Membrane potential definition

As demonstrated in work on the ion transport through a membrane [20,21], the relation among AFwi-w2 in curve 1, AFwi/lm in curve 2, and AFlm/w2 in curve 3 at a definite current value can be approximated by Eq. (2), when Wl, W2, and LM contain sufficient concentrations of ions as in the case of Eq. (3). This equation suggests that the membrane potential in the presence of sufficient electrolytes in Wl, W2, and LM is... 

Mitochondria perform a crucial series of reactions in the human organism. The inner mitochondrial membrane is relatively impermeable to charged substances, although uncharged substances, such as Oz and CO, move across with ease. To perform their functions, however, mitochondria must transport various charged substances in and out of its matrix. This is done via transport particles called porters or translocases (see Chapter 9 for definitions). They are not directly associated with the hydrolysis of ATP some depend for their operation on concentration gradients of one sort or another, and others depend on the mitochondrial membrane potential. 

Less well-defined but particularly important in terms of the function of non-neuronal cells are so-called receptor-operated channels [6,7]. By definition these are channels in the plasma membrane which open in response to hormone-receptor interaction without a change in membrane potential. The mechanism of their opening may either be by a direct coupling of receptor (possibly via a G protein) with the channel, or by an indirect coupling via the generation of an intracellular messenger such as cAMP or the putative messenger, inositol 1,3,4,5-tetrakisphosphate. 

Fig. 3. A. Generalized potential energy profile of an ion channel through a membrane showing definitions of symbols used. Barriers are assumed to be symmetric and equally spaced. B. Linear decrease in barrier height from outside (OUT) to inside (IN). This can give rise to anomalous (outwardly directed) rectification in a Na+ channel. Minima are all drawn at the same level for convenience, since their heights do not affect the current-voltage relationship. C. Linear increase in barrier heights from outside to inside. This accentuates normal, inwardly directed rectification in a Na+ channel. See text.

From equation (3), besides the charges, the membrane potential is affected by the surface area (A) and the thickness (d), vrhich have been demonstrated, 22 We have measured the capacitance of the glass membrane as a function of pH as discussed in the last part of this paper. The membrane electrodes behave definitely as capacitors. 

The great potential and numerous advantages of carbon membranes will definitely lead to their wide application in the gas separation industry over the eoming years. 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

There are several major classes of Ca channels (1) receptor-operated Ca channels in plasma membranes (2) ligand-gated Ca " channels in intracellular membranes and (3) voltage-dependent Ca channels that are usually found in plasma membranes or the invaginations of the plasma membrane that are known as transverse tubule membranes. Receptor-dependent or receptor-operated Ca channels (ROCCs) are primarily opened in response to activation of their associated receptors and, by definition, exhibit a certain amount of selectivity for Ca " over other cations. Several potentially different types of ROCCs have been characterized including ATP-sensitive channels in smooth muscle [1], mitogen and IP3-sensitive... 

It should be recalled that the term surface potential is used quite often in membranology in rather a different sense, i.e. for the potential difference in a diffuse electric layer on the surface of a membrane, see page 443.) It holds that 0 = 0 + X (this equation is the definition of the inner electrical potential 0). Equation (3.1.2) can then be written in the form... 

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