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Water Potential AP Lab (potato lab)

Background Information

Water Potential

Water potential is a measure of potential energy of water relative to pure water. This quantifies the tendency of water to move from one area to another through osmosis. Understanding water potential is essential to study water movement within biological systems and their environment.

Water potential is usually abbreviated by Greek letter Y (psi). Water potential is mainly affected by two factors; Solute potential (Ys or Yp) and Pressure potential (Yp). These values can be calculated with the following formula: Y=Ys+Yp. Solute potential is the tendency of a solution to gain water. Solute dissolved in water increases the concentration and decreases the pressure in the solution creating a pull in the surrounding less concentrated solution. This pull contributes water potential of a solution. It actually is a component of water potential of a solution. 

We use some reference values to measure magnitude. For example we take 0 degree celcius as freezing point of water and every other temperature measured in reference to this value. Same thing goes for solute potential. Pure water is considered to have 0 bars pressure. As the solute is added, the pressure decreases. So, solute potential can never be higher than zero. Solute potential can be calculated by the following formula: Ys=-iCRT, where i is he ionization constant (no units), C is the concentration of the solution (in mol/Liter), R is the ideal gas constant (in liter x bar/mol x Kelvin) and T is the temperature (in Kelvin). 

Pressure potential is the physical push on the solution increasing the pressure, so it is always higher than zero. In biological systems this pressure is created by cell wall. As the cell gains water it swells and if there is cell wall surrounding the cell, it will start exerting pressure on water pushing it out of the cell. This two factor (solute potential and pressure potential) determines the water potential of a solution. The difference in water potential between two area will determine what direction the water will move. This concept is very important for biological systems because many life processes (opening-closing stomata, water movement in kidneys, water movement in plants from roots to leaves, transpiration etc.) depend on water potential of interacting areas.



The purpose of this experiment to calculate the water potential of plant cells and observe the effects of solutions with different water potentials. We will use potato cubes for plant cells. 

Materials (per table)

Test tubes, 6                              Test tube rack, 1                                        DIstilled water, 150 mL                               Various concentrations of sucrose solution
Potatoes, 1 large                         French fries cutter or                               Rubber stopper or                                     Sucrose, 
                                                   cork borer                                                 parafilm


1. Prepare sucrose solutions 0 M - 1 M increasing by 0.2 M. (Preparing solutions prior to lab will save you time. Keep solutions in a sealed container to prevent water loss through evaporation.)
2. Cut three potato cubes for each test tube. Make sure potato cubes for the test tube labelled 0 M easily fit to the test tube. (because these potato cubes will gain water and swell since they are in the pure water)
3. Mass three potato cubes and record their value to your data table as initial mass to their corresponding concentrations. 
4. Place potato cubes into the test tubes.
5. Add 20 ml of sucrose solution as they are labelled. Add distilled water to the test tube labelled 0 M, add 0.2 M sucrose solution to the test tube labelled 0.2 M and so on.
6. Seal each of the test tubes with rubber stopper.
7. Leave test tubes aside overnight.
8. Empty the test tubes and mass the potato cubes from each test tube. Record their value as final mass in your data table to their corresponding concentrations.
9. Calculate the % change in mass and draw a graph for % change in mass vs concentration.
10. Find the water potential value using graph.