Cell membranes are static protein–lipid–protein sandwiches.

Early membrane models tried to explain two things at once: cells clearly had a fatty barrier, but that barrier also had to control what entered and left.

The influential Danielli-Davson model pictured the membrane as a sandwich. A layer of lipids sat in the middle, with continuous layers of protein coating both outer surfaces. Later electron-microscope images seemed to support a neat three-layer structure.

Several experiments made that picture harder to maintain.

Freeze-fracture microscopy splits a frozen membrane apart along its interior. Instead of showing proteins only as surface coatings, it revealed particles embedded within the membrane itself. Biochemical work likewise showed that many membrane proteins extend deeply into the lipid layer.

Cell-fusion experiments supplied another clue. When two cells with differently labeled membrane proteins were fused, the proteins gradually mixed across the combined surface. The membrane was not a rigid sheet; many of its components could move sideways.

Singer and Nicolson brought these findings together in their 1972 fluid-mosaic model: a flexible lipid bilayer with proteins embedded in or associated with it. Modern membranes are even more organized and crowded than that simple picture suggests, but the universal static sandwich did not survive.