What is Water Potential?

Water potential the force responsible for movement of water in a system (tells us in which direction water will flow) Has the symbol psi Is measured in bars or megapascals                                   

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

The water potential of pure water is given the value ZERO (bars) 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.

Pure water: = 0 bars

water potential = solute potential + pressure potential

Adding solute decreases water potential! The more solute there is present in a solution the more negative it becomes (because there is less water). So, solute potential will be a negative number if not pure water. So hypertonic solutions have negative solute potentials.

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

This will occur until the water potential inside the cell equals the water potential outside of the cell (equilibrium).

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)  

Sometimes you’ll be asked to do a little extra You’ll need to determine SOLUTE potential

Units will cancel out to equal bars. How do you go from molarity of a solution to the solute potential to figure the wp? Another equation solute potential = -iCRT I = ionization constant - no units, usually between 1 and 2 (1 for sucrose, 2 for NaCl) *increasing ions increases the –I, which lowers the solute potential C = molar concentration of sucrose – moles/liter (in this case where no net gain/loss of water occurs) *remember, the more we add, the lower the solute concentration R = pressure constant (0.0831 liter/bars/mole 0K for sucrose) T = temperature Kelvin (273 + C) Units will cancel out to equal bars.

practice The molar concentration of a sugar solution in an open beaker has been determined to be 0.2M. Calculate the solute potential at 22 degrees Celsius. solute potential = -iCRT

Answer -(1)(0.2 moles/liter)((0.0831 liter/bars/moleK)(295K) Get rid of units and you’ll be left with only bars -(1)(.2)(.0831 bars)(295) Water potential = -4.9029 or -5 bars

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

So, how do you get the water potential? Once you determine the solute potential, plug into the equation to determine the water potential. The pressure potential will be zero since water is at equilibrium (no net movement in or out).

More practice – Biological systems rely heavily on the properties of water movement. Excretion, digestion, and blood pressure are just a few examples of situations where water balance is important. Suppose you have a semi-permeable membrane that ONLY water can pass. On one side of the membrane you have 0.1 M CaCl2. On the other side of the membrane, you have 0.1 M Glucose. CaCl2 ionizes in water to produce 3 ions. Glucose does not ionize in water. Temp is 25 degrees C. Calculate the water potential for each side of the membrane Describe which way water will move and explain your answer

Answer Solute potential for CaCl2 solution = -(3)(0.1)(0.0831)(298) = -7.43 Water potential for CaCl2 solution = -7.43 (open container so no pressure potential) Solute potential for glucose solution = -(1)(0.1)(0.0831)(298) = -2.48 Water potential for glucose solution = -2.48 (open container so no pressure potential)

Answer: Water will move from the right side (glucose solution) to the left side (CaCl2 solution) because water moves from a high water potential (in this case -2.48) to a low water potential (in this case -7.43).