Potential differences in membranes

Schematic representation of the major pathways for the transport of Ca across cellular membranes. PM, plasma membrane ER(SR), endoplasmic reticulum (sarcoplasmic reticulum) M, mitochondria A P, difference in membrane potential. The transport proteins shown are 1 and 2, PM and ER(SR) Ca -ATPases 3 and 4, PM and ER(SR) receptor-mediated Ca " channels 5 and 6, PM and M (inner-membrane) Na /Ca exchangers 7 and 8, PM and M voltage-sensitive Ca channels. In addition, some not-well-defined passive transport pathways are indicated by dashed arrows.

The membrane potential depolarization in pmal mutants could be explained if the mutant enzymes were less active in pumping protons across the membrane. This notion was supported by kinetic studies on ATP hydrolysis by these enzymes that showed small but significant decreases in Vmax (15). However, a dilemma arose when whole cell medium acidification experiments were performed, which reflect the action of the H+-ATPase in vivo. The rate of glucose-induced proton efflux by pmal mutant cells was found to be considerably better than that of wild-type cells (Figure 2). Only when high external K+ was included in the medium to minimize differences in membrane potential between mutant and wild-type cells did the activity of the... 

The electrogenic potential may be determined by measuring the change in potential due to temperature change of the system or introduction of specific inhibitors (such as ouabain) for Na transport. Since that part of the membrane potential due to passive ionic processes across the membrane (as in the squid axon membrane) is less sensitive to temperature than that due to an active transport process, one can deduce from the temperature coefficient of the membrane potential the contribution due to an active transport process or since ouabain is supposed to inhibit the active transport process, one can measure the active transport contribution to the membrane potential as the difference in membrane potential in two environments (one, a normal physiological solution the other, a normal physiological solution containing ouabain). 

We have shown that the heterogeneity of PS II reaction centers is essential for explaining picosecond fluorescence kinetics data of intact photosynthetic systems. For the first time differences between a- and p-centers have been reported, concerning their primary photophysics. The significant differences in the rate constants and related parameters between PS II and PS Up can not be fully explained only with their difference in antenna size (8) and dieir difference in membrane potential (9). Of course even more complex models to fit the fluorescence decay data cannot be excluded. 

Driving Force Gas moves across a membrane in response to a difference in chemical potential. Partial pressure is sufficiently proportional to be used as the variable for design calculations for most gases of interest, but fugacity must be used for CO9 and usually for Hg... 

FIGURE 30.3 Changes in membrane potential cp of a cell membrane occurring upon application of depolarizing current pulses of different amplitude / (a,b) below threshold (c) excitation of the membrane during an above-threshold pulse. 

Space-clamped (HH) equations relate the difference in electrical potential across the cell membrane (V) and gating variables (0 < m, n, h < 1), for ion channels to the stimulus intensity (7J and temperature (T), as follows ... 

A series of 4,4-disubstituted quinazolin-2-ones derived from HTV nonnucleoside reverse transcriptase inhibitor leads have shown good in vitro potency and in vivo efficacy [28]. Using FLIPR assays on cell lines with different resting membrane potentials, TTA-Q3 (10) and TTA-Q6 (11)... 

Several different changes in mitochondria occur during apoptosis. These include a change in membrane potential (usually depolarization), increased production of reactive oxygen species, potassium channel activation, calcium ion uptake, increased membrane permeability and release of cytochrome c and apoptosis inducing factor (AIF) [25]. Increased permeability of the mitochondrial membranes is a pivotal event in apoptosis and appears to result from the formation of pores in the membrane the proteins that form such permeability transition pores (PTP) may include a voltage-dependent anion channel (VDAC), the adenine nucleotide translocator, cyclophilin D, the peripheral benzodiazepine receptor, hexokinase and... 

Further investigations with bimanyl-labeled K-Ras4B peptides demonstrated that relatively small differences in membrane charging (approximately 10 mol %) are sufficient for an electrostatic enrichment in the more negative environment [230]. With the farnesyl group as a hydrophobic anchor, the peptide is still mobile and can swap between vesicles but may find its target membrane with the sensitive surface potential-sensing function of its lysine residues. 

