Semi-Permeable Membrane Lab
Name: ______________________________
PURPOSE OF THE LAB: (What is the question we are looking to answer?):
PROCEDURE:
1.
Obtain a piece of cellophane dialysis tubing soaking in water. This acts as the semi-permeable
membrane in this experiment. Use a piece of string to tie a very TIGHT knot at a point about 1 cm from
one end of the tube.
2.
Rub the untied end between your thumb and forefinger to open it up. Make sure the dialysis tubing is
still wet when you are trying to open the end.
3.
Fill the bag with the sugar/starch solution in bottles. Tie the top of the bag with a piece of string. The
bag should now look like a mini hot dog. RINSE the bag under water to remove any excess sugar
(glucose) or starch that may have spilled on the outside of the bag.
4.
Place the bag on a paper towel to remove the excess solution. Then, weigh the bag (be sure to press the
ON TARE button on the electronic scale before weighing) and record the weight below.
 Initial weight of the bag: ____________ grams
5.
Place the bag in a beaker that is filled HALF FULL of distilled water. Wait for 8-10 minutes. While
waiting, answer these questions:
 State your hypothesis as to this experiment in regards to the size of the molecules:
 What does it mean to be semi-permeable? (Think about it. What does it mean if you use the prefix
“semi-“? What does permeable mean? Put it together now!):
 Based on your understanding of water, sugar molecules and starch molecules, make a hypothesis as
to how each of the molecules will move in the experiment. For each molecule, draw a picture and
make a prediction as to what will happen. Make sure you explain why you think this will happen:
6.
Test for the presence of sugar (glucose) in the soaking water:
 With a dropper, collect some water from your beaker, and put it in a test tube. (Two squirts of water
from the dropper is enough.) Add to the test tube 5 drops of Benedict’s solution. (This is blue.)
Place the test tube in a beaker of BOILING water for 2-5 minutes. If you observe an ORANGE
color change in your test tube, this indicates the presence of simple sugars (monosaccharides or
disaccharides). Record your result here:
o The color of the sugar test was: __________________________________. This indicates that
_____________________________________________________________________.
7.
While waiting for the results of the sugar test, add a small squirt (enough to make the water a darkish
yellow) of iodine to your beaker (the tubing should still be in there). Iodine will turn dark blue-black in
the presence of starch. Let this sit for about 5 minutes. Note what happens to the colors in the beaker
and record the results.
o The color of the iodine test in the water was _____________________. This means what?:
o The color of the iodine test in the bag was ______________________. This means what?:
8.
Remove the bag from the beaker and place on a paper towel to remove excess water. Weigh the bag
now. Be sure to press the on tare button before you weigh it. Record the weight.
 Weight of the bag after: ____________ grams
9.
Clean up. You may throw away the membrane. Dump the liquids out and clean out the beaker. Put all
materials away. Then answer the summary questions.
Semi-Permeable Membrane Lab Summary Questions
1. Record on lines 1 and 2 the names of the two
substances that you put into the tubing. Record
on lines 3 and 4 the names of the liquids that
you put in the beaker.
1
2
3
4
2. Did the starch molecules pass through the membrane into the beaker? Explain how you figured this out.
3. Explain what happened to the iodine molecules.
4. Did the sugar (glucose) molecules pass through the membrane into the beaker? How did you know?
5. According to your results, how does the size of the starch molecules compare with the size of the sugar
molecules? How does the size of the starch molecules compare with the iodine molecules?
6. The membrane that you used in this lab is semi-permeable. What does this mean, now that you see the
results of the experiment?
A cell membrane is also semi-permeable. What does this mean about how the cell deals with transporting
materials?:
7. What happened to the weight of the baggie over the time of the lab? Explain why this happened:
8. If you tested materials for the content of macromolecules in them, what test would you perform to determine
if the food is high in simple sugars:
What test would you perform to determine if the food is high in starches?:
9. Which type of molecules move in accordance to passive transport?
Which type of molecules would need to move with the assistance of active transport in the cell?