Membrane
Asymmetry
The lipid bilayer defines the boundary between the cytoplasm and the environment surrounding a cell. One layer of the lipid bilayer faces the cytoplasm and the other faces the external environment. These two different layers are referred to as the inner leaflet and outer leaflet, respectively. Figure 10.14 shows that the lipid composition of the inner and outer leaflets can vary considerably and are not in an equilibrium state, where the compositions of the leaflets would be the same. Because the two leaflets of the membrane must deal with different surroundings, it seems to make sense that they are usually quite different in composition and structure.
One feature of lipid bilayers that helps to maintain the asymmetric distribution of lipids within it is the relatively slow way in which glycerophospholipid molecules can "flip" from one side of the bilayer to the other in the absence of enzymatic action. This flipping is called transverse diffusion and is a process that can be enhanced by a class of enzyme called translocases.
Two other features of membrane structure illustrate its asymmetry. First, carbohydrates are always attached on the outside surface of the cell. Second, membrane protein orientation (flipping) does not occur, so that proteins always maintain the same polarity.