Biological membrane, electrical properties

The so-called London dispersion or van der Waais interactions are those between molecules that have neither a net charge nor a permanent dipole moment. This interaction is essentially due to the interactions between a transient electrical dipole in one molecule and its induced electrical dipole in the other molecule. TTiis type of electrical interaction plays an important role in biological systems (e.g., in surface tension, stability of biological membranes, condensation properties, adhesion and fusion of biological cell membranes, enzyme-substrate recognition, etc.). 

In 1848 du Bois-Reymond [21] suggested that the surfaces of biological formations have a property similar to the electrode of a galvanic cell and that this is the source of bioelectric phenomena observed in damaged tissues. The properties of biological membranes could not, however, be explained before at least the basic electrochemistry of simple models was formulated. The thermodynamic relationships for membrane equilibria were derived by Gibbs in 1875 [29], but because the theory of electrolyte solutions was formulated first by Arrhenius as late as 1887, Gibbs does not mention either ions or electric potentials. 

BIOELECTROCHEMISTRY. Application of the principles and techniques of electrochemistry to biological and medical problems. It includes such surface and interfacial phenomena as the electrical properties of membrane systems and processes, ion adsorption, enzymatic clotting, transmembrane pH and electrical gradients, protein phosphorylation, cells, and tissues. 

Membranes (Biology)—Electric properties—Congresses. 2. Electrophysiology—Congresses. 

In understanding the electrical characteristics of biological materials such as tissue which is a complex, inhomogeneous, anisotropic, and nonlinear material, we feel it is necessary to first understand the electrical properties of the simpler biological fluids. Additionally since the fields in the vicinity of membranes may be quite large ( xlO 4 volts/cm) it is important to understand the high field behavior as well as the more commonly measured small signal characteristics. 

In this chapter we turn our attention to the properties of solutes. We will compare chemical potentials in the aqueous phases on the two sides of a membrane or across some other region to predict the direction of passive solute fluxes as well as the driving forces leading to such motion. We will also show how the fluxes of charged species can account for the electrical potential differences across biological membranes. 

Fluxes of many different solutes occur across biological membranes. Inward fluxes move mineral nutrients into cells, while certain products of metabolism flow out of cells. The primary concern in this section is the passive fluxes of ions toward lower chemical potentials. First, we indicate that the passive flux density of a solute is directly proportional to the driving force causing the movement. Next, the driving force is expressed in terms of the relevant components of the chemical potential. We then examine the consequences of electroneutrality when there are simultaneous passive fluxes of more than one type of ion. This leads to an expression describing the electrical potential difference across a membrane in terms of the properties of the ions penetrating it. 

The electric properties of biological membranes and proteins depend on the potential distribution and the electrostatic interaction energy of their charges (see e.g.. Refs [1 ]). The potential distribution J/(r) at position r around fixed point charges with a distribution p(r) in a uniform medium of relative permittivity is described by the Poisson equation,... 

Pure lipid membranes are electrical insulators with a specific capacitance of 1 tiF/cm, which separate two electrolytic compartments. The conductance of biological membranes is maiifly determined by highly specialized proteins that act as ion chaimels. For supported membranes to mimic the electrical properties of a biological membrane, it is necessary to measure its electrical characteristics. Even very small defects that are not... 

Electrical properties of membranes. Biological membranes serve as barriers to the passage of ions and polar molecules, a fact that is reflected in their high electrical resistance and capacitance. The electrical resistance is usually 10 ohms cm, while the capacitance is 0.5-1.5 microfarad (pF) cm . The corresponding values for artificial membranes are 10 ohms cm and 0.6 - 0.9 pF cm . The lower resistance of biological membranes must result from the presence of proteins and other ion-carrying substances or of pores in the membranes. The capacitance values for the two types of membrane are very close to those expected for a bilayer with a thickness of 2.5 nm and a dielectric constant of 2. 4 The electrical potential gradient is steep. 

The studies of elementary films formed in inverse emulsions and stabilized by different synthetic and natural surfactants revealed that the membrane electric conductivity experiences a sharp increase upon the addition of some biologically active surfactants. For instance, membrane conductivity may increase by five orders of magnitude when trace amounts of valinomycin antibiotic are introduced into the outer aqueous medium of lipid membrane. At the same time the membrane becomes permeable to potassium and hydrogen ions but impermeable to sodium ions. A sharp decrease in electric resistance of synthetic membranes is observed when proteins and enzymes with suitable substrates are introduced into them. By studying the properties of such membranes one may model important biological processes, e.g. the transfer of neural impulses. 

In order to elucidate the physicochemical properties of such a biological membrane interface, several model membrane systems (lipid monolayer, lipid bilayers, and protein-incorporated lipid model membrane systems) which mimic biological membrane interfaces have also been studied.In particular, many properties at the membrane surface are intimately related to the electrical potential originating from the fixed charge or electrical polarization of the membrane constitutents. 

The biological membrane contains complex molecular components such as proteins and lipids. These have intrinsic electrolyte properties which include the presence of fixed ions. Consequently, the assumption of an electrically neutral membrane, which was made in the Planck model, requires modification. In the following section, the properties of membranes containing a uniform distribution of fixed charges will be discussed. 

---