Using Waves at the Beach to Describe Concentration Gradients

Written by Jennifer A. Metzler, Ball State University

When discussing passive versus active transport and the difference between an input of cellular energy, I ask students to imagine they are at the beach or at a wave pool. Since passive transport is going down a concentration gradient, I tell them to liken it to having the waves at their back and moving into shore. It is not a problem for them at all and they do not need to expend any energy as they are going with the flow. With active transport going against the concentration gradient, I tell them to imagine turning around and having the waves hit their chest and try to move away from shore. In this case they must expend energy as they are going against the flow of the waves.

Using Food and Drink to Describe Osmosis

Written by Jennifer Wiatrowski, Pasco-Hernando Community College

I use two different analogies to relate osmosis to the real world:

a. I use two large beakers (beakers A and B), a jug of water and lemonade mix or fruit punch mix (these are dry powders). I then tell the class I am going to mix two classes of a refreshing beverage (nice in a hot climate like Florida). I tell them that I will make the first beverage very sweet, and pour in a large amount of lemonade or punch mix. The second beverage will not be nearly as sweet and I pour in a small amount of the powder. I then fill each of the beakers to equal volumes. I then ask the class if the two beakers have equivalent solutions. (They say no!). So, although the two beakers appear to have equal volumes, the amount of water is varies between the beakers. This shows students that the amount of water in a given space is influenced by the amount of solute. I then ask the students to imagine connecting the beakers together with a selectively permeable membrane and then ask them which way the water would flow  (from beaker A to B or B to A?)

b. I then talk about Martha Stewart. Martha always says that you should never toss your salad until your guests arrive or you are ready to serve it (I was recently informed by my students that “toss the salad” has taken on additional meaning……so this gets a good laugh out of the class). So, at the beginning of class, I take a bag of salad and some dressing and mix them. I then put it aside for awhile. In the meantime we talk about hypertonic and hypotonic. I then ask to think about the scenario with the salad and the dressing. They eventually reason that the salad is hypotonic to the dressing (or the dressing is hypertonic to the salad) and the consequence of this is a watery salad as the dressing pulls water out of the greens. This would make Martha HORRIFIED.  To finish, we take look at the salad we mixed earlier.