Understanding diffusion and osmosis are foundational for explaining a variety of biological topics including the physiology of the urinary system and the effect of soil salinization on ecosystems. All life is dependant on solutions. Water, often called the "universal solvent" because it will dissolve a large number of substances, makes up more than half the mass of a cell. The chemical reactions that are part of living things all happen in solution. A solution has two components; a solute and a solvent. The solute is the dissolved part of the solution. The solvent is the liquid component and, in living things, is water.
Osmosis is generally defined as the movement of a solvent across a semipermeable membrane (a membrane through which some things can pass, while others cannot). In biology, this is water moving across a biological membrane (review membrane structure with this game). Factors such as size, charge, and the presence of specific protein channels determine what solutes can pass through a membrane.
Most students are able to pick up on basic diffusion relatively quickly, but find osmosis more difficult. Diffusion can be demonstrated in ways that are easy to directly observe (a drop of food coloring spreading through a beaker of water, or the smell of a body spray in the hallway). When observing the results of osmosis though, students often incorrectly assume that it is the movement of solutes, rather than the solvent (water) leading to the observed changes. Discussing dynamic equilibrium and net movement can also be confusing, as visualizations are often static images.
The Diffusion and Osmosis simulation aims to help students by providing a moving visualization of both solute and solvent. The simulation features a beaker with two sides separated by a semipermeable membrane. Students can control the starting concentration of three different-sized molecules on each side of the beaker, as well as temperature.
In addition to the three molecules, water is also represented as part of the simulation. When the simulation runs, all the substances, including water, move and collide with each other. The membrane separating the two sides of the beaker has holes that the substances may pass through depending on size. While students watch the molecule movement throughout the simulation, a graph records solute concentrations. This allows students to directly analyze concentration, unlike most practical labs, where data is in the form of mass change or qualitative assessment of visual appearance.
The simulation is not intended to replace hands-on lab activities. Rather, the activity is designed as either an introduction or follow-up to traditional osmosis lab options. The goal is to help students visualize what is happening at a molecular level in the practical labs that they perform.