What's Up in Biology? Labs Involving Transport Across a Membrane!

This is one of my favorite units to teach!

I love it when I get to this point in my biology curriculum.  We have covered the skills of scientific measurement, scientific method, and microscopy.  Now the students are prepared to use these skills on more complex topics.  These concepts (cellular transport, structure of a membrane, movement across the membrane, active and passive transport, cytolysis and plasmolysis , etc.) are so much fun to teach!  There are great labs that can be done during this unit, and it leads to even more complex topics such as cell communication, photosynthesis (thylakoid membranes) and respiration (cristae membranes)!  To me, the most fun in teaching a biology class comes when the student has a knowledge base to draw upon.  It is like a "connect the dot" puzzle.  The student has all the dots, and you (as the teacher) are leading them from dot to dot, helping them to make those important connections.

I do a series of labs involving transport across the membrane.  These labs are very simple, but I have found that my students really love them.  I start with a very simple lab called "Diffusion Through a NonLiving Membrane".  The picture to the right shows the end of the lab.  Sorry, I forgot to take a picture at the beginning!  We use dialysis tubing filled with either a glucose or a starch solution which is then placed in a beaker of water.  In the beaker containing the starch bag, iodine was added to the water in the beaker.  In the beaker containing the glucose bag, students used a glucose test strip to determine if glucose was moving across the membrane.

Results are recorded after 20 minutes, and again after 24 hours.  The results are dramatic and my students immediately grasp the concepts taught by this lab:  (1) The smaller the molecule, the faster it can cross the membrane.  (2) Some substances are too large to cross the membrane.  (3) Movement of materials occurs in both directions.  (4) Water is the substance that most dramatically affects the volume of a cell.

This is such as simple way to get the basic concepts of osmosis and diffusion across to our students.