Water Potential

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

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

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

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

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

Animal cell

Plant cells Cell wall can exert pressure Cell wall can exert pressure Prevents cell bursting. Prevents cell bursting. Cell wall exerts a large enough pressure Cell wall exerts a large enough pressure No additional water can enter the cell No additional water can enter the cell Even if the cell still has a higher solute concentration than it’s surroundings. Even if the cell still has a higher solute concentration than it’s surroundings.

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

Plant cell

Lab

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

ψ = − 0.23 MPa Fig. 36-8a (a) 0.1 M solution Pure water H2OH2O ψ P = 0 ψ S = 0 ψ P = 0 ψ S = − 0.23 ψ = 0 MPa

Fig. 36-8b (b) Positive pressure H2OH2O ψ P = 0.23 ψ S = − 0.23 ψ P = 0 ψ S = 0 ψ = 0 MPa

Fig. 36-8c ψ P = ψ S = − 0.23 (c) Increased positive pressure H2OH2O ψ = 0.07 MPa ψ P = 0 ψ S = 0 ψ = 0 MPa 0.30

Fig. 36-9a (a) Initial conditions: cellular ψ > environmental ψ ψ P = 0 ψ S = − 0.9 ψ P = 0 ψ S = − 0.7 ψ = − 0.9 MPa ψ = − 0.7 MPa 0.4 M sucrose solution: Plasmolyzed cell Initial flaccid cell:

Fig. 36-9b ψ P = 0 ψ S = − 0.7 Initial flaccid cell: Pure water: ψ P = 0 ψ S = 0 ψ = 0 MPa ψ = − 0.7 MPa ψ P = 0.7 ψ S = − 0.7 ψ = 0 MPa Turgid cell (b) Initial conditions: cellular ψ < environmental ψ

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