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Saturday, September 22, 2012

Very Simple Diffusion and Osmosis Experiment


The concept of cellular transport (diffusion, osmosis, hypotonic, hypertonic, active transport, passive transport) is fundamental to a biology class.  There are so many great ideas for labs that teach and explore these concepts.  Just this week, our biology students completed an activity that is so very simple, but it really illustrates the concept of semipermeable membranes.


Students are given 2 pieces of dialysis tubing.  One is filled with a starch solution and the other is filled with a glucose solution.  Each is placed into a cup containing tap water.  The cup that contains the starch tube also has iodine added to the water in the cup.

After setting up the experiment, students are asked to wait 20 minutes before recording their results.  The students immediately recognize that the dialysis tube containing starch is beginning to turn purple.  The iodine molecules in the cup of water are diffusing across the dialysis tube membrane and are causing the starch molecules to turn dark blue or black.  In addition to this observation, students are given a glucose test strip and asked to test for the presence of glucose in water of the second cup.

Students allow their experiment to sit overnight.  Observations are made and results are recorded again after 24 hours.  All students quickly determine that:  (1) Starch did not leave the bag.   (2) Iodine moved into the bag.   (3) Glucose moves out of the bag slowly.  (This is evidenced by the fact that the initial test for glucose after 20 minutes is negative, but the test is positive after 24 hours.)  (4) Much water moves into the bag.

From these observations, students are asked to make predictions about the size of the molecules, and must place the molecules in order of their size from smallest to largest.

The lab that I use with my students can be found here:  Diffusion Through a Non-Living Membrane.

FREEBIE:  You might also want to try this lab:  The Effect of Concentration on the Rate of Diffusion. This is a free download....Enjoy!

1 comment:

  1. Good blog. Very easy to understand the point of each piece of text.

    ReplyDelete