The article by Lankford and Friedrichsen (2012) that appeared in the August issue of The American Biology Teacher (74:392-399) does an excellent job summarizing a variety of hands-on activities that can be used to teach diffusion and osmosis. These "representations" will provide a valuable addition to a teacher's repertoire. I was particularly impressed that the authors warned teachers that convection, rather than diffusion, was also responsible for molecular motion in some of their representations. However, I urge teachers to use caution when defining osmosis as water moving from an area of greater water concentration to an area of lesser water concentration. Although this is a simplistic way for students to understand the concept of osmosis, it is not always true. A simple example shows why.
Consider an osmometer constructed of a glass tube attached to a semipermeable membrane sac that is suspended in a beaker of water. Because the conditions on both sides of the membrane are initially the same (i.e., at equilibrium), there will be no net movement of water in either direction and, thus, the level of water in the tube will not change. If an impermeable solute such as sucrose is placed inside the membrane sac, there will be a net movement of water from the beaker into the membrane sac, causing water to move up the glass tube; the height and rate provide a measure of osmosis. At some point, osmosis stops and the water level in the tube remains constant when a new equilibrium is achieved between the water in the membrane sac and the water in the beaker. If water moves from a higher concentration to a lower concentration, then osmosis should never stop because the concentration of water in the beaker, which has no dissolved solute, can never equal that inside the membrane sac, given that the latter will always contain some solute.
The reason osmosis stops is that the pressure inside the osmometer increases and counteracts the tendency for osmosis. If additional pressure is exerted on the system, water will even move from the membrane sac back into the beaker, as occurs with a reverse osmosis apparatus. Because osmosis is a function of both solute concentration and pressure, it should never be defined strictly as a movement of water from an area of higher concentration to an area of lower concentration.
There is an easy solution. Teachers simply need to define osmosis as the movement of water from an area where it has a high free energy to an area where it has a lower free energy (a concept plant physiologists call "water potential"). A student need only remember that solutes lower the free energy of water, essentially by restricting the ability of the water to move freely, much as a defensive football player restricts a running back from freely moving about the field. The more solutes that are added to water, the lower its free energy will be. On the other hand, pressure usually increases the free energy of water. Because there is never an exception to the movement of water from higher to lower free energy, this is a preferable way to explain the concept.
Returning to the osmometer example, the addition of solutes to the water inside the membrane sac decreases the free energy of the water in relation to that of pure water in the beaker, causing water to move into the membrane sac. As the water enters, it dilutes the solute concentration, increasing the free energy of the water to a small degree. Even more importantly, as water moves into the membrane sac, the pressure in the system increases, which in turn increases the free energy of the water until ultimately it is equal to that of pure water in the beaker. At this point, osmosis stops.
Unfortunately, plant and animal biologists have historically used different terminology and approaches to diffusion and osmosis. The main reason for this is that animal cells lack a wall; as a consequence, animal physiologists generally do not need to consider the impact of pressure on osmosis. Hopefully, someday plant and animal biologists will adopt the same terminology for water relations that apply in all situations. In the meantime, excellent articles like the one by Lankford and Friedrichsen can help teachers present these challenging concepts.
Stephen G. Saupe
Professor of Biology and Curator of the Bailey Herbarium
College of Saint Benedict/Saint John's University
Collegeville, MN 56321