Diffusion Through A Membrane
Part of Curriculum Unit:Diffusion & Osmosis - Trading Places
Abstract help
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.
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.
National Standards help
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.
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.
Pre-requisite Skills help
* 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).
* 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).
Teacher Information help
* 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.
* 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.
Student Activity help
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
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
Assessment help
* Completed laboratory write up.
* Oral review of each group when they are finished with the lab.
* Oral review of each group when they are finished with the lab.
Enrichment / Alternative Activity help
Perform diffusion and osmosis activities using living cells - see unit plans.
Technology Requirements/Integration help
* Computers (if students type up their laboratory activities)