The fluid mosaic model is an explanation of the structure of cell membranes. They can be thought of as a two dimensional fluid that is composed of a lipid bilayer. The lipid bilayer is composed of two sequential layers of amphiphatic phospholipids. The phospholipids are composed of a polar phosphate head and a non-polar lipid tail. These phospholipids organize via self assembly where the heads line up together and the tails line up together so that each layer has each phospholipid lined up in the same orientation. Then two layers come together and orient themselves such that the tail portions are together on the interior of the membrane and the head portions are on the exterior of the membrane. In addition, numerous proteins and lipoprotein complexes fuse onto the surface of the membrane and imbed themselves within the membrane.