A pure bilayer is a simple model for the many different membranes of complex composition that surround cells and organelles in living creatures. The real membranes are a mixtures of a variety of lipids and proteins. Generally, these lipids have a phospholipid headgroup and two long hydrocarbon tails. The details of the headgroup and the tails determine which type of lipid we are dealing with. In model systems, the headgroup is usually phosphatidylcholine (PC) or phosphaditylethanolamine (PE), and tails generally lauroyl (L), myristoyl (M), oleoyl (O) or palmitoyl (P) chains. Combining these elements, we get lipids like DPPC (di-palmitoyl-phosphatidylcholine), DMPC (di-myristoyl-phoshatidylcholine), POPC (palmitoyl-oleoyl-phosphatidylcholine) and DLPE (di-lauroyl-phosphatidylethanolamine). Obviously, in real life things are far more complicated and a biochemistry person will be able to use many more types of lipids, but for now it is hard enough for us to deal with these four different types.

Cool computer graphics of a bilayer
A bilayer consisting of 128 DPPC lipids and a few thousand water molecules (shown schematically). The structure is taken from (D.P. Tieleman, H.J.C. Berendsen. 1996. Molecular dynamics simulations of fully hydrated DPPC with different macroscopic boundary conditions and parameters, J. Chem. Phys. 105, pp. 4871-4880)

The cool picture gives an impression of what a model bilayer looks like from our point of view. The gray atoms in the middle are the carbon atoms from the tails of the lipids. The blueish spheres are the carbons from the PC headgroup, the red spheres are oxygens and the yellow spheres are phosporous atoms. The white/red speckled noise are actually watermolecules, drawn at a smaller scale so they do not cover all of the headgroups of the lipids. It should be clear now where the word 'bilayer' comes from, a membrane has two layers of lipids that face each other with their tails.

Courtesy of the Biocumputing Web Page (University of Calgary)