψ_s = -iCRT

Pressure Potential

• Physical pressure exerted on a solution

• Greater pressure = higher WP (direct) = less water movement

• As pressure increases in the cell, it drives water out —> Turgor pressure from cell wall

Plants have _______ that prevent _____

rigid cell walls…..bursting

Turgor Pressure _______ inside plant cells as water moves

Increases

If pressure becomes great enough, then _____.

no additional water will be able to move in