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2. 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.

3.  It is measured in units of pressure, can be measured in kPa, MPa, bar.  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. 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. (*the wall ‘pushes’ back and exerts ‘positive’ pressure) 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

6. Pressure Potential (con’t) Plant cells that are not ‘full’ are called flaccid Open containers and animal cells (no cell walls) have Pressure Potential of 0.

7. 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 (distilled or deionized) 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 (value becomes more negative), and water will tend to move into the solution.

8. 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 -0.23MPa, and water moves into the solution. Water moves from a place of high water potential to a place of low water potential.

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

12. 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 = 0.0831 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) (0.0831 liter bar/mole K) (300 K) = -7.48 bar Water potential= -7.48 + 0, so water potential = -7.48 Yikes, what's that??????

13. Water Potential problems When calculating ‘Solute Potential’ (-iCRT), the R value will depend on the pressure units given in the problem. If ‘bars’ or ‘atm’ is used, R= 0.0821 L-atm (*bars) / mol 0 K. If kPa is used, R= 8.315 (see below) UNIVERSAL GAS CONSTANT R=0.0821 L  atm/mol  K R=8.315 dm 3  kPa/mol  K

14. A few notes: R value will depend on the units used for pressure. (see last slide) The ionization constant will vary, depending on if the solute can ionize in solvent or not. Ex- For NaCl (ionic bond), when dissolved in water it forms 2 ions (Na+ and Cl-), so the value is 2. Ex- For glucose (covalent bond), does not ionize in water, so the value is 1.

15. Let’s Take a class (self graded) quiz!! (A

16. Practice problem The value for Ψ in root tissue was found to be -3.3 bars. If you take the root tissue and place it in a 0.1 M solution of sucrose at 20°C in an open beaker, what is the Ψ of the solution, and in which direction would the net flow of water be?

17. Solution: Ψ s= -iCRT -(1)(0.1M)(0.0831 L bars/mol K)(293 K)= -2.43 bars Yp=0, so Y=-2.43. The movement will be into the root cells. Goes from higher (less negative) to lower (more neg). *M = mol/L HW- Do the Water Pot. Prac. Prob. sheet

18. Big Idea of Water Potential So, we can now define osmosis as the movement of water molecules from a region of higher water potential (hypotonic) to a region of lower water potential (hypertonic) through a semi- permeable membrane. *Hypotonic has less neg value for solute potential; Hypertonic has more neg value

19. Molarity Another way of expressing concentration, the way that we will use most in this course, is called molarity. Molarity is the number of moles of solute dissolved in one liter of solution. The units, therefore are moles per liter, specifically it's moles of solute per liter of solution.

20. Be very careful to distinguish between moles and molarity. So when you're given a problem or some information that says the concentration of the solution is 0.1 M that means that it has 0.1 mole for every liter of solution; it does not mean that it is 0.1 moles molarity = moles of solute liter of solution "Moles" measures the amount or quantity of material you have; "molarity" measures the concentration of that material.

21. What is the molarity of a solution made by dissolving 2.5 g of NaCl in enough water to make 125 ml of solution? molarity = moles of solute liter of solution 2.5 g NaCl x 1 mole NaCl 58.5 g NaCl = 0.0427 mole molarity = 0.0427 mole NaCl 0.125 L =0.34 M NaCl 1 mole of any substance has a mass = to the atom’s atomic mass. Molarity is number of moles per Liter of solution

22. 1- An AP Biology student dissolves 98.4 g of FeSO 4 in enough water to make 2.000 L of solution. What is the molarity of the solution? 0.324 M EXAMPLE PROBLEMS 1 mole. 151.91g x 98.4g = 0.647 moles ÷ 2.00L= 0.324 mol/L Return to Water Potential Discussion