Name_______________________________
Period_____________
Date __________
Diffusion across a Selectively Permeable Membrane
Introduction:
Each cell is surrounded by a selectively permeable membrane which regulates what gets into and out
of the cell. A selectively permeable membrane allows some types of molecules to diffuse across the
membrane and prevents other types of molecules from crossing the membrane. Small molecules
and water can move freely through a semi-permeable membrane, but larger molecules will pass
through more slowly or perhaps not at all.
In the following experiment, cellophane dialysis tubing serves as an excellent representation of the
cell membrane. For the purpose of this experiment the enclosed tube may be considered a single
living cell in a greatly enlarged form.
★ Pre-Lab Questions (answer all the ★ items)
1. What problems do you think a cell would have if it could not control what got into or out of it?
2. What macromolecule group does starch belong to? Based on your prior lab knowledge, how
did we test for the presence of starch (remember iodine?)
3. What does it mean for the cell to be selectively permeable?
4. Based on the lab procedure how do you test for the presence of glucose?
★ Which of the following four molecules do you think will be able to cross the membrane?
Molecule
Molecular Formula
Glucose
C6H12O6
Starch
polysaccharide made up of
many glucose molecules
Iodine
I3-
Water
H2O
Will it cross the
selectively permeable
membrane?
Why or why not?
★ The Initial State diagram shows the locations of glucose, starch and iodine at thePre-lab
beginning of the
experiment. Make predictions about which molecules can cross the membrane tocomplete?
predict the
locations of each type of molecule after diffusion. In the Final State diagram, write the letters for each
type of molecule in the places where you think they will be found at the end of the experiment.
Initial State
Final State
Beaker
Synthetic
membrane
I I
I
G S G S
S G S G
I I
Key
I—Iodine
G—Glucose
S—Starch
I
To test whether glucose crossed the synthetic membrane to move into the solution in the beaker, you
will use a glucose test strip.
Water is the main component of both the solution inside the tube and the solution in the beaker
outside the tube. If water can cross the selectively permeable membrane, water could diffuse into the
tube and out of the tube.
★ Decide on a method to test whether more water has moved into the tube or out of the tube or there
was no change in the amount of water in the tube. Describe your method here.
Procedure:
1. Obtain one 20 cm piece of dialysis tubing and two pieces of string. The tubing
should be soaked in water for a minute or more.
2. Fold the bottom of the piece of tubing 1 cm up and tie the folded part with a piece
of string tightly to create a bag.
3. To open the other end of the tube, rub the end between your fingers until the
edges separate. Using the marked pipet, measure 5 mL of each glucose solution
and starch solution to be added to the “cell”.
4. After the tube is filled, fold 1 cm of tubing and tie off the top of the tube with string.
See the figure 1.
Figure 1
5. Rinse off the tube in fresh water. Be sure to rinse the string and squeeze any excess water from
the string. Why is this step important? Record the mass of the “cell”
6. Fill the 400 ml beaker ¾ full with tap water. Add 10 drops of Iodine solution and stir well. Be
careful to avoid staining your clothes or fingers with the iodine. (Iodine will turn blue-black in the
presence of starch).
7. Test the solution in the beaker for the presence of glucose by dipping a glucose test strip into it.
After 30 seconds, compare the color on the strip to the color chart on the side of the bottle.
8. Complete the Initial Set-up Status information on your data table.
9. Place the “cell” into the beaker of solution and allow it to remain undisturbed for 15 minutes.
10. After the 15 minutes, remove the tubing from the beaker, blot dry the outside of the bag and take
and record your observations in the data table for final status of the solution and the bag.
11. Retest the solution in the beaker and in the bag with a NEW glucose test strips and record this
data in your table.
12. Record your remainder of your observations in the data table.
13. Record your observations of the colors of the solution in the tube and the solution in the beaker in
the “Initial State” row in the table below. Also record the beginning value of the measure you are
using to evaluate movement of water.
Data Table
Initial
Contents
Bag
Beaker
Initial Solution
Color
Final Solution
Color
Initial Presence
of Glucose
15 %
Glucose &
1% Starch
H2 O &
Iodine
Initial mass
The Cell
Final mass
Final Presence
of Glucose
Name ______________________________
Period __________ Date _______________
Diffusion Across a Selectively Permeable Membrane
Data Analysis Questions:
1. Did starch move out of the tube? How do you know?
2. Did iodine move into the tube? How do you know?
3. Did glucose move out of the tube? How do you know?
4. Did water move into or out of the tube? How do you know?
Lab Conclusions:
1. Use your observations to fill in this chart describing which molecules crossed the selectively
permeable membrane. Circle any results that differ from your predictions on page 1.
Molecule
Molecular Formula
Glucose
C6H12O6
Starch
polysaccharide made up of
many glucose molecules
Iodine
I3-
Water
H2O
Did it cross the
selectively permeable membrane?
2. What characteristic of the molecules in your experiment appears to determine which ones can
cross the selectively permeable membrane? (Include an explanation for any results that differed
from your predictions.)
3. Explain how your observations support the conclusion that dialysis tubing is a selectively
permeable membrane.
4. There was water on both sides of the membrane, in the tube and in the beaker. Why did water
move in the direction that it did?
5. Each living cell is surrounded by a selectively permeable cell membrane which allows water to
move into or out of the cell by diffusion. The diffusion of water across a selectively permeable
membrane plays such an important role in biology that this process has been given a special
name, osmosis. What will determine whether osmosis moves water into a cell or out of a cell?
6. Living cells can use energy provided by ATP to pump an ion or molecule across the cell
membrane in a process called active transport. Active transport can move a substance from a
region of lower concentration to a region of higher concentration; this is useful for a cell to
maintain the correct concentration of ions and substances like glucose. What type of molecular
transport was demonstrated in your investigation—diffusion or active transport? How do you
know?