Name:_________________________________
Diffusion across a Model Cell Membrane Lab
Question: How do certain molecules and ions get in to and out of cells?
Introduction: Cells need to be able to acquire and get rid of molecules and ions in order to survive. Cells are
surrounded by a selectively permeable membrane. Selectively permeable means that some things can
pass through the membrane, while others cannot. Diffusion is one way that substances can move across
the cell membrane. In diffusion, molecules or ions move from areas of higher to areas of lower
concentrations. In the cell, diffusion across the cell membrane depends on multiple factors, one of
which is size.
Lab Overview: In this lab activity, you will test whether or not glucose (mono saccharide, meaning single
sugar), starch (poly saccharide, meaning many sugars), and Iodine ions (I3-) can diffuse across a model
of a cell membrane. A tied off piece of dialysis tubing will represent your cell and its membrane.
Dialysis tubing contains microscopic pores through which some substances can diffuse. Your model
cell will contain glucose, starch, and water. You will submerge your cell in an Iodine solution (Iodine
and water). When Iodine and starch come in to contact, the mixture turns a blackish-purple.
Pre-Lab Questions. Answer these before you begin your experiment.
1. Your model cell is the circle in this
image. Label where each substance
initially occurs at the beginning of this
experiment (glucose, starch, Iodine,
water). Using what you know about
diffusion, draw arrows to indicate which
direction glucose, starch, and Iodine
might diffuse.
2. How will you know if Iodine diffused
across the membrane and in to the model
cell? Be specific.
Group Materials:
1 glass beaker
Starch powder
Timer (watch/phone)
12 cm of dialysis tubing
Lugol’s Iodine
Glucose test strips
20 mL of 1 Molar glucose solution
Water
Forceps/tweezers
String
Methods:
Making your model cell:
1. Add 200 mL of water and 2 mL of Lugol’s Iodine to your beaker. Gently stir.
Worksheet created by JYL 2014
2. Briefly run your dialysis tubing under water, make a tight knot in one end (you can use string if you
need to), and then gently rub the other end between your fingers to open it.
3. Add 1 small scoop of starch powder to your 20 mL of 1 Molar glucose solution. Carefully pour this in to
your dialysis tubing.
4. Use 1 glucose test strip to measure the % glucose inside your model cell (dialysis tubing), and record the
% glucose in the data table below. Use another strip to make sure no glucose is in your beaker.
5. Carefully tie a tight knot to close the dialysis tubing and gently shake. Your model cell is complete!
6. Rinse your closed cell in water to wash away any starch or glucose that may have spilled.
Testing for diffusion:
7. Make your initial observations about your cell (see first data table).
8. Place your cell into your beaker and start your timer.
9. Record your observations about your cell and the solution in the beaker after 1 minute, 5 minutes, and
10 minutes. Use forceps/tweezers to pick up your cell by a tied off end to do this.
10. At the same points in time, use glucose test strips to determine the % glucose on the outside of your cell.
Record in the data table provided.
Data and Results.
Color of
Solution
Data Table: Starch and Iodine
Inside Your Cell
Is starch
Is iodine
Color of
present?
present?
Solution
Outside Your Cell
Is starch
Is iodine
present?
present?
Initial
Observations
After 1
minute
After 5
minutes
After 10
minutes
Data Table: Glucose (Using Test Strips)
Initial % Glucose inside cell
Initial % Glucose outside cell
% Glucose outside cell after
1 minute
% Glucose outside cell after
5 minutes
% Glucose outside cell after
10 minutes
Conclusions.
1. Draw what happened during your experiment. Use arrows
and labels to indicate in which direction the glucose, starch,
and iodine diffused, if they diffused across your cell
membrane.
2. Did each substance diffuse across your membrane? Why or why not?
Worksheet created by JYL 2014