As luck would have it, Gorter and Grendal chose just about the right degree of monolayer compaction to yield consistent ratios of cell to monolayer surface areas of about 1:2. Should the lipid have been packed more tightly? If so, then there's not enough lipid to provide a bilayer membrane and something else must be occupying the membrane surface (in the absence of lipid). Is there any independent and better means of determining the appropriate packing density of extracted lipid in a model monolayer?
A little over two decades ago a Dutch group of scientists (Zwall, et al., 1975) investigated the sensitivities of membrane lipids, in situ and in monolayers, to various phospholipases. Several lipases did not digest phospholipid in intact erythrocytes but readily hydrolyzed the same lipids when spread in monolayers at lower surface pressures. Compressing the monolayer lipid beyond about 31 dynes/cm dramatically inhibited digestion to the degree seen in intact cells, however, possibly because at this surface pressure the monolayer lipids are compacted about as much as they are in the native membrane. Interestingly, at this level of compaction, the ratio of monolayer surface area to erythrocyte surface area is about 1.5:1!
If this ratio is more reasonable estimate than the traditional 2:1, then possibly plasma membranes are "mostly" but not entirely lipid bilayers. If so, what occupies the remainder of the cell surface?