Name_______________________________________ Block ________ Date __________________
Diffusion Lab
Cell membranes separate a cell’s organelles from the outside environment. Membranes must
permit some substances across and at the same time prevent others from entering the cell. In our lab
we will be using dialysis tubing, which is selectively permeable just like the cell membrane. We will
create a concentration gradient, or a difference in concentration. This means that substances inside
and outside of the tubing will not be distributed evenly. We will then be observing to see which
substances are able to cross the membrane (tubing) and which substances are not.
We will be using an iodine solution to test for the presence of starch. If starch is present, iodine
solution will turn dark bluish purple, and if no starch is present it will remain brown. You will also be
using glucose test strips, which test for the presence of glucose. If the pad on the test strip remains
blue, glucose is not present; if the pad on the test strip changes color, glucose is present.
Pre-Lab Questions – Answer in complete sentences.
1. What does it mean to say that the cell membrane is selectively permeable?
2. What is a concentration gradient?
3. How is iodine solution used to test for the presence of starch?
4. How are glucose test strips used to test for the presence of glucose?
5. If you had a solution where the iodine turned purple and the glucose test strip did not change
color, what substance(s) would be present? How do you know?
Materials
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15 cm of dialysis tubing
1 beaker or plastic cup
4 glucose test strips
Sugar solution (glucose or dextrose)
Starch solution
Iodine solution
Procedure
1. Copy the data table onto the right-side page of your notebook. Do not take up the entire page; you
will be adding sketches of your observations to this page as well
2. Obtain your dialysis tubing and your plastic cup.
3. Tie the dialysis tubing closed on one end, approximately 3 cm from the end.
4. Measure 5 ml of the starch solution in the graduated cylinder and pour it into the tied tubing.
5. Measure 5 ml of the glucose solution in the graduated cylinder and pour it into the tied tubing.
6. Obtain a glucose test strip and test the inside of the tube for glucose. Record your results in the
Data Table on your lab handout as well as in your notebook.
6. Tie off the other end of the tubing to seal it.
7. Rinse the tubing thoroughly with water and set it on a paper towel.
8. Fill the cup with water to within 3 cm from the top.
9. Obtain a glucose test strip and test the water for glucose. Record your results in the Data Table on
your lab handout as well as in your notebook.
10. Add 10 drops of iodine solution to the cup and stir. Observe the color of the iodine solution in the
cup. Record your observation in the Data Table on your handout and in your notebook.
11. Place the sealed dialysis tubing into the cup.
12. In your notebook (on the same page as the data table), sketch a diagram in color showing the lab
setup, labeling what substances in the cup were inside and outside of the tubing.
13. After 10 minutes, remove the tubing from the cup.
14. Observe the color of the iodine solution both inside and outside of the tube. Record your results
in the Data Table on your lab handout as well as in your notebook.
15. Obtain a glucose test strip and test the water again for glucose. Record your results in the Data
Table on your lab handout as well as in your notebook.
16. Obtain a glucose test strip. Carefully cut open the tubing and test the liquid inside for glucose.
Record your results in the Data Table on your lab handout as well as in your notebook.
17. Clean up by throwing out the dialysis tubing, rinsing the cup, and hanging it on the drying rack.
18. In your notebook (on the same page as the data table), sketch a diagram in color showing the lab
setup after the 10 minutes have passed, labeling what substances were inside and outside of
the tubing.
DataTable
Cup (Outside
Tube) – Initial
Iodine Solution
Color
Glucose Test
Strip (+ or - )
What Substances
are Present?
Cup (Outside
Tube) – After 10
minutes
Dialysis Tubing
(Inside) – Initial
No Lugol’s present
Dialysis Tubing
(Inside) – After 10
minutes
Analysis Questions – Answer in complete sentences and in your own words.
1. At the beginning of the lab, where was the concentration of glucose the greatest?
Did the glucose move across the membrane? If so, in which direction did it move (into the tube or
out of the tube)? How do you know?
2. At the beginning of the lab, where was the concentration of iodine solution the greatest?
Did the iodine solution move across the membrane? If so, in which direction did it move (into the
tube or out of the tube)? How do you know?
3. What evidence supports the statement that starch molecules did NOT move across the
membrane?
4. Using what you know about carbon compounds, explain why a simple sugar like glucose can
move across the membrane, but starch molecules cannot. (Keep in mind that both glucose and
starch are carbohydrates. You may want to refer to your notes on carbon compounds).
5. What process moved each of the substances through the membrane? What caused this process
to occur?
6. Why would we be able to assume that glucose was still inside the tubing, even if we hadn’t tested
for it? Hint: Think about the end result of diffusion.