Osmosis and Diffusion Lab Instructions:
Refer to background information found in AP Biology Investigative Labs: An Inquiry-Based Approach,
investigation 4 Osmosis and Diffusion.
Procedure 1: Plasmolysis
Plasmolysis is the shrinking of the cytoplasm of plant cell in response to diffusion of water out of the cell
and into a hypertonic solution (high solute concentration) surrounding the cell. During plasmolysis the
cellular membrane pulls away from the cell wall. In the next activity you will examine the details of the
effects of highly concentrated solutions on diffusion and cellular contents.
Materials for Procedure 1
Onion
Slides
Lens paper
15% NaCl solution
Microscope
Cover slips
Water
Droppers
Procedure 1
1. Prepare a wet mount of a small piece of the epidermis of an onion. Observe under 100X
magnification. Sketch and describe the appearance of the onion cells in your lab notebook.
2. Add 2 or 3 drops of 15% NaCl solution to one edge of the cover slip. Draw this salt solution
across the slide by touching a piece of paper towel to the fluid under the opposite edge of the
cover slip. Sketch and describe the appearance of the onion cells in your lab notebook. Explain
what has happened.
3. Add 2 or 3 drops of water to one edge of the cover slip. Draw this water solution across the
slide by touching a piece of paper towel to the fluid under the opposite edge of the cover slip.
In your lab notebook, describe what happened.
Procedure 2: Observing Osmosis in Living Cells
In this part of the exercise you will use potato cores placed in different molar concentrations of sucrose
in order to determine the water potential of potato cells.
Materials for Procedure 2
Potatoes, sweet potatoes or yams
Cork borers
Balance
Cups
Sucrose solutions
Metric ruler
Procedure 2
1. Pour 100 mL of each solution (0.0 M Distilled Water, 0.2 M sucrose, 0.4 M sucrose, 0.6 M
sucrose, 0.8 M sucrose, 1.0 M sucrose) into a labeled cup.
2. Use a cork borer to cut four potato cylinders for each cup. Do not include any skin on the
cylinders.
3. Determine the mass of the four cylinders together and record the mass in a data table in your
lab notebook. Put the four cylinders into the appropriate cup.
4. Cover the beaker with plastic wrap to prevent evaporation. Let it stand overnight.
5. Remove the cores from the beaker, blot them gently on a paper towel, and determine their total
mass.
6. Record the final mass in your lab notebook. Calculate the percentage change as you did in
procedure 1. Do this for each solution.
7. Graph both your data for the percentage change in mass.
8. Determine the molar concentration of the potato core. This would be the sucrose molarity in
which the mass of the potato core does not change. To find this draw a line of best fit on the
graph for your data. The point at which this line cross the x-axis represents the molar
concentration of sucrose with a water potential that is equal to the potato tissue water
potential. At this concentration there is no net gain or loss of water from the tissue. Indicate
this concentration of sucrose in your lab notebook.
Procedure 3: Modeling Diffusion and Osmosis
In this experiment you will create models of living cells using dialysis tubing. Like cell membranes,
dialysis tubing is made from a material that is selectively permeable to water and some solutes. You will
fill your model cells with different solutions and determine the rate of diffusion.
Materials for Procedure 3
Distilled or tap water
1 M sucrose
1 M NaCl
1 M Glucose
5% ovalbumin (egg white protein)
20 cm-long dialysis tubing
Cups
Balances
Things to think about…
How can you use weights of the filled cell models to determine the rate and direction of diffusion?
What would be an appropriate control for the procedure you just described?
Suppose you could test other things besides weights of the dialysis tubes. How could you determine the
rates and directions of diffusion of water, sucrose, NaCl, glucose and ovalbumin?
Will protein diffuse? Will it affect the rate of diffusion of other molecules?
Will the water potential of your solutions (outside and inside the cell) impact the direction of osmosis
and/or diffusion?
Procedure 3
1. Design your own experiment using selected materials from the list above. In your lab notebook
record your question, hypothesis and experimental design (including IV, DV, and constants).
2. Dialysis tubing can be used to model cells by tying a knot in one end of the tubing. Fill the tubing
with the desired liquid and knot the other end leaving enough space for diffusion into the cell.
Allow your experiment to run for a minimum of 30 minutes.
3. Record data in your lab notebook. Calculate the percent change in weight using the following
formula: (final – initial)/ initial X 100.
4. Record conclusions in your lab notebook. Use these questions to assist you in the process:
 Which pair(s) that you tested did not have a change in weight? How can you explain
this? Will water potential calculations for each solution support your findings?
 If you compared 1 M solutions, was a 1 M NaCl solution more or less hypertonic that a
1 M sucrose solution? What is your evidence? What about 1 M NaCl and 1M glucose
and 1 M sucrose?
 Does the protein solution have a high molarity? What is evidence for your conclusion?
 How could you test the diffusion of glucose?
 Based on what you learned from your experiment, how could you determine the solute
concentration inside a living cell?
5. Write an individual lab report for this part of the lab activity.
Procedure 4: Water Potential Calculation
Complete the attached exercise, “Calculation of Water Potential from Experimental Data.”