It's easy to construct good bimolecular models of amphipathic lipids, to actually make phospholipid micelles and extended monolayers and bilayers and test their properties, and to formulate reasonable hypotheses about the bilayer nature of plasma membranes. But are plasma membranes really bilayers?
Well, yes, they are . . . and isn't that answer obvious? What's all the fuss? After all, bilayers are reasonable structures for amphipathic lipids (see Sections 3 and 4) and real plasma membranes do contain lots of amphipathic lipids (Section 1). Therefore . . . plasma membranes are bilayers! So what's the problem? Asking the question seems in this context simply academic nit-picking?
On the contrary, asking such a question is to be scientific, because answering it entails the testing of a reasonable hypothesis. Both operations - speculating and testing - are necessary and important features of any science. In the last several chapters we've engaged in speculation. But how is testing done? Specifically, how can we test the accuracy of the bilayer model of plasma membranes?
Many different ways as it turns out! And you will encounter several as you continue examining membrane structure and function. Most of these tests are at best indirect, however, where experimental data or the results of observations are consistent with, but do not unequivocally prove, the existence of a lipid bilayer membrane. Rarely, are direct tests producing unequivocal results possible, and our view of the lipid bilayer membrane is supported more by the gradual accretion of consistent evidence than by dramatic demonstration. As your study of membrane structure and function deepens notice how each new concept or piece of structural or functional information is consistent with the bilayer model. Such consistency over time "proves" the membrane is a bilayer, and for more than five decades membranes have been taken to be lipid bilayers.
As soon as new information and data become inconsistent with this (or any) model, however, it will weaken and be displaced by a better one, sometimes in a slow, gradual manner and occasionally very rapidly and catastrophically! Such is the tentative or occasionally stochastic nature of science and scientific explanation!