Membranes that enclose all livings cells are two phospholipid sheets. 10,000 stacked together would equal one sheet of paper fluid mosaic model shows proteins embedded in a fluid lipid bilayer. Lipid layer that forms the foundation of a cell"s membranes is a bilayer formed of phospholipids. Primarily glycerol phospholipids and sphingolipids such as sphingomyelin. Early model portrayed the plasma membrane as a sandwich: a phospholipid bilayer between two layers of globular protein. Aka fluid mosaic model: a mosaic of proteins floats in or on the fluid lipid bilayer. Two categories of membrane proteins based on association with membrane. Peripheral proteins: associated with the surface of the membrane cellular membranes consists of four component groups. Glycerol phospholipids, integral membrane protein, and peripheral proteins. Cellular membrane have an organized substructure the lack of double bonds in saturated fats allow them to pack together tightly, and therefore the membrane is less fluid.