I have taken this classic biology lab activity illustrating the principles of diffusion and osmosis and adapted it as an online activity. I did this lab many times with my 10th grade regular bio class at Kelly High School in Chicago, but it can be used successfully with kids ranging from middle school to AP Bio. Students can read through the background here and make their own graphs, analyze these data, and draw conclusions. 

-Aaron Reedy

Background

Molecules are constantly in motion as a result of a cell's stored kinetic energy, which causes them to bump into each other and move in random new directions. Diffusion is the movement of molecules from an area of where there are many (high concentration) to an area where there are fewer (low concentration). Osmosis is the diffusion of water through a semipermeable membrane. It is important to remember that a semipermeable membrane allows the solvent (usually water) to pass through, but restricts the movement of a solute (a thing dissolved in the solvent). Water will move across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.

When each side of a membrane has equal solute concentration, the solution is said to be isotonic and water molecules will be equally likely to move in both directions across the membrane. In the case of a hypertonic solution, there is more solute outside the cell than inside the cell. Hypertonic solutions cause water molecules to move out of the cell and into the region of higher solute concentration. Conversely, in hypotonic solutions there is a higher solute concentration inside the cell than outside, and water molecules move into the cell. Whenever possible, water will always move from an area of high water concentration/low solute concentration to an area of low water concentration/high solute concentration.

In this activity, we are going to explore osmosis by looking at a dataset produced with a classic classroom experiment. The experiment uses pieces of potato that are placed in six different solutions of water each with a different solute concentration. The solute is sucrose and the concentrations are measured in units of molarity. The solutions range from no solute to a high concentration of solute and are 0.0 (distilled water), 0.2, 0.4, 0.6, 0.8 and 1.0 molar sucrose.

Pieces of potato are cut to similar sizes, weighed, and then placed in one of the six solutions overnight. The next day, the potato pieces are removed from the solutions, blotted dry, and their final masses are recorded. 

Each row in this tidy dataset contains an observation for a single potato piece. Each column in the dataset is a variable and the cells in that column are the values of that variable. The variables recorded for each potato piece are Lab Group Name, Sucrose Concentration (Molarity), Initial Mass (g), Final Mass (g), and Mass Change (%).

To see a video clearly illustrating and explaining the general procedure for this lab, watch Paul Andersen’s Bozeman Science video walkthrough:

The activity

1. Click the yellow Make a graph button to visualize your data. Choose the scatter plot icon and Show Sucrose Concentration (Molar) on the X-axis and Mass Change (%) on the Y-axis.  You can add descriptive statistics like means and medians by checking the box just to the right of the graph. 

Observe patterns in the data:

2.  What are the independent and dependent variables in this experiment?

3. How does Change in Mass (%) change with Sucrose Concentration (Molarity)? 

4. Which substance moved across the cellular membrane in this activity? What is the specific name of the movement in terms of this substance? 

5. Now, change the variable called Sucrose Concentration (Molarity) to a Numeric variable with the dropdown menu right below the variable name near the top of the page. Then add it back to the graph again by clicking the Show button. Finally, add a regression line of best fit by checking the box just to the right of the graph.

What is your best estimate for the natural solute concentration inside a potato cell? Explain how your data is evidence for that estimate. 

6. Which solution is closest to being isotonic with respect to a potato cell? Which solutions were hypertonic/hypotonic? How do you know? 

Challenge question:

7. Using the principles illustrated with these data, explain why you can’t drink seawater when lost at sea.

For a quick explanation of diffusion and osmosis, we highly recommend Paul Andersen’s AP Biology Lab 1: Diffusion and Osmosis video. The explanation of the potato lab starts at 5:36.

Answer key available to teachers upon request. Email info@dataclassroom.com

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