About seven decades ago, Gorter and Grendel, the "founding fathers" of our current model of plasma membrane structure, predicted quite simply that if a plasma membrane were really a bilayer then its surface area should be half that occupied by all its amphipathic lipids spread out in a monolayer. To test this prediction, they measured the surface areas of different mammalian erythrocytes microscopically. Independently, they also extracted the lipids from erythrocyte membranes (from the samples used for measurements of cell surface areas), spread the lipids out at an air/saline interface and measured the respective monolayer surface areas. Comparison of these two measurements produced a cell:monolayer surface area ratio of  approximately 1:2 for a number of different mammalian red cells, thereby confirming their bilayer model of the plasma membrane. 

Another way of testing the bilayer hypothesis entails an assessment of whether cell membranes are fluid: that is, in part, determining whether the amphipathic lipids behave as suggested by their spectroscopical images.  We will explore one experimental approach to answering this question in the chapter on Membrane Fluidity, but before we leave this section on lipid structure and membrane organization let's ask one more question of the Gorter-Grendel data. 

Why weren't the cell:monoloayer ratios exactly 1:2? Also, as we've already seen (opposite page), bilayers vary in their degree of lateral compaction.  Not surprisingly, monolayers do as well and any given amount of lipid can cover a range of surface areas!  How could Gorter and Grendel standardize this feature in their experiment?  Is a 2:1 ratio reasonable? Do their measurements and their model ignore crucial features of plasma membranes?

Where specifically is all the membrane protein? Continue on to Section 5 and subsequent sections to examine this question and to explore the nature of integral and peripheral proteins.