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Understanding Water Potential: Movement and Equilibrium

Learn about water potential and its role in describing water movement, particularly in and out of cells, as well as how it determines the rate and direction of osmosis. Explore the factors that affect water potential in plants and understand how it contributes to equilibrium between solutions of different concentrations.

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Understanding Water Potential: Movement and Equilibrium

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  1. Water Potential Click

  2. Water potential is a concept that helps to describe thetendency of water to move from one area to another, particularly into or out of cells. • Water molecules move randomly. • When water is enclosed by a membrane some of the moving water molecules will hit the membrane, exerting pressure on it. • This pressure is known as water potential.

  3. It is measured in units of pressure. The unit used will be bars. Can be measured in MPa (megapascals) or kPa (kilopascals). • Pure water has a water potential of zero. • A solution will have a lower concentration of water molecules so it will have a negative water potential.

  4. Water Potential • We look at water movement in terms of water potential. (ψ psi) • Two factors: • Solute concentration and pressure • Pure water ψ =0 • The addition of solute lowers the water potential. (negative number) Water potential determines the rate and direction of osmosis.

  5. ψp ψp • Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid. • When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall. • The cell wall gives very little and so pressure starts to build up inside the cell. • This has the tendency to stop more water entering the cell and also stops the cell from bursting. • When a plant cell is fully inflated with water, it is called turgid. • (Pressure potential is called turgor pressure in plants) ψp ψp

  6. Water moves from a place of high water potential to a place of low water potential. This is an open container, so the ψp = 0 This makes the ψ = ψs The ψs =-0.23, so ψ is -0.23 MPa, and water moves into the solution.

  7. Can a solution with a molarity of 0.2 be in equilibrium with a solution with a molarity of 0.4? • YES! • Two solutions will be at equilibrium when the water potential is the same in both solutions. This does not mean that their solute concentrations must be the same, because in plant cells the pressure exerted by the rigid cell wall is a significant factor in determining the net movement of water.

  8. Solute (osmotic) potential (ψs)= –iCRT Example Problem: The molar concentration of a sugar solution in an open beaker has been determined to be 0.3M. Calculate the solute potential at 27°C degrees. Round your answer to the nearest hundredth. What is the water potential?

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