Electrical potential membrane

Figure 5. Relationship of depolarization of membrane electrical potential (E) in excised roots of barley 250 flM benzoic acids against their partition coefficients (log P). The numbers correspond to specific benzoic acid derivatives (1) salicylic, (2) -hydroxybenzoic, (3) protocatechuic, (4) gentisic,...

CalcitrioPs action primary function is in regulating plasma calcium concentration. In health, the plasma total calcium concentration is tightly controlled at 2.35-2.55 mmol/1. Only the ionized or free fraction, amounting to about 50% of the total, is physiologically active in for example, maintenance of membrane electrical potential and bone formation. The hormone causes increased bone resorption via activation of osteoclasts (see Section 9.4) and increased intestinal absorption of calcium following the synthesis of a specific binding protein in mucosal cells. As described in Section 4.7, some... 

The voltage difference, sometimes referred to as the transmembrane potential or the membrane electric potential, that is due to the difference of concentrations of ions on either side of the membrane. Typically, such potentials are measured in millivolts (mV) and usually have values between —30 and —70 mV. In most cells, the interior of a cell is negative with respect to the exterior. 

The gating may be controlled by the membrane electrical potential, by a hormone, by the specific ligand, or by other means. Some pores may be small enough to allow only small molecules such as HzO to pass through. Others may be large enough to allow for nonspecific simple diffusion of molecules of low molecular mass. Structures are known for only a few. 

Unlike transport across the membranes of the ER, transport across plasma membranes of bacteria often requires both hydrolysis of ATP and energy provided by the membrane electrical potential.33 38 44-48 Secretion into the periplasmic space has been well characterized but less is known about transport of proteins into the external membranes of E. coli48 A16 kDa periplasmic chaperone may be required.483... 

In this case, the energy required for the transport of the molecule across the membrane is derived from the coupled hydrolysis of ATP, for example the movement of Na+ and K+ ions by the Na+/K+-ATPase. All cells maintain a high internal concentration of K+ and a low internal concentration of Na+. The resulting Na+/K+ gradient across the plasma membrane is important for the active transport of certain molecules, and the maintenance of the membrane electrical potential (see Topic N3). The movement across the membrane of Na+, K+, Ca2+ and H+, as well as a number of other molecules, is directly coupled to the hydrolysis of ATP. 

The 1952 Hodgkin-Huxley model for membrane electrical potential is perhaps the oldest and the best known cellular kinetic model that exhibits temporal oscillations. The phenomenon of the nerve action potential, also known as excitability, has grown into a large interdisciplinary area between biophysics and neurophysiology, with quite sophisticated mathematical modeling. See [103] for a recent treatise. 

We have seen that chemical and biological interactions lead to mathematical models displaying a variety of linear and nonlinear behavior relaxation to fixed points, multistability, excitability, oscillations, chaos, etc. Despite the different origin of the models, and the diverse nature of the variables they represent (chemical concentrations, population numbers, or even membrane electric potentials) the mathematical structures are quite similar, and it is possible to understand some aspects of the dynamics in one field (e.g. the chemical oscillations in the BZ reaction) with the help of models from other fields (for example the FN model of neurophysiology, or a phytoplankton-zooplankton model). This possibility of common mathematical description will be used in the rest of the book to highlight the similarities and relationships between chemical and biological dynamics when occurring in fluid flows. 

In the exocrine pancreatic cells. In many animal cells, the combined force of the Na" concentration gradient and membrane electric potential drives the uptake of amino acids and other molecules against their concentration gradient by lon-llnked symport and antiport proteins (see Section 7.4). And the conduction of action potentials by nerve cells depends on the opening and closing of Ion channels In response to changes In the membrane potential (see Section 7.7). 

Here we discuss the origin of the membrane electric potential In resting cells, how Ion channels mediate the selective movement of Ions across a membrane, and useful experimental techniques for characterizing the functional properties of channel proteins. 

The magnitude of the membrane electric potential is the same (59 mV for a tenfold difference in ion concentrations), except that the right side is now positive with respect to the left (Figure 7-13c), opposite to the polarity obtained across a membrane selectively permeable to Na ions. 

In this example, the Na concentration gradient and the membrane electric potential contribute almost equally to the total AG for transport of Na Ions. Since AG Is <0, the inward movement of Na Ions Is thermodynamically favored. As discussed in the next section, certain cotransport proteins use the inward movement of Na to power the uphill movement of other ions and several types of small molecules into or out of animal cells. The rapid, energetically favorable movement of Na ions through gated Na channels also is critical in generating action potentials in nerve and muscle cells. 

A FIGURE 7-20 Transmembrane forces acting on Na ions. As with all Ions, the movement of Na ions across the plasma membrane is governed by the sum of two separate forces—the ion concentration gradient and the membrane electric potential. At the internal and external Na concentrations typical of mammalian cells, these forces usually act in the same direction, making the inward movement of Na" ions energetically favorable. 

The electrochemical gradient across a semipermeable membrane determines the direction of ion movement through channel proteins. The two forces constituting the electrochemical gradient, the membrane electric potential and the ion concentration gradient, may act in the same or opposite directions (see Figure 7-20). 

Moreover, mitochondria, chloroplasts, and bacteria utilize essentially the same kind of membrane protein, the FqFi complex, to synthesize ATP. The FqFi complex, now commonly called ATP synthase, Is a member of the F class of ATP-powered proton pumps (see Figure 7-6). In all cases, ATP synthase Is positioned with the globular Fi domain, which catalyzes ATP synthesis, on the cytosolic face of the membrane, so ATP Is always formed on the c rt osolIc face of the membrane (Figure 8-2). Protons always flow through ATP synthase from the exoplasmic to the c rt osolIc face of the membrane, driven by a combination of the proton concentration gradient ([H ]exopiasmic > [H ]cytosolic) and the membrane electric potential (exoplasmic face positive with respect to the c rt osolIc face). 

A burst of ATP synthesis accompanied the transmembrane movement of protons driven by the 10,000-fold H" concentration gradient (10 M versus 10 M). In similar experiments using "inside-out" preparations of submitochondrial vesicles, an artificially generated membrane electric potential also resulted in ATP synthesis. 

V/cm. One hypothesis is that the positive charges in the am-phipathic matrix-targeting sequence could simply be elec-trophoresed, or pulled, into the matrix space by the inside-negative membrane electric potential. 

Electro- dialysis Cation- and anion—exchang e membrane Electrical potential difference Electric charges of particle Removing salts, acids, and bases from fermentation broths, separation of amino acids, etc. 

Membrane electrical potential With respect to biological membranes, a voltage difference that exists across a membrane owing to differences in the concentrations of ions on either side of the membrane. 

Electrodialysis Cation and anion exchange membranes Electrical potential gradient Electrical charge of particle and size Desalting of ionic solution... 

Fig. 7 Effect of membrane electrical potential on the photocurrent under polychromatic visible light excitation and rate constant for the charge recombination reaction, observed during the sensitization of Ti02 membrane films by RUL3 complex.

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