Membranes are not simply made of lipid molecules (the fluid mosaic model)

The structure of the plasma membrane is complex:

The major constituents are lipids, including phospholipids. However, the bilayer is interrupted by the inclusion of membrane proteins. Membrane proteins have hydrophobic surfaces which allow them to associate in the hydrophobic core of the membrane. These surfaces also exclude them from the aqueous phase (cytoplasm or exterior of the cell).

The cytoplasmic face of the membrane is covered with a mesh of cytoskeletal proteins. Cytoskeletal proteins form protein chains which give the cell it's shape. Some cytoskeletal proteins are motors which move cellular contents through the cytoplasm

The "fluid mosaic" model of the membrane states that the membrane is fluid-that is, that the molecules that make up the membrane are in constant motion rather than being fixed in space. The membrane is a mosaic-made up of lipids of various kinds, and of various proteins. Increased saturation of the fatty acids in phospholipids tends to disrupt the ordered structure of the membrane since saturation "kinks" the hydocarbon chain.

The two layers of the membrane (the inner and outer "leaflet") are isolated from each other. Lipids move extremely quickly within one a leaflet. However, they can not move easily from one leaflet to the other since the hydrophobic head group "anchors" them. Specific proteins catalyze the movement of lipids between the leaflets. Because of the specificity of those proteins the two leaflets are different in the composition.

The proteins in the membrane may associate in a variety of ways.

They may span both leaflets or they may reside entirely in one leaflet; both of these are termed "integral membrane proteins".

They may associate only peripherally with the hydrophilic head groups, or with integral membrane proteins (those inserted within one or both leaflets). Like the phospholipids, the membrane proteins present in the two leaflets may differ. Since the proteins can be different at each end the two sides of the membrane may differ functionally.