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)