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This activity helps the students understand how substances can cross the cellular membrane without using specific transporters. It demonstrates the principles of diffusion in relation to size of particles and concentration of the substance. Beforehand, students should have a basic understanding of diffusion and cell membrane properties.

Access the Student Journal for Activity 5

Materials Needed :

1 beaker/plastic cup per group (3 groups total)
Corn starch
Tincture of Iodine (important: must be tincture of iodine)
Plastic sandwich bags- cheap is best (not the kind that zip shut)
1 spoon
Measuring cup or 50mL syringe
Eye dropper
Timer or Clock


Activity #5 - Properties of Diffusion

1.  Fill your beaker or plastic cup with about 1 cup (240mL) of water.

 2.  Place a spoonful of starch into your plastic bag.  Make sure not to spill any of the starch on the outside of the bag.  Put 50mL of water in the plastic bag.  Try to get as much air out of the bag as possible without spilling any of the solution and then tie the bag so it will not leak.  Be careful not to put too much pressure on the bag or it might tear.

 3.  Using the eye dropper, place the number of drops of iodine into the water as directed by your teacher (10, 25, or 50).

 4.  Place the bag with the starch solution into the beaker, making sure that the starch solution is covered by the iodine mixture.

5.  Watch the solution for 10 minutes and record what happens.

6.  Record your findings for 10 minutes. Note:

     When did an initial color change occur?

     What was the final color of the starch solution?

      How much of the solution turned color?


     1. Why did the starch solution change color?

     2. Why didnít the water in the beaker change color?

     3. Which substance is the plastic bag permeable to?

     4. How is the plastic bag like the cell membrane? How is it different from the cell

     5. How did changing the concentration of the iodine solution affect the color of the
          starch solution and affect the time in which the starch solution changed color? Why?



If you need additional groups, you may want additional varying concentrations or vary the concentration of the starch solution in the bags. Make sure each group gets to see what the other groups did and compare results.


Experiment designed by JeanMarie Stanley, Texas A&M University

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