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Published byGilda Motta Modified over 5 years ago
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Water Potential Measures the tendency of water to leave one place for another
Ψ “psi” Used by botanist to predict water movement into or out of a cell.
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Question If we soak potato CELLS in sucrose which way will water move?
Water always moves from a higher water potential to a lower water potential.
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Look on the Formula Sheet
Ψ = Ψp Ψs Pressure Potential Water Potential Solute Potential Water always moves to a lower, more negative, water potential.
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Ψs= Solute Potential Ψs Solutes are always negative
More solutes = more negative Which way will water move? What is happening to the pressure potential in cell?
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Ψp= Pressure Potential
Usually Positive
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Water Potential in the Potato Cells
Revise your definition: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential to an area of lower water potential Water always moves to a more negative water potential.
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Water Potential Can we figure out the water potential of a cell in a solution at equilibrium?
Ψ= Ψp + Ψs Where there is no % change in mass, the solution in the beaker has the same water potential as the potato cells. Dynamic Equilibrium
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Water Potential at Equilibrium
Ψ of Solution in Beaker = Ψ of Potato = (Ψ= Ψp + Ψs) = (Ψ= Ψp + Ψs) Potato cells
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Water Potential Solution Potato Cells (Ψ= Ψp + Ψs) = (Ψ= Ψp + Ψs)
(open beaker) Ψ=Ψs+(Ψp= 0) = Ψ= Ψp + Ψs Ψ=Ψs = Ψ= Ψp + Ψs Ψs = Ψ= Ψp + Ψs Pure water at atmospheric pressure has a water potential of zero.
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Solute potential of a solution at equilibrium equals water potential of potato cells.
Ψs = (Ψ= Ψp + Ψs ) Ψs = (Ψ= Ψp + Ψs ) Ψs = Ψ ? We don’t know individual components of water potential for the potato cells. We can calculate Ψs and then we will know Ψ.
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CB Workbook, p.118
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To Calculate Ψs Ψs = -iCRT (Formula Chart)
i= Ionization constant (sucrose is 1.0 because it does not ionize). C= Molar Concentration (from line of best fit where the line crosses the x axis) R= Pressure Constant ( liter bars/mole °K T= Temperature °K (273 + °C)
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An example Ψs = -iCRT Ψs= -(1.0)(0.36 mole/liter)( liter bar/mole °K)(295 °K) -8.83 bars This equals the entire Ψ of the cell.
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