Transmembrane potential difference

Discuss the significance of biological transmembrane potential differences. How are such potential differences created and maintained ... 

Experimentally it is the transmembrane potential difference that is observed by the use of potential-sensitive fluorescent dyes. Two of the components of the transmembrane potential difference are the intra- and extracellular potential differences (Fig. 17.2). If (f)x is the potential in the interior of the cell and 0O the exterior potential, then the transmembrane potential, Em, is given, in the case of a symmetric membrane where D for the two sides cancels, by... 

Kg. 17.2 Schematic representation of transmembrane potential profile. Em, transmembrane potential difference Eo DL, exterior diffuse double layer potential difference Ei DL, interior diffuse double layer potential difference ED potential difference due to membrane molecular dipoles EDi = EDo, symmetric membrane potential EDiff diffusion potential difference. 

The transmembrane potential difference can be described by the Goldman equation that relates t tm to the permeabilities of the membrane to specific ions and the concentrations of such major ions on either side of the PM ... 

As outlined in Chapter 3, cell excitability can in part be determined by the maintenance of gradients of Na+, K+ and CP ions. Differential plasma membrane (PM) permeabilities to such ions and the gradients of ion concentration contribute to the transmembrane potential difference (t tm), which is typically about —0.1 volt (V) (inside with respect to the outside). In addition, the cytosolic free concentration of Ca2+ is extremely low (0.1 pM in resting cells and about 10 pM in excited cells) as compared to concentrations of Na+, CP and K+ of about 10, 10 and 100 mM, respectively, in the cytosol and about 100, 100 and 10 mM, respectively, in the extracellular milieu. These huge ion gradients are maintained through the operation of ion pumps such as the adenosine 5 -triphosphate (ATP)-energized Ca2+ pump (Ca2+-ATPase) and the Na+ and K+ pump (Na+, K+-ATPase). 

The transmembrane potential difference roles in signaling and energy transduction... 

Finally, its also worth emphasising that related to the transmembrane potential difference, a transmembrane gradient of electric charge known as the Donnan potential is also known to exist. The Donnan potential arises from the inability of larger charged macromolecules or other hxed charges to move across a membrane and for counterions to compensate this gradient. 

Other electrochromic membrane probes of note include, me-rocyanine 540 used to study the transmembrane potential in liver mitochondria although some problems are evident (5, 30, 31). Overall, measurements of transmembrane potential differences in living cells are well established with the methods fairly reliable and robust. Many of these membrane potential measurement strategies are generic and hold for plasma membranes of neurones and other excitable tissues (6). 

These involve the use of techniques to suppress the fast inward Na+ current. Either a voltage can be applied so that the transmembrane potential difference is clamped above the level at which the Na+ current is activated. (36) or tetrodotoxin (37) -which specifically inhibits the Na+ current, can be added, or the membrane can be depolarized by raising the external K+ (38,39). Under each of these conditions the slow inward current can be activated by adding aminophylline or isoproterenol, (39) and by electrical stimulation. 

Fig. 1. Profile of the electrical potential across the PM. yfp is the electrical potential at the PM surface. is the transmembrane potential difference from bulk phase to bulk phase as measured by microelectrodes, i nisurf the transmembrane potential difference from surface to surface. The upper solid line represents the profile after the addition of solutes to the exterior medium that depolarize the outer face, thereby altering i m.surf hut not E. The figure is redrawn from Kinraide (2001).

The formation of protons was detected experimentally during the peroxidation of lipids of bilayer lipid membranes.68,69 It was shown by direct measurements of the aqueous solution pH that X-ray irradiation of the aqueous solution on one side of the bilayer leads to ejection of protons into the solution. As a result, a transmembrane potential difference is generated on BLM. Since after X-ray irradiation peroxide anions are formed in the solution, peroxidation of lipids can be initiated by the anion radical O. 70... 

Electrical activity in plant and animal cells are caused by two main factors first, there are differences in the concentrations of ions inside and outside the cell and second, there are molecules embedded in the cell membrane that allow these ions to be transported across the membrane. The ion concentration differences and the presence of large membrane-impermeant anions inside the ceU result in the existence of a polarity the potential inside a cell is typically 30 to 100 mV lower than that in the external solution. It is important to note that almost all of this potential difference occurs across the membrane itself. The bulk solutions both inside and outside the cell are, for the most part, at a uniform potential. This transmembrane potential difference is in turn sensed by molecules in the membrane that control the flow of ions. 

FIGURE 22.3 Circuit representation of membrane current. The conductance can be nonlinear as indicated by the powers p and q on the state variables m and hi, respectively. These state variables are typically time-varying functions of the transmembrane potential difference, V. The battery, , represents the electrochemical gradient of the ionic species responsible for the current. 

Transmembrane potential difference The potential difference between the inside and the outside of a cell manifests itself very close to membrane. In many cases the potential of the external fluid is... 

Voltage clamp This refers to the experimental procedure of using active electronic circuits to hold the transmembrane potential difference at a fixed value by pumping current into the cell. 

Three membrane potentials appear to be associated with cellular membranes but can also be present in artificial membranes. The cellular membrane potential that has received the most experimental attention is associated with a gradient of electrical charge across the phospholipid bilayer known as the transmembrane potential difference (with the symbol Ai/ ). These charge gradients are engineered and... 

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