Introduction
Water passes into and out of cells by a special type of diffusion called osmosis. Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. All of our cells are surrounded by a selectively permeable membrane through which water molecules can pass. In this simple experiment, your students will use an egg membrane to model how osmosis works in animal cells.
Next Generation Science Standards
- LS1.A: Structure and Function. Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell.
- Science and Engineering Practices: Developing and Using Models
- Crosscutting Concepts: Structure and Function
Considerations
This activity works best for students working in groups of 2 to 3 and takes place over 3 days as follows:
Day 1: Dissolving Eggshells
Day 2: Setting up the experiment
Day 3: Recording data and completing a lab report
Materials
Per Student Group:
- 2 Fresh Eggs
- White Vinegar (about 600 mL)
- 2 Containers (large enough to hold an egg and completely cover it with liquid; 600-mL beakers work well)
- Large Spoon
- Distilled Water (about 300 mL)
- Corn Syrup (about 300 mL)
- Balance
- 2 Small Paper Plates
- Grease Pencil
Preparation and Procedure
The first step is to dissolve the eggshell and expose the membrane. To do this, students soak the eggs in vinegar for 24 hours. Vinegar contains acetic acid that reacts with the calcium carbonate in the shell. When students first place the eggs in vinegar, have them observe the tiny bubbles forming around the eggs. This is evidence that a chemical reaction is taking place. The procedures below include the steps for dissolving the shells and completing the experiment.
- Use the grease pencil to label one container and one paper plate “Egg 1” and the other container and paper plate “Egg 2.” Carefully place an egg into each container.
- Pour enough vinegar over each egg to completely cover it.
- Observe the eggs for a few minutes and note any changes.
- Leave the eggs in their containers for 24 hours.
- Observe the eggs the next day and record your observations.
- Slowly pour the vinegar out of each container. Be very careful not to rupture the egg membranes.
- Carefully remove the eggs using the tablespoon, rinse them with water, and place each on its own labeled paper plate. Set the containers aside for now.
- Measure and record the mass of each egg, then place each egg back into its original container.
- Pour distilled water into the Egg 1 container until the egg is completely covered.
- Pour corn syrup into the Egg 2 container until the egg is completely covered.
- Put the 2 containers in a safe place overnight. Note: Have students make a prediction about what they think will happen to the mass of each egg.
- After 24 hours, observe each egg and record your observations.
- Slowly pour the water or syrup out of each container. Be very careful not to rupture the egg membranes.
- Carefully remove the eggs using the spoon, rinse them with water, and place each on its own labeled paper plate.
- Measure and record the mass of each egg. Calculate and record the change in mass.
Sample Data Table
| Egg | Mass After Soaking in Vinegar (g) | Mass After Soaking in Water or Corn Syrup (g) | Change in Mass |
|---|---|---|---|
|
1 |
75.5 |
110.0 |
+34.5 |
|
2 |
83.5 |
66.5 |
-17.0 |
Conclusion
Students should observe that the egg in distilled water was plump and gained mass, while the egg that was in corn syrup was shriveled and lost mass.
After the experiment, share with your students that egg white is about 90% water and discuss with them how the egg membrane (like a cell membrane) is selectively permeable. It lets some molecules move through—such as water, while it blocks larger molecules—such as sugar.
From students’ understanding of osmosis and diffusion, they should be able to explain that placing the egg in distilled water caused water to move from outside of the egg, where the concentration was higher, to inside of the egg, where the concentration was lower. The reverse happened for the egg placed in corn syrup. Because corn syrup contains a high amount of sugar, water molecules moved from the inside of the egg to an area of lower concentration outside of the egg.
Extensions
- Have students think of a way to make the shriveled egg plump again.
- Have students plan and conduct investigations using other solutions such as salty water, and also with food coloring.
- Have students create a drawing showing how osmosis works. They may also create a physical model using candy pieces to represent water molecules.
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