Each type of integral membrane protein has a single, specific orientation with respect to the cytosolic and exoplasmic faces of a cellular membrane. All molecules of any particular integral membrane protein, such as glycphorin, lie in the same direction. This absolute asymmetry in protein orientation give the two membrane faces their different characteristics. In contrast to phospholipids, proteins have never been observed to flip-flop across a membrane. The asymmetry of membrane proteins is established during their biosynthesis and maintained throughout the proteins lifetime.
Membrane asymmetry is most obvious in the case of membrane glycoproteins and glycolipids.
In the endoplasmic reticulum, they are found on the interior, or lumenal, membrane surface. The lipid compositions of the two membrane leaflets are quite different in all the plasma membranes analyzed thus far. In the human erythrocyte and in a line of canine kidney cells grown in culture, all of the glycolipids, and almost all of sphingomyelin and phosphatidycholine are found in the exoplasmic leaflet. In contrast, lipids with neutral or negative polar head groups, such as phosphatidylethanolamine and phospatidylserine are preferentially located in the cytosolic leafet. The relative abundance of a particular phospholipid in the two leaflets of a plasma membrane can be determined based on its susceptibility to hydrolysis by phospholipases added to the cell exterior.