ABSTRACT
This chapter provides the structure and organization of the two main constituents of biological membranes the lipids and the proteins. The recognition that biological membranes are two-dimensional fluids was a major advance in understanding membrane structure and function. Cell membranes are crucial to the life of the cell. The plasma membrane encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment. Lipid molecules constitute about 50% of the mass of most animal cell membranes, nearly all of the remainder being protein. Eucaryotic plasma membranes contain especially large amounts of cholesterol up to one molecule for every phospholipid molecule. A lipid bilayer also has other characteristics that make it an ideal structure for cell membranes. Black membranes are used to measure the permeability properties of synthetic membranes. Analysis of membrane lipids by mass spectrometry has revealed that the lipid composition of a typical cell membrane is much more complex than originally thought.