During the process, the solute diffuses into the intercellular space and, depending on the characteristics of the solute, it may pass through the membrane and enter the intracellular space. Differences in chemical potentials of water and solutes in the system result in fluxes of several components of the material and solution water drain and solute uptake are the two main simultaneous flows. Together with the changes in chemical composition of the food material, structural changes such as shrinkage, porosity reduction, and cell collapse take place and influence mass transfer behavior in the tissue. 

This is a process mainly used in power plants for removal of dissolved polar compounds. Solute is exchanged between two liquids through a selective semipermeable membrane in response to differences in chemical potential between the two liquids. The process removal efficiency for TDS is about 62-96%. 

The opsins probably perform a variety of different tasks, but their known roles are as photosensors or photoisomerases (Foster Bellingham 2002). Photosensory opsins such as the rod and cone opsins, P opsin and VA opsin use light to activate a phototransduction cascade that ultimately results in a change in membrane potential of the photoreceptor cell. By contrast, photoisomerases are involved in photopigment regeneration. The best described photoisomerase is... 

The open state of the Ca channels of the cell membrane is controlled by different signals. We know of voltage-gated Ca channels that are opened by a depolarization or change in membrane potential. There are also Ca chaimels that are controlled by G-protein-mediated signal transmission pathways, and ligand-gated channels (see Chapter 16). It is also reported that InsPs can activate Ca " channels in the cell membrane. 

DIALYSIS. The process of separating compounds or materials by the difference in their rates of diffusion through a colloidal sentipermeahle membrane. Thus, sodium chloride diffuses eleven limes as last as tannin ami twemy-une times as fast as albumin. When the process is conducted under the influence of a difference in electrical potential, as from electrodes on opposite sides of the semipermeable membrane, it is called clectrodialysis. 

If the solute carries a net charge, there is an additional thermodynamic effect of moving the charge across any difference in electric potential that exists between the solutions on the two sides of the membrane. The free energy change then is... 

Discontinuous systems. The membrane is regarded as a surface of discontinuity, hindering the movement of the different ions and molecules. The driving forces are in this case the differences in electrical potential, pressure and chemical potential (765, 166) [see equation (4)]. 

We consider first transport of water and salt across a clay membrane. The salt dissociates into v+ cations of valence z+ and z/ anions of valence 2 We assume that all solutions are ideal, so that the the chemical contributions to the electrochemical potentials within the solution on side i of the membrane are //,vv = piVw + RT In xiw ca p,tVw — RT(xi+ + xt-) and Hi p,V I RT In Xi , where the subscripts (w, ) indicate water and ions, R is the gas constant, T the absolute temperature, p the pressure, xw,x are mole fractions and VW,V are partial molar volumes. There will be an additional electrical contribution Fz , where 0 is the electrical potential and F is the Faraday. Transport through the membrane depends upon the difference in electrochemical potentials across the membrane. If these differences are small, there will be linear transport relations of the form [2]... 

There is difference in electric potential across the membrane. So there is an electric field in the membrane, but there are no electric fields in the two bulk phases. The electrical work required to move charge dQ across the center of the membrane is (cj>B — charge transport because the potential difference is constant. Since dnCx = — dncp, equation 8.3-3 can be used to show that at phase equilibrium,... 

FIGURE 3.1 Concentration profiles in a passive sampling device. The driving force of accumulation is the difference in chemical potentials of the analyte between the bulk water and the sorption phase. The mass transfer of an analyte is governed by the overall resistance along the whole diffusional path, including contributions from the individual barriers (e.g., aqueous boundary layer, biofilm layer, and membrane). 

Pervaporation is a concentration-driven membrane process for liquid feeds. It is based on selective sorption of feed compounds into the membrane phase, as a result of differences in membrane-solvent compatibility, often referred to as solubility in the membrane matrix. The concentration difference (or, in fact, the difference in chemical potential) is obtained by applying a vacuum at the permeate side, so that transport through the membrane matrix occurs by diffusion in a transition from liquid to vapor conditions (Figure 3.1). Alternatively, a sweep gas can be used to obtain low vapor pressures at the permeate side with the same effect of a chemical potential gradient. 

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