Figure 3.2. Diagram of phospholipid bilayer: the head groups, represented as
small circles, are to the outside of the bilayer, and the hydrocarbon tails are in-
side. There would be water above and below the bilayer.Phospholipid bilayers form sheets in three dimensions that can
fold to form enclosed surfaces separating two aqueous environments,
such as the inside and outside of a cell. Indeed, phospholipid bilayers
constitute the membranes forming the boundary layers around all
cells for all of life on Earth. The cell membranes of bacteria, of plants,
of mushrooms, and of brain neurons all have the same fundamental
structure: a bilayer of phospholipid molecules. It is one of the most
beautiful and elegant structures in the known universe!
These phospholipid bilayer membranes are very tiny structures,
with thicknesses of only about 5 nanometers (5 x 10-9 meters, or
five billionths of a meter). The phospholipid bilayer membranes of
cells contain a diversity of protein molecules that serve a variety of
functions vital to cells. In nerve cells, among the various membrane
proteins are ion channels, ion pumps, neurotransmitter receptors,
and neurotransmitter reuptake transporters. In an actual biological
membrane, the density of membrane proteins can be quite high, and
the proteins will have a variety of wild and crazy shapes (Fig. 3.3).
The molecules that make up a phospholipid bilayer are constantly
jiggling as a result of thermal vibration, and there can be quite a lot of
mobility of the proteins and other molecules within a biological mem-