Self-assembly of a lipid bilayer

Your cell membranes are made of these little molecules, however they are not bound together. So how does the membrane stay together? The molecules have 2 ends, one hydrophobic and the other hydrophilic. The membrane is made of 2 layers of the molecules (like a sandwich), with the hydrophilic ends of the molecules (the bread) on the outside of the membrance and the hydrophobic ends touching inside. This happens because the two sides of the membrane (the outside and inside the cells) are water, which the hydrophobic ends hate (because they are fat/oil).

The question becomes, does this layering occur naturally? Or do the molecules have to be coerced into this arrangement. Of course the answer is, yes, it happens naturally. This video is simply a physical simulation and visualization.

If you look carefully, everything for the whole video is replicated 9 times. These simulations typically use periodic boundary conditions: if something goes out the left side, it is put in at the right side, and so forth. This significantly decreases boundary effects that would arise from putting interfaces into the simulation. Only one of the 9 cells is really being simulated, but they replicate it once in each direction for visualization so you can see what is happening through the boundary conditions.