Membranes, whether cell membranes or the cellophane dialysis membranes often used in labs, allow the passage of certain molecules and prevent the passage of others. The cellophane dialysis membrane permits slow passage of water and other small molecules through itself but does not allow other larger molecules to pass through. This slow passage of water molecules is a type of diffusion, initiated by random Brownian movement. This laboratory serves as a good starting point for activities showing diffusion and osmosis in a non-living system. Students will be able to apply what they learn here to what would happen in living cells.

Scientists conduct investigations for a wide variety of reasons. For example, they may wish to discover new aspects of the natural world, explain recently observed phenomena, or test the conclusions of prior investigations or the predictions of current theories. Cells have particular structures that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world. Inside the cell is a concentrated mixture of thousands of different molecules which form a variety of specialized structures that carry out such cell functions as energy production, transport of molecules, waste disposal, synthesis of new molecules, and the storage of genetic material.

* Students must know how to make length and volume measurements in the metric system. * Students will make observations and draw conclusions. * Students will follow standard safety practices. * Students will work cooperatively in groups. * Students will possess typing skills (if laboratory reports are done using computers).

* Make up solutions for student use ahead of time and have "stock" solutions placed in a central location. * Have students fill their dialysis bags over a sink. I often help them pour to expedite this procedure. * Hand out small pieces of Tes-tape to conserve this resource. * Check and "collect" the dialysis bags at the end of they activity (they are known to "leave" the room with some students). * Review the test results prior to starting; I often write them on the board as a reminder.

* Completed laboratory write up. * Oral review of each group when they are finished with the lab.

Diffusion Through A Membrane Purpose: To provide background for the understanding of diffusion in living cells and of the action of osmosis. Background: Membranes, whether cell membranes or the cellophane dialysis membranes that we shall use, allow the passage of certain molecules and prevent the passage of others. The cellophane dialysis membrane permits slow passage of water and other small molecules through itself but does not allow other larger molecules to pass through. This slow passage of water molecules is a type of diffusion, initiated by random Brownian movement. Materials: 80% glucose solution graduated cylinder starch solution dialysis tubing iodine solution beaker Tes-tape string water metric ruler eyedropper Procedures and Interpretations: Use your textbook or notes to answer the following questions: 1. Define “diffusion”. 2. Define “osmosis”. 3. Is osmosis a kind of diffusion or is diffusion a kind of osmosis? 4. What is meant by a semipermeable membrane? 5. Why must a cell membrane be semipermeable? A. Tie a knot very tightly at a point about 1 cm from one end of a piece of cellophane dialysis tubing and fill the tube to within 5 cm of the top with soluble starch solution. (Perform this procedure over a sink!) B. Add 20 drops of glucose solution to the cellophane bag. C. Tie the end of the cellophane tube tightly with string and rise it under running water to remove any glucose or starch that may have spilled on the outside. Trim off any excess string. D. Place 200 ml of water in a beaker and add 10 ml of iodine to it. E. Place the filled cellophane tube in the water-iodine solution. The development of a bluish-black color is a test for starch. F. After 15 minutes, test the external solution for glucose by placing a small strip of Tes-tape in the water-iodine solution. A yellow or green color on the Tes-tape indicates the presence of glucose. 6. What is the chemical test for the presence of starch? (be sure to include what color the chemical turns). 7. What is the chemical test for the presence of glucose? (be sure to include what color the chemical turns). 8. On the basis of the chemical tests for starch and glucose, what must have happened to the iodine in your experimental set up? (where did it go?) 9. On the basis of the chemical tests for starch and glucose, what must have happened to the starch in your experimental set up? (where did it go?) 10. What happened to the glucose? (where did it go?) 11. What materials have diffused through the membrane? 12. What substance did not diffuse through the membrane? 13. How might you determine if water moved across the membrane? Molecules of iodine are smaller than molecules of glucose: a molecule of starch is made up of many molecules of glucose bonded together. 14. Suggest a hypothesis to account for the failure of starch to diffuse through the membrane. 15. What did you imagine the structure of the cellophane tube to be when you made this hypothesis? (HINT: Is the membrane a solid structure or does it contain tiny pores?) If a membrane permits some substances to pass through and prevents or slows down the passage of other materials, it is said to be semipermeable. 16. Can materials diffuse through a semipermeable membrane in both directions at the same time? 17. What materials diffused in opposite directions in this experiment? 18. What part of the cell did the dialysis bag represent? 19. What type of materials would a cell allow to diffuse in? (HINT: What does a cell need from the environment in order to survive?) 20. What type of materials would a cell allow to diffuse out? (HINT: What type of waste materials does a cell produce?) As you do these experiments with the dialysis tubing, remember t

* Computers (if students type up their laboratory activities)