Investigation: How Can Diffusion Be Observed? - Intro Biology


Introduction: In this lab, you will observe the diffusion of a substance across a semipermeable membrane. Iodine is a known indicator for starch. An indicator is a substance that changes color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch.

Prelab Observations: Describe what happened when iodine came into contact with starch.

checkbox 1.  Obtain a plastic bag that is filled with cornstarch and water and tied at the top.   This bag represents the animal cell’s cytoplasm and cell membrane.    Like the cell membrane, the bag is semipermeable , meaning that some substances can pass through it. .  Be careful when handling the bag.    

The outside of the bag would be like the cell’s _____________________________

The white contents inside the bag would be like the cell’s  __________________________

Does the bag leak, or do the contents seem contained? ________________________

The cell membrane can let some things through it, this property is called: _____________________

Do you think the bag will also let some things through it?  __________________________

checkbox 2.  Obtain a beaker that is filled with iodine.  This represents the solution you will place your “cell”  (bag) in. 

What color is the iodine? ______________________  * Be careful, iodine can stain your fingers and clothes.

How much iodine is in the beaker?  (Use the marks on the beaker to measure in milliliters)  _________              

checkbox 3.  Place the bag in the beaker, do not submerge the part of the bag that is tied.    Predict what you think will happen to the starch inside the bag?


How much did the iodine- water raise when you placed the bag in the beaker? ________

checkbox 4.  What is diffusion?   ______________________________________________________________

Molecules tend to move from areas of __________ concentration to areas of __________ concentration.

checkbox 5.  Examine the bag and beaker and consider the idea of CONCENTRATION. 

Which is more concentrated in starch? [ beaker or  bag ]   _____________________

Which is more concentrated in iodine? [ beaker or  bag ]    _____________________

checkbox 6.  A solution can be HYPERTONIC,  HYPOTONIC, or ISOTONIC.    

If there is MORE starch in the bag, it is called ____________________________

Since the beaker has LESS starch, it is called: ____________________________

checkbox 7.  Record your observations.

  Starting Color Color after 15 min
Solution in Beaker    
Soulition in Bag    


1. Based on your observations, which substance moved, the iodine or the starch? How do you know?


2. Explain how the bag is semipermeable, like the cell membrane.


3.. Sketch the cup and bag in the space below. Use arrows to illustrate how diffusion occurred in this lab.


4. What would happen if you did an experiment in which the iodine solution was placed in the baggie, and the starch solution was in the beaker? Be detailed in your description.


5. Why is it not a good idea to store iodine in a plastic bag?