There is protein in biological membranes. Wettability of the surface of an actual biological membrane is greater than the surface of an artificial membrane consisting only of a phospholipid bilayer. Membranes are coated on both sides with proteins, which generally absorb water. Not all membranes are identical or symmetrical. Membranes with different functions also differ in chemical composition and structure.
Membranes are bifacial with distinct inside and outside faces.
A membrane with an outside layer of proteins would be an unstable structure. Membrane proteins are not soluble in water, and, like phospholipid, they are amphipathic . Protein layer not likely because its hydrophobic regions would be in an aqueous environment, and it would also separate the hydrophilic phospholipid heads from water. In 1972, S.J. Singer and G.L. Nicolson proposed the fluid mosaic model which accounted for the amphipathic character of proteins. Hydrophilic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure.