Water PotentialWater Potential

Water potentialWater potential

(psi)(psi)

• Tendency of a solution to take up Tendency of a solution to take up waterwater

• Water to diffuse from one area to Water to diffuse from one area to anotheranother

Water potentialWater potential

• Two components Two components

• 1. Pressure1. Pressure

• Physical forces, can be positive or negativePhysical 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 2. Solute concentration (or osmotic potential)potential)

• Always negativeAlways negative

Water potentialWater potential

• Water potential is the sum Water potential is the sum

• Solute potential Solute potential ss (osmotic (osmotic potential)potential)

• Pressure potential Pressure potential pp

• Water potential is expressed as:Water potential is expressed as:

= = ss + + pp

Water potentialWater potential

• Pure water Pure water =0=0

• Adding solute lowers potentialAdding solute lowers potential

• Less free water moleculesLess free water molecules

• Water moves from a higher water Water moves from a higher water potential to a lower water potentialpotential to a lower water potential

• Less concentrated (hypotonic) to a Less concentrated (hypotonic) to a more concentrated (hypertonic)more concentrated (hypertonic)

Animal cellsAnimal cells

• Water movement depends only on Water movement depends only on solute concentrations.solute concentrations.

• Hypertonic solution:Hypertonic solution:

• Water moves out & the cell shrinksWater moves out & the cell shrinks

• Hypotonic solution:Hypotonic solution:

• Water moves in & the cell swellsWater moves in & the cell swells

• Bursting (Bursting (LysisLysis) can happen.) can happen.

Animal cellAnimal cell

Plant cellsPlant cells

• Cell wall can exert pressureCell wall can exert pressure

• Prevents cell bursting.Prevents cell bursting.

• Cell wall exerts a large enough Cell wall exerts a large enough pressure pressure

• No additional water can enter the cellNo additional water can enter the cell

• Even if the cell still has a higher solute Even if the cell still has a higher solute concentration than it’s surroundings.concentration than it’s surroundings.

Plant cellsPlant cells

• Flaccid:Flaccid:

• Limp-lost waterLimp-lost water

• Turgid:Turgid:

• Firm-gained waterFirm-gained water

• Plasmolysis:Plasmolysis:

• Plant cell shrinks from cell wallPlant cell shrinks from cell wall

• Lost waterLost water

Plant cellPlant cell

LabLab

                                               

             

                                                 

                         

Fig. 7-UN3Fig. 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. 36-8aFig. 36-8a

(a)

0.1 Msolution

Purewater

H2O

ψP = 0

ψS = 0ψP = 0ψS = −0.23

ψ = 0 MPa

Fig. 36-8bFig. 36-8b

(b)Positivepressure

H2O

ψP = 0.23

ψS = −0.23

ψP = 0

ψS = 0ψ = 0 MPa ψ = 0 MPa

Fig. 36-8cFig. 36-8c

ψP =  ψS = −0.23

(c)

Increasedpositivepressure

H2O

ψ = 0.07 MPa

ψP = 0

ψS = 0ψ = 0 MPa

0.30

Fig. 36-9aFig. 36-9a

(a) Initial conditions: cellular ψ > environmental ψ

ψP = 0 ψS = −0.9

ψP = 0 ψS = −0.9

ψP = 0ψS = −0.7

ψ = −0.9 MPa

ψ = −0.9 MPa

ψ = −0.7 MPa0.4 M sucrose solution:

Plasmolyzed cell

Initial flaccid cell:

Fig. 36-9bFig. 36-9b

ψP = 0ψS = −0.7

Initial flaccid cell:

Pure water:ψP = 0ψS = 0ψ = 0 MPa

ψ = −0.7 MPa

ψP = 0.7ψS = −0.7ψ = 0 MPa

Turgid cell

(b) Initial conditions: cellular ψ < environmental ψ

• Ψs = -iCRTΨs = -iCRT

• i = ionization constanti = ionization constant

• Sucrose=1.0 (sucrose does not ionize Sucrose=1.0 (sucrose does not ionize water)water)

• C = Molar concentration (from C = Molar concentration (from experiment)experiment)

• R = Pressure constant R = Pressure constant (R=0.0831(R=0.0831 liter bars/mole liter bars/mole K)K)

• T = temperature in K (273 + C)T = temperature in K (273 + C)