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

 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.

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.

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

Water potential ( ψ ) = pressure potential ( ψ p ) + solute (osmotic) potential ( ψ s ) Pressure potential ( ψ p ): In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake Solute potential ( ψ s ): The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. *As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

Water potential (ψ) = pressure potential (ψ p ) + solute potential (ψ s ) (osmotic) This is an open container, so the ψ p = 0 This makes the ψ = ψ s The ψ s =-0.23, so ψ is MPa, and water moves into the solution. Water moves from a place of high water potential to a place of low water potential.

CCan a solution with a molarity of 0.2 be in equilibrium with a solution with a molarity of 0.4? YYES! T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.

Solute (osmotic) potential ( ψ s )= –iCRT i =The number of particles the molecule will make in water; for NaCl this would be 2; for sucrose or glucose, this number is 1 C =Molar concentration R =Pressure constant = liter bar/mole K T =Temperature in degrees Kelvin (273 + °C) of solution 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? Answer:-7.48 Solute potential= -iCRT = -(1) (0.3 mole/1) ( liter bar/mole K) (300 K) = bar Water potential= , so water potential = -7.48