Instructor’s Copy
Lab Worksheet – Thou Shalt Not Pass
Introduction: The cell membrane acts as a barrier to some substances while allowing other
substances to enter or leave the cell. When a membrane allows a particular substance to pass
through it, it is said to be permeable to that substance. If a membrane allows the passage of some
molecules or ions but blocks others, the membrane is selectively permeable. Cell membranes of
living organisms are selectively permeable. The cell membrane acts as a selective barrier between
the internal and external environments of the cell. The permeability of the cell membrane can
change in response to changes in the cell’s external and internal environments. Some substances,
such as water, oxygen, and carbon dioxide, can pass freely through a cell membrane. Other
substances have a more limited access to the cell.
In this activity, a nonliving cellophane membrane will be used as a model for a living cell
membrane. Cellophane membranes are selective, but unlike living membranes, their selectivity does
not vary. Water and other small molecules can pass through, but large molecules are blocked. The
diffusion of molecules through the membrane is caused by random molecular movements and
collisions.
Materials:
cellophane dialysis tubing or zip-lock baggie
starch solution
saturated glucose solution
Lugol’s iodine solution
glucose test strip
25 ml graduate
250 ml beakers
pipette
Procedure:
A. Obtain a 15-cm length of cellophane dialysis tubing which has been soaking
in water or a plastic zip-lock baggie. If you are using tubing, fold one end of
the tubing back on itself and tie it with a piece of string. (See Figure 1)
Fig. 1
B. Fill the tube about half full with starch solution. (If using a baggie, add about 25 ml of starch.)
Add 10 ml of glucose solution.
C. Tie the top of the tube tightly with string. (Close the zip-lock baggie completely.) Rinse the
tubing or baggie with running water to remove any spillage from the outside.
D. Pour 10 ml of Lugol’s iodine solution (a test for starch) into a 250 ml beaker half filled with tap
water. Mix it.
E. Place the cellophane tubing (or baggie) in the beaker.
F. After 20 minutes, observe the colors of the solutions in the beaker and in the tubing.
G. With a pipette, take several drops of solution from the bottom of the beaker. Put the drops on a
glucose test strip.
Observations:
1. What color changes do you observe in the beaker and in the tubing?
The liquid inside the tubing turns black while the liquid in the beaker remains orange.
2. What color change do you observe on the glucose test strip? The strip changes color to indicate
the presence of glucose.
Analysis:
1. What does Lugol’s iodine solution test for? It turns black to indicate starch
2. What caused the color change that you observed in step F? the iodine moved into the tubing
3. Was there any evidence of starch in the solution in the beaker? How could you tell?
No starch was in the beaker because the iodine remained orange.
4. What substance was indicated by the test strip to be present in the solution in the beaker?
Glucose was present in the beaker.
5. Which substances diffused through the cellophane tubing? In which direction?
Glucose moved from the tubing to the beaker and iodine moved from the beaker into
the tubing. (Make sure students understand that there is still iodine in the beaker and sugar
in the tubing. Also make sure they understand that water is moving also.)
6. What substance did not diffuse through the cellophane? Starch did not diffuse.
7. If there were no iodine in the beaker, would you observe any color changes during the
experiment? Where? What would the color change indicate?
The only color change would be the test strip which would indicate the movement of
glucose into the beaker from the tubing.
8. Which structure of a typical cell does the cellophane tubing (or baggie) represent?
The selectively permeable cell membrane.
Answer the questions regarding the following situation.
Container 1
Container 2
Bag with syrup (colorless)
Bag with starch solution
(cloudy white)
Beaker with water and iodine
(orange)
The bags are permeable to water and iodine only. Iodine is a starch indicator, it turns black
in the presence of starch. Osmosis and diffusion are both possible, with both water and substances
flowing from areas of high to areas of low concentration by passive transport.
Question
Will the bag get larger or smaller?
What color will be in the bag?
What color will be in the beaker?
Container 1
Larger
Orange
Orange
Container 2
Maybe slightly larger
Black
Cloudy white