  The Fluid Mosaic Model by Singer & Nicolson proposes that biological
membranes consist of a fluid lipid layer into which integral proteins are
embedded. The lipid membrane acts as a two-dimensional liquid in which the
proteins can diffuse and interact. Until today, this view seems very reasonable
and is the predominant picture in the literature. However, there exist broad
melting transitions in biomembranes some 10-20 degrees below physiological
temperatures that reach up to body temperature. Since they are found below body
temperature, Singer & Nicolson did not pay any further attention to the melting
process. But this is a valid view only as long as nothing happens. The
transition temperature can be influenced by membrane tension, pH, ionic
strength and other variables. Therefore, it is not generally correct that the
physiological temperature is above this transition. The control over the
membrane state by changing the intensive variables renders the membrane as a
whole excitable. One expects phase behavior and domain formation that leads to
protein sorting and changes in membrane function. Thus, the lipids become an
active ingredient of the biological membrane. The melting transition affects
the elastic constants of the membrane. This allows for the generation of
propagating pulses in nerves and the formation of ion-channel-like pores in the
lipid membranes. Here we show that on top of the fluid mosaic concept there
exists a wealth of excitable phenomena that go beyond the original picture of
Singer & Nicolson.
