Planar Diffusion of  Lipid and Protein

Imagine you are viewing the surface of a model bilayer which consists of phospholipid and many fewer embedded (or integral) proteins.  If your viewing instrument were a very high powered fluorescent microscope, if some lipid and protein conveniently fluoresced red and blue respectively, and if these particles were diffusing in a liquid environment, then you might observe something resembling what's simulated on the facing page.  (Of course, a more realistic model bilayer would likely have 100 or more lipids for every protein; here the lipid/protein ratio is 5!) 

To view the simulation, press the "Start" panel at the top.  Unlike earlier simulations in this series, notice that particles don't "bounce" when they strike the compartment boundary; rather they leave and are replaced by other particles entering the compartment at random locations along the boundary.  Thus, this "compartment" is a "window" - a microscopic field of view - and not a closed container.

"If/Then" statements like the one above represent hypotheses.   To be scientifically useful, however, they must also be testable.  The red and blue particles "seem" to be moving in a random manner, but how can we assess or test this impression?  That they move at all certainly supports the concept the bilayer is fluid, but it would be more useful to determine whether their movement is random and due to diffusion.  Why?

In approaching this problem consider an imaginary circle in the center of the microscopic field of view.  If you were to count the number of each type of particle inside the circle at each instant of time, what would you expect to find?  Red and blue particle counters are displayed at the bottom right of the simulation adjacent to strip chart recorders, and the recorders move from right to left indicating fluctuations in these counts over time.

Consider how the circle scenario might provide a means for assessing particle diffusion and the liquid nature of the lipid bilayer.  Flip the page to examine a simulation of an actual experiment.