The excitable fluid mosaic

TL;DR

This paper proposes that biological membranes are more dynamic and excitable than previously thought, with phase transitions influencing membrane behavior, protein sorting, and nerve pulse propagation, extending the classical fluid mosaic model.

Contribution

It introduces the concept that membrane phase transitions can induce excitable phenomena, adding a new layer to the classical fluid mosaic model of membranes.

Findings

Membranes exhibit broad melting transitions near physiological temperatures.

Phase behavior influences membrane elasticity and function.

Membrane excitability can generate nerve-like pulses and ion-channel-like pores.

Abstract

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.