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Understanding Water Potential: Movement and Components

Water potential is the force responsible for water movement in a system, determined by solute and pressure potentials. Learn the equation and key concepts.

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Understanding Water Potential: Movement and Components

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  1. What is Water Potential?

  2. Water potential • the force responsible for movement of water in a system • Has the symbol psi • measured in bars, megapascals, or kilopascals (use units in prompt or default to bars)

  3. Has two components: • Solute potential (also called osmotic potential) Psis which is determined by solute concentration • Pressure potential Psip which results from exertion of pressure on membranes/walls as water moves in or out; can be positive or negative

  4. The water potential of pure water is given the value ZERO • Because pure water has the highest concentration of water molecules, and thus the highest water potential, the water potential of all other solutions must be lower than zero i.e. negative.

  5. Pure water: = 0

  6. Adding solute decreases water potential! • The more solute there is present in a solution the more negative it becomes. Solute potential will be a negative number if not pure water.

  7. water potential = solute potential + pressure potential

  8. Water moves from areas of higher water potential to areas of lower water potential (i.e. towards the more negative, concentrated region). water always "falls" from a high to a low water potential

  9. This will occur until the water potential inside the cell equals the water potential outside of the cell.

  10. If this makes no sense whatsoever the key information to learn is: • The equation given • the water potential of pure water is zero • water moves from areas of higher water potential to areas of lower water potential (i.e. towards the more negative region)

  11. water moves  equilibrium water moves 

  12. If a cell’s ΨP = 3 bars and its ΨS = -4.5 bars, what is the resulting Ψ? -1.5 bars 2. The cell from question #1 is placed in a beaker of sugar water with ΨS = -4.0 bars. In which direction will the net flow of water be? -1.5 bars to -4 bars out of the cell/ the cell is plasmolyzed

  13. 3. If a cell’s Ψ = 4 bars and its Ψs = -5.5 bars, what is the resulting Ψp? Ψp = 9.5

  14. How do you go from molarity of a solution to the solute potential? • Another equation solute potential = -iCRT I = ionization constant (1 for sucrose) (2 for NaCl) C = molar concentration of sucrose (in this case where no net gain/loss of water occurs) R = pressure constant (0.0831 liter/bars/mole 0K for sucrose) T = temperature Kelvin (273 + C) Units will cancel out to equal bars.

  15. So what is the solute potential of a 0.1 M solution of sucrose at 22 C? • Solute potential = -iCRT • i (ionization constant) = 1 • C molarity 0.1M • R = 0.0831 (from handbook) • T = temp K (273 + C of solution) Ωs= - (1) (0.1) (0.0831) (295) Ωs = - 2.45 bars

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