Diffusion: Molecules on the Move
Dialysis tubing is an excellent way to simulate the cell membrane’s selective permeability. In this experiment, sealed dialysis tubing containing a test solution of albumin, glucose, sodium chloride, and starch is placed in a cup of distilled water. After 30 to 45 minutes, the tubing is removed from the cup and its contents, and the cup of water is tested to determine whether the components have diffused across the membrane.
Diffusion: Molecules on the Move
Materials
Teacher Preparation
- 10% Glucose Solution
- 5% Albumin Solution
- Distilled Water
- 2% Starch Solution
- 5% Sodium Chloride Solution
- 500-mL Erlenmeyer Flask
- Dialysis Tubing, 5 ft
- Spool of Thread
- Pair of Scissors
Student Procedure
- 5 Plastic Dropping Pipets, 1 mL
- 1 to 2 mL Biuret Solution
- 8 Test Tubes
- 1 Test Tube Rack
- 1 to 2 mL Benedict's Solution
- 1 to 2 mL Silver Nitrate Solution, 0.1 M (2%)
- 1 to 2 mL Iodine-Potassium Iodide Solution (IKI)
- 2 Plastic Cups, 8 oz
- 1 China Marker
- Dialysis Tubing
- Thread
- Test Solution
- Distilled Water
- Boiling Water Bath (hot plate, beaker, water)
Preparation and Procedure
Teacher Preparation
- Put on lab coat or apron, eye protection, and gloves.
- Cut a 15-cm piece of dialysis tubing for each workstation.
- Cut two 15-cm pieces of thread for each workstation.
- Soak the dialysis tubing in distilled water for 30 to 60 minutes.
- Prepare a beaker of test solution as follows, adjusting volumes to provide each student group with 2 to 4 mL of test solution:
- Add equal volumes each of the following components to an Erlenmeyer flask:
- 10% glucose
- 0.5% albumin (0.25 g albumin to 50 mL water or 0.50 g albumin to 100 mL water)
- 2% starch
- 5% sodium chloride
- Distilled water
- Mix the solution and label the beaker "Test Solution".
- Add equal volumes each of the following components to an Erlenmeyer flask:
- Provide each workstation with the Student Procedure items in the Materials list.
- Prepare a boiling water bath consisting of a beaker filled with water on a hot plate. The water in the beaker should just be boiling. Place test tube tongs with the hot water bath.
Student Procedure
- Put on lab coat or apron, eye protection, and gloves.
- Take a piece of dialysis tubing and open it by rolling it between your thumb and index finger.
- Seal one end of the tubing by tightly twisting it and tying it off with a piece of thread.
- Fill the tubing with test solution using a plastic dropping pipet. Leave enough room to tie off the other end of the tubing.
- Gather the open end of the tubing between your fingers to remove any air. Twist it tightly and tie it off with a piece of thread.
- Rinse the tubing with distilled water to remove any traces of test solution.
- Place it into a plastic cup and fill the cup with distilled water.
- Allow the tubing to sit in the cup for 30 to 45 minutes.
- Remove it from the cup and rinse it with distilled water.
- Open one end of the tubing and pour its contents into a cup. Label the cup "Mixture".
- Label 8 test tubes; number the first "1" and continue to the last, "8."
- Test for the presence of test solution components in the mixture as follows. Record the results of all your tests.
- Add 10 drops of Benedict's solution and 2 mL solution from the Mixture cup to test tube 1. Place the test tube in a boiling water bath for 5 minutes. If the solution changes to a green, yellow, orange, or red color, then glucose is present.
- Add 10 drops of biuret solution and 2 mL of solution from the Mixture cup to test tube 2. If the solution changes to a purple color, then protein (albumin) is present.
- Add 10 drops of silver nitrate solution and 2 mL of solution from the Mixture cup to test tube 3. If a milky white precipitate forms, then chloride is present.
- Add 10 drops of IKI solution and 2 mL of solution from the Mixture cup to test tube 4. If the solution changes to a blue-black color, then starch is present.
- Repeat step 12, using test tubes 5 to 8 in place of tubes 1 to 4, and using water from the cup instead of solution from the Mixture cup.
Conclusion
In this experiment, students should see that glucose is the only molecule able to move across the cell membrane. This is primarily due to the size of the molecules, as both albumin and starch are larger than glucose. After performing this experiment, your students should have a good start to understanding of the movement of molecules as it relates to cell membranes’ selective permeability.