A more disorganized bilayer is seen when a moistened or partially dried liposomal preparation is examined crystallographically (opposite page). The image resembles the more ordered, crystalline structure seen previously but differs from it in thickness and lateral extension. The image seems both thinner and wider. How so?
Lateral and transmembrane compaction of a bilayer reflects the lateral motion of the acyl residues. Dehydration compresses the bilayer, forcing the acyl residues into close approximation, increasing their Van der Waal interactions and severely limiting their movement. When hydrated, however, the hydrocarbon tails are more free to move, and their "wiggling" allows their tails to interdigitate slightly with those of the opposing monolayer and causes the bilayer to spread laterally. The transformation accompanying rehydration of a crystalline bilayer faintly resembles what happens when ice melts!
Hydrated and at room temperatures, wiggling phospholipids move about in the plane of their respective monolayers (but their polar heads greatly reduces their flipping from one monolayer to the other). This limited mobility produces what we now call a fluid bilayer. Are cell membranes also fluid?
With their enveloping lipid bilayers, liposomes look like very simple, minature cells. But are plasma membranes around real cells really bilayers?
A good scientific way of answering this question entails assembling and organizing relevant information, making a specific, testable prediction, and then testing the accuracy of the prediction. So what do you know so far about plasma membranes? They contain lots of amphipathic lipid (Sections 1 and 2)! And what do you know about amphipathic lipids? They'll assemble in aqueous environments into stable, organized structures such as micelles, monolayers, bilayers and liposomes (Sections 3 and 4). Now, make a testable prediction, and turn the page to examine the results of a hallmark prediction and experiment!