  We investigate a model of an asymmetric bilayer consisting of sphigomyelin,
phosphatidylcholine, and cholesterol in the outer leaflet, and
phosphatidyl\-ethanolamine, (PE), phosphatidylserine, and cholesterol in the
inner leaflet. Composition fluctuations are coupled to membrane fluctuations
via the bending energy which depends upon the local spontaneous curvature
dominated by the PE in the inner leaflet. This brings about a microemulsion in
that leaflet with a characteristic wavelength of 37 nm as shown by the PE-PE
structure function. Thus the inner leaflet will respond to an external
perturbation most strongly at this length. However the correlation length is
approximately 1 nm so that composition variations are not seen in the inner
leaflet itself. Depending upon the strength of the coupling between leaflets,
the outer leaflet is either a normal fluid or is also a microemulsion. In this
model then, the lipids of the plasma membrane would not evince inhomogeneities,
but would be primed to create them from the inner leaflet in response to
non-lipid perturbations.
