Problem: What are
some factors that influence the movement of materials through
a semi-permeable membrane?
Diffusion is the net movement of molecules from a region
of high concentration to a
region of low concentration. This differs from osmosis
which is the movement of water
across a semi-permeable membrane from an area of higher
concentration of H2O to an
area of lower concentration of H2O.
Diffusion Through a Membrane
To study the cell membrane, you will employ a simple
procedure. Experiments often
utilize a simplified version of the situation that is
being investigated, a model or a
system that is acceptably similar to the actual case,
but which is usually easier for the
experimenter to control.
Materials: glucose test strip,
cup or beaker. dialysis tubing, glucose solution, starch
solution,
and water
Note: You will be expected to wear goggles during
this experiment.
Procedure
1. Obtain a beaker (about 400 ml) or a cup as provided
by your instructor and
fill it approximately 2/3 full of water
(preferably distilled water).
2. Use a glucose test strip to test the water for the
presence of glucose. Record your
results.
3. Take an approximately six inch length of membrane tubing
and soak it in warm water
until it becomes pliable.
4. Tie one end and place about an inch of starch solution
in the bag.
5. After putting the starch solution in the bag, add about
an inch of glucose solution
to the bag.
6. Use your fingers to squeeze the bag on the outside
to mix the starch and glucose
solution thoroughly in the bag.
7. Place 30-35 drops of Lugol's solution (iodine)
in the water in the beaker or cup.
(Enough iodine should be added
until the water in the cup is light brown.
8. Test the bag to be certain that the starch solution
does not leak from either end
and that there are no holes in it.
Wash the bag with water to remove any starch
or glucose adhering to its surface;
give special attention to the tied ends.
9. Place your bag which has now been tied on both
ends and rinsed into the cup or beaker
containing the iodine solution.
10. Wait for approximately 10 minutes and record your
observations.
A "possible" arrangement to help you organize
your data appears below.
| Material Tested |
Result before running the experiment |
Result after running the experiment |
| iodine (Lugol's) |
|
|
| glucose solution |
|
|
| starch solution |
|
|
Your procedure needs to include a drawing of the experimental
setup for this
activity with the components labeled.
Things to test or comment on in your data.
a.) Is there glucose in the water in the cup after
10 minutes? (Use the glucose test strip
to test this.)
b.) Was there a change in the color inside the bag?
c.) Was there a change in the color of the water
in the cup outside the bag?
Hint: If there
was a significant change, you did something wrong with
this experiment.)
Questions which should be answered in
your conclusion.
1. Did glucose leave the inside of the membrane
and go into the beaker? How did you
prove this?
2. Did starch leave the inside of the membrane and
go into the beaker? How did you
prove this?
3. Did iodine enter the membrane from the water
in the beaker? How did you
prove this?
4. Based on this investigation, form a tentative
conclusion in reference to how the
size of molecules influences
their ability to diffuse through a semi-permeable membrane.
Explain/justify your answer.
5. You didn't investigate the diffusion of water
(osmosis) in this experiment, but if you did,
do you think water would diffuse
into the bag from the cup or out of the cup into the
bag? How could you
set up an experiment to test your hypothesis about the
movement
of the water?
