Phospholipid membranes as molecular environments

These assemblies are thought to be stabilised by the physical interactions that underlie the phenomenon of membrane phase-separation elicited by sterols such as 

3 The physical origins of the transmembrane V or Af) surface ( s) and dipolar ( p) membrane potentials 

A number of quite different techniques have evolved to make measurements of the various electric potentials associated with membranes but particular measurements must rely on the physical differences between the potentials. In order to avoid interpretative difficulties due to interference between each potential, clearly it is necessary to have a very clear understanding of the physical nature of both the potential as well as the measurement technique. In addition, however, membranes also possess other properties which often complicate assignments of specific values for the membrane potentials. 

Membranes represent formidable barriers to the movement of ions (and electrons). Thus, transport of ions as net charges across the insulator offered by the membrane will establish an electrical potential difference (a voltage = V) across the membrane (Vm or Aij/). This phenomenon is well known and can be described by Equation. (5.1)  

The transport of electrical charge across a membrane may take the form of cation, anion (both inorganic and organic) or electron transport. It can be passive due to membrane leaks (either inherent or facilitated by protein channels, ionophores, detergents etc). In these latter cases the transport is driven by the difference in electrochemical potential of the particular ion between the two phases separated by the membrane. In biological membranes, however, the charge transport is typically 

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