A "FRAP" Experiment 

When exposed to very intense light many fluorescent molecules are bleached (that is, lose their fluorescense), and it's possible to modify a fluorescence microscope such that a  region of the molecules in a field of view can be bleached while the fluorescence of the field is monitored.  Consider the consequence of bleaching the circular region of the simulated bilayer on the opposite page.  If the bilayer is fluid, then bleached molecules will gradually diffuse out of the region and be replaced with unbleached ones; the fluorescence of the region will initially drop to zero and then gradually return.  Alternatively, the components of a "static", crystalline membrane would not move and the fluorescence of the bleached region would not recover.  (One can imagine a variety of responses in real membranes - both normal and pathological - in which the recovery of fluorescence following a photobleaching episode lies somewhere in between these two extremes.) 

You can perform this experiment (often referred to as FRAP - Fluorescence Recovery After Photobleaching) in the following way.  First, allow the simulation to run a bit, and estimate the steady-state numbers of fluorescent particles in the circular region.  Then, position the cursor anywhere within the circle and left-click the mouse: a bright yellow disc appears, signifying the flash.  What happens next?  How quickly does the blue and red fluorescence recover?  Estimates of recovery rates, as well as the times required for half of the fluorescence to recover, provide useful indices for assessing diffusion and the effects of such parameters as temperature, lipid composition and membrane organization.  How could you measure these two parameters?

Also, consider what happens when a FRAP experiment is conducted with a real cell?   Would you expect fluorescent lipid and integral protein to move in a manner similar to what you observed in the model bilayer?  To test your expectations, imagine a field of view like the one on the opposite page, of a portion of the plasma membrane of a flattened fibroblast in culture.  Imagine further your microscope has a video camera and is linked to a computer with image storage and analysis software that will plot the level of fluorescence in the circular region, and turn the page.