Cell Biology
Cell Structure
Key Question:
How does the process of diffusion occur at the cell membrane?
How does a plastic bag behave like a cell membrane?
Pre-knowledge: Iodine is used to indicate the presence of starch. The red-brown color of the
iodine turns purple black if it is exposed to starch.
Vocab: Write out the definitions: diffusion, osmosis, concentration gradient, semi-permeable,
permeable, cell membrane, cytoplasm.
What is a semi-permeable membrane?
Describe the process of diffusion.
Objective: The activity is to model the process of diffusion using a sandwich bag of cornstarch
solution (a cell) and the iodine bath (fluids around the cell). Note: The bag is made of a thin semipermeable plastic.
Question: How does a plastic bag filled with cornstarch solution behave like a cell in the human
Hypothesis: (Consider the following question when writing your hypothesis--If (manipulated
variable, then (responding variable), because (why!?).
If a bag of cornstarch solution is placed in a solution of iodine, which will diffuse through the
membrane? The iodine molecules into the bag or the starch molecules out of the bag or both or
neither? Why?
 200 mL beaker
 Plastic baggie filled with 100 mL cornstarch solution, tied with rubber band
 Iodine solution bath
 Tap water
Caution Iodine Stains!
Procedure & Data Table
(Follow teacher instructions and safety procedures).
1: Observe the bag of cornstarch.
2: Slowly and carefully place the bag of cornstarch (cell) into the iodine bath (body fluid). Make sure
the knot or twist tie is above the liquid level in the beaker. WAIT 10 minutes. Record your
observation. The beaker needs to sit overnight, with the cornstarch solution in the bag.
Clean Up: Put your beaker with the baggie in your classroom cabinet and wipe down your work area,
After 24 hours: Step Three: Observe the bag of cornstarch (cell) into the iodine bath (body fluid).
Make sure the knot or twist tie is above the liquid level in the beaker. Fully record your observation
in your notebook.
Starting Color
Observation after 15
Observation after 24
Solution in Beaker
Solution in Bag
FINISH LAB BY COMPLETING THE QUESTIONS BELOW. Write the question to the left of the
Cornell notes “line”, answer on the right side of line.
Analysis and Conclusion: Discussion Questions
1.Did the starch solution change color in the beakers? Why (discuss concentration gradient)?
2.Did the iodine solution change color in the beakers? Why? (discuss concentration gradient)?
3. Which substance(s) the plastic bag is permeable to?
4. What do you think is the reason for the outcome? (discuss concentration gradient)?
5. How is the plastic bag like the cell membrane?
6. How is the plastic bag different than the cell membrane in relation to movement of
substances? (look at the picture of a membrane and think CRITICALLY)

Diffusion Through a Membrane Lab