Water Potential

Water potential Tendency of a solution to take up water Tendency of water to diffuse from one area to another  (psi)

Water potential Two components to water potential 1. Pressure Physical forces, can be positive or negative If positive increased pressure. If negative decreased pressure. 2. Solute concentration (or osmotic potential) Always negative

Water potential Water potential of a solution is the sum Solute potential s (osmotic potential) Pressure potential p Water potential is expressed as: = s + p

Water potential Pure water =0 Adding solute lowers potential Less free water molecules Water moves from a higher water potential to a lower water potential Less concentrated (hypotonic) to a more concentrated (hypertonic)

Animal cells Water movement depends only on solute concentrations. Hypertonic solution the water moves out and the cell shrinks Hypotonic solution the water moves in and the cell swells Bursting (Lysis) can happen.

Animal cell

Plant cells Cell wall can exert pressure and prevents lysis. When the pressure inside the cell becomes large enough No additional water can enter the cell Even if the cell still has a higher solute concentration.

Plant cells Flaccid: Limp-lost water Turgid: Firm-gained water Plasmolysis: Plant cell shrinks from cell wall Lost water

Plant cell

Lab                                                                                                                                         

Environment: 0.01 M sucrose “Cell” 0.01 M glucose 0.01 M fructose Fig. 7-UN3 Environment: 0.01 M sucrose 0.01 M glucose 0.01 M fructose “Cell” 0.03 M sucrose 0.02 M glucose

(a) 0.1 M solution Pure water H2O ψP = 0 ψS = 0 ψP = 0 ψS = −0.23 Fig. 36-8a (a) 0.1 M solution Pure water Figure 36.8a Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP = 0 ψS = −0.23 ψ = 0 MPa ψ = −0.23 MPa

(b) Positive pressure H2O ψP = 0 ψS = 0 ψP = 0.23 ψS = −0.23 ψ = 0 MPa Fig. 36-8b (b) Positive pressure Figure 36.8b Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP = 0.23 ψS = −0.23 ψ = 0 MPa ψ = 0 MPa

(c) H2O ψP = 0 ψS = 0 ψP = ψS = −0.23 ψ = 0 MPa ψ = 0.07 MPa Fig. 36-8c (c) Increased positive pressure Figure 36.8c Water potential and water movement: an artificial model H2O ψP = 0 ψS = 0 ψP =   ψS = −0.23 0.30 ψ = 0 MPa ψ = 0.07 MPa

(a) Initial conditions: cellular ψ > environmental ψ Fig. 36-9a Initial flaccid cell: ψP = 0 ψS = −0.7 0.4 M sucrose solution: ψ = −0.7 MPa ψP = 0 ψS = −0.9 ψ = −0.9 MPa Plasmolyzed cell ψP = 0 ψS = −0.9 Figure 36.9 Water relations in plant cells ψ = −0.9 MPa (a) Initial conditions: cellular ψ > environmental ψ

(b) Initial conditions: cellular ψ < environmental ψ Fig. 36-9b Initial flaccid cell: ψP = 0 ψS = −0.7 ψ = −0.7 MPa Pure water: ψP = 0 ψS = 0 ψ = 0 MPa Turgid cell ψP = 0.7 ψS = −0.7 Figure 36.9 Water relations in plant cells ψ = 0 MPa (b) Initial conditions: cellular ψ < environmental ψ

Ψs = -iCRT i = ionization constant (for sucrose this is 1.0 because sucrose does not ionize water) C = Molar concentration (from experiment) R = Pressure constant (R=0.0831 liter bars/mole K) T = temperature in K (273 + C)