Electrochemical Properties of Biomembranes

Biological membranes carry electric charges of different origin. Because of the low polarity and hence low dielectric constant in the core of the lipid bilayer, the charges try to avoid the inner membrane zone. Therefore, essentially all the charged groups reside in the peripheral regions at both sides of the manbrane. 

The permeability of the biological membrane is different for different ions and this gives rise to an electric potential difference between the bulk phases at the inner and outer sides of the membrane, respectively. The origin of this potential difference, referred to as the (trans)membrane potential A /,n, is discussed more extensively in Section 19.4. As a rule, the potential at the inner side is lower implying a negative value of In biological membranes A / is typically in the range of a few tens of mV. 

FIGURE 19.3 Schematic representation of the electrostatic potential across a biomembrane. 

The whole membrane, surrounded by an electrolyte solution at both sides, may be considered as a condenser with a capacitance C given by 

The capacitance of the membrane is relatively constant, because s and d are essentially invariant. The capacitance can be experimentally assessed a typical value for the capacitance of a cell membrane is 10 mF m. Taking 10 nm for the membrane thickness results in a value of 9 for the dielectric constant. This value is much higher than that for the dielectric constant of pure lipids, which is about 3.5. The reason for the difference is probably due to the polar head groups of the lipids and the heterogeneity of the membrane, notably its (glyco)proteins and water content. 

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