


Exercise 1 - Osmosis Across a Membrane

2. Pour approxiamately 15-20 ml of each of the following solutions into separate bags.

Distilled Water 0.4 M sucrose 0.8 M sucrose
0.2 M sucrose 0.6 M sucrose 1.0 M sucrose

4. Blot the bags to remove any sugar that may have spilled, check the bags for leaks.
5. Record the mass of each baggie in the data table.
6. Fill six beakers with enough distilled water to cover your bags. Place a bag in each one (keep track of which bag is in which beaker)
7. Let the bag sit for 20-30 minutes.
                                             -------- While this is running, set up potatoes for exercise 3.
                                             -------- Make a prediction about what you think will happen.

Predict what you think will happen during the experiment.


8. After 20-30 minutes, remove the baggies from the water, and carefully blot dry and record the final weight.



Contents in Bag Initial Mass Final Mass Mass Difference Time in Beaker % Change in Mass Average
Distilled Water            
0.2 M            
0.4 M            
0.6 M            
0.8 M            
1.0 M            


blank graph













5. Did you results match your predictions? Propose an explanation for why your results (either overall or an individual bag) may have differed from what you were expecting.


EXERCISE 2 - Determining the Water Potential of Potato Cells

In animal cells, the movement of water into and out of the cell is influenced by the relative concentration of solute on either side of the cell membrane. If water moves out of the cell, the cell will shrink. If water moves into the cell, the cell may swell or even burst. In plant cells, the presence of a cell wall prevents the cells from bursting, but pressure does eventually build up inside the cell and affects the process of osmosis. When the pressure inside the cell becomes large enough, no additional water will accumulate in the cell even. So movement of water through the plant tissue cannot be predicted simply through knowing the relative solute concentrations on either side of the plant cell wall. Instead, the concept of water potential is used to predict the direction in which water will diffuse through living plant tissues.

In a general sense, the water potential is the tendency of water to diffuse from one area to another. Water potential is expressed in in bars, a metric unit of pressure equal to about 1 atmosphere and measured with a barometer.

Consider a potato cell is placed in pure water. Initally the water potential outside the cell is 0 and is higher than the water potential inside the cell. Under these conditions there will be a net movement of water into the cell. The pressure potential inside the cell will increase until the cell reaches a state of equilibrium.


2. Determine the mass of all 4 potato cylinders together and record.
4. Remove the cylinders from the beakers and record the mass. Determine the temperature of the room. ________
5. Complete the table and graph your results.

Distilled Water        
0.2 M        
0.4 M        
0.6 M        
0.8 M        
1.0 M        


What is the Molar concentration of the cores? ___________

7. Calculate the solute potential ( Ψ ) for the sucose solution using the formula below. Show your work! ___________

Solute Potential Formula

Ψ = -iCRT

i = ionization constant (for sucrose, this is 1 because sucrose does not ionize in water)
C = molar sucrose concentration at equilibrium (determined from graph)
R = pressure constant (0.0831 liter bar/mole °K )
T = temperature °K (273 + °C )

8. Explain water potential and describe how it affects osmosis.


9. Explain how to determine the molarity of a potato.


10. Design an Experiment:

If you are given a solution of sucrose that has an unknown molarity (.2, .6, .8, 1.0), how could you use potatoes, distilled water, or other known solutions to determine the molarity of your unknown? - Be clear in your design, use another page and staple to this one.