Water Potential

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Water Potential
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Water potential

• Tendency of a solution to take up water
• Tendency of water to diffuse from one area to
  another
• ? (psi)

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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

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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

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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)

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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.

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Animal cell
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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.

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Plant cells

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

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Plant cell

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Lab
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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
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Fig. 36-8a
(a)
0.1 Msolution
Purewater
H2O
?P 0?S -0.23
?P 0?S 0
? -0.23 MPa
? 0 MPa
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Fig. 36-8b
(b)
Positivepressure
H2O
?P 0.23?S -0.23
?P 0?S 0
? 0 MPa
? 0 MPa
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Fig. 36-8c
(c)
Increasedpositivepressure
H2O
?P  ?S -0.23
?P 0?S 0
0.30
? 0.07 MPa
? 0 MPa
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Fig. 36-9a
Initial flaccid cell
?P 0?S -0.7
0.4 M sucrose solution
? -0.7 MPa
?P 0 ?S -0.9
? -0.9 MPa
Plasmolyzed cell
?P 0 ?S -0.9
? -0.9 MPa
(a) Initial conditions cellular ? gt
environmental ?
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Fig. 36-9b
Initial flaccid cell
?P 0?S -0.7
Pure water
? -0.7 MPa
?P 0?S 0
? 0 MPa
Turgid cell
?P 0.7?S -0.7
? 0 MPa
(b) Initial conditions cellular ? lt
environmental ?
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• ?s -iCRT
• i ionization constant (for sucrose this is 1.0
  because sucrose does not ionize water)
• C Molar concentration (from experiment)
• R Pressure constant (R0.0831 liter bars/mole
  K)
• T temperature in K (273 C)