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Diffusion—Molecules on the Move

By Angela M. White
Carolina Teaching Consultant

Traditionally, when students perform the diffusion and osmosis experiment using potato slices, they are given a step-by-step procedure that leads to changes in the slices' flexibility. Why not reverse the process for observing osmosis so that it has more of an inquiry-based approach?

Start by giving each group 3 potato slices: one that has been soaked for 30 to 60 min in tap water, one soaked for the same amount of time in distilled water, and one soaked for the same amount of time in salt water. The goal is for students to determine the type of environment each slice has been subjected to and the direction of water flow (into or out of the slice).

You may choose to augment the experiment by performing a demo using another vegetable or by showing images of what happens when cells are placed into the 3 different environments. Ask students to feel and bend each potato slice and record their observations. Next, give each group time to discuss what it believes happened to each potato slice and to develop an experiment to test its hypothesis.

Students will use the following materials to perform their experiments:

  • Whole Potato
  • Knife
  • Electronic Balance
  • Weigh Boats
  • 3 250-mL Beakers
  • Paper Towels
  • Distilled Water
  • Timer
  • Table Salt
  • Forceps
  • Sink (with water faucet)

Each group must first receive your seal of approval before testing its hypothesis. Remind students to put on a lab coat or apron, eye protection, and gloves before starting their experiments. Warn them to use caution when cutting potatoes, or cut the potatoes yourself if you think your students might injure themselves. Once each group has finished its experiment, have it record and analyze the results. Lead a class discussion about how each group approached this problem and ask the class if it has reached any conclusions about the process of water moving into and out of plant tissue. After the discussion, you can introduce the class to a more advanced experiment: the cell membrane simulation.

The cell membrane simulation

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 water are tested for the presence of each component in the test solution.

Teacher preparation

Materials

  • 10% Glucose Solution
  • 0.5% Albumin Solution
  • Distilled Water
  • 2% Starch Solution
  • 5% Sodium Chloride Solution
  • 500-mL Erlenmeyer Flask
  • 5 ft Dialysis Tubing
  • Spool of Thread
  • Pair of Scissors
  • 5 1-mL Plastic Dropping Pipets (per workstation)
  • 1 to 2 mL Biuret Solution (per workstation)
  • 8 Test Tubes (per workstation)
  • 1 Test Tube Rack (per workstation)
  • 1 to 2 mL Benedict’s Solution (per workstation)
  • 1 to 2 mL Silver Nitrate Solution, 0.1 M (2%) (per workstation)
  • 1 to 2 mL Iodine-Potassium Iodide Solution (IKI) (per workstation)
  • 2 8-oz Plastic Cups (per workstation)
  • 1 Glass Pencil (per workstation)

Procedure

  1. Put on lab coat or apron, eye protection, and gloves.
  2. Cut a 15-cm length of dialysis tubing for each workstation.
  3. Cut two 15-cm lengths of thread for each workstation.
  4. Soak the dialysis tubing in distilled water for 30 to 60 min.
  5. Prepare a beaker of test solution as follows, adjusting volumes to provide each workstation with 2 to 4 mL of test solution:
    • Add equal volumes of water and each of the following components to an Erlenmeyer flask: glucose, albumin (0.25 g albumin to 50 mL water or 0.50 g albumin to 100 mL water), starch, and sodium chloride.
    • Mix the solution and label the beaker "Test Solution."
  6. Provide each workstation with the "per workstation" items in the "Materials" list.

Student Instructions

Materials

  • Dialysis Tubing
  • Thread
  • Plastic Dropping Pipets
  • Biuret Solution
  • Test Tubes
  • Test Tube Rack
  • Benedict’s Solution
  • Silver Nitrate Solution
  • Test Solution
  • Distilled Water
  • Iodine-Potassium Iodide Solution (IKI)
  • 8-oz Plastic Cups
  • Glass Pencil

Procedure

  1. Put on lab coat or apron, eye protection, and gloves.
  2. Take a length of dialysis tubing and open it by rolling it between your thumb and index finger.
  3. Seal one end of the tubing by tightly twisting it and tying it off with a piece of thread.
  4. Fill the tubing with test solution using a plastic dropping pipet. Leave enough room to tie off the other end of the tubing.
  5. 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.
  6. Rinse the tubing with distilled water to remove any traces of test solution.
  7. Place it into a plastic cup and fill the cup with distilled water.
  8. Allow the tubing to sit in the cup for 30 to 45 minutes.
  9. Remove it from the cup and rinse it with distilled water.
  10. Open an end of the tubing and pour its contents into a cup. Label the cup "Mixture"
  11. Label 8 test tubes; label the first "1" and continue to the last, "8."
  12. Test for the presence of test solution components as follows. Record the results of all of your tests.
    • Add 10 drops of Benedict's solution and 2 mL solution from the Mixture cup to test tube 1. If the solution changes to an orange 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.
  13. Repeat step 12 substituting test tubes 5 to 8 for tubes 1 to 4 and using water from the cup instead of solution from the Mixture cup.

Conclusion

After performing these 2 experiments, your students should have a good practical understanding of diffusion and osmosis as they relate to cell membranes’ selective permeability. For further exploration of these topics, see the Carolina Science™ catalog or visit Carolina.com for the kits, chemicals, and apparatus needed.

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