Electrochemical potential gradient diffusion

If a diffusion potential occurs inside the membrane, the relation between mass transport and electrochemical potential gradient — as the driving force for the diffusion of ions — has to be examined in more detail. This can be done by three different approaches ... 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Some Transporters Facilitate Diffusion of a Solute down an Electrochemical Potential Gradient... 

Some transporters facilitate diffusion of a solute from a region of relatively high concentration or down a favorable electrochemical potential gradient. Such transporters do not require energy since the transport is in the thermodynamically favorable direction. 

Concepts of local equilibrium and charged particle motion under - electrochemical potential gradients, and the description of high-temperature -> corrosion processes, - ambipolar conductivity, and diffusion-controlled reactions (see also -> chemical potential, -> Wagner equation, -> Wagner factor, and - Wagner enhancement factor). 

The flux of electrolytes through ion exchange membranes based on a difference of chemical potential is low, except for the fluxes of acids and alkalis, compared with the flux in the presence of an electrochemical potential gradient. Therefore, to separate neutral salts by diffusion dialysis is economically limited, except for special cases. 

Diffusion of charged species is by independent paths. In other words, it is assumed that the flux of species / is proportional to its electrochemical potential gradient solely and is independent of the gradient in the electrochemical potential of the other components. 

Given these assumptions, the flux of the defects can be related to the rate of growth of the layer. Assuming one-dimensional diffusion, the defect and electronic flux densities (particles per square meter per second) subject to an electrochemical potential gradient dfii/dx are given by Eq. (7.37), or... 

The problem considered here is slightly different from the one just examined. Consider, for simplicity, an MO oxide subjected to an electrochemical potential gradient d-q o/dx which in turn must result in the mass transport of MO units from one area to another. Typically this occurs during sintering or creep where as a result of curvature or externally imposed pressures, the oxide diffuses down its electrochemical potential gradient (see Chaps. 10 and 12). To preserve electroneutrality and mass balance, the fluxes of the M and O ions have to be equal and in the same direction. 

PASSIVE DIFFUSION Simple diffusion of a solute across the plasma membrane involves three processes partition from the aqueous to the lipid phase, diffusion across the Upid bilayer, and repartition into the aqueous phase on the opposite side. Diffusion of any solute (including drugs) occurs down an electrochemical potential gradient of the solute and is dependent on both its chemical and electrical potential. 

FACILITATED DIFFUSION Membrane transporters may facilitate diffusion of ions and organic compounds across the plasma membrane this facilitated diffusion does not require energy input. Just as in passive diffusion, the transport of ionized and nonionized compounds across the plasma membrane occurs down their electrochemical potential gradient. Therefore, steady state will be achieved when the electrochemical potentials of the compound on both sides of the membrane become equal. 

Fig. 21.13. Transport of compounds across the inner and outer mitochondrial membranes. The electrochemical potential gradient drives the transport of ions across the inner mitochondrial membrane on specific translocases. Each translocase is composed of specific membrane-spanning helices that bind only specific compounds (ANT adenine nucleotide translocase). In contrast, the outer membrane contains relatively large unspecific pores called VDAC (voltage-dependent anion channels) through which a wide range of ions diffuse. These bind cytosolic proteins such as hexokinase (HK), which enables HK to have access to newly exported ATP.

The electrochemical potential gradient is therefore the driving force for both diffusion and migration, which are generally coupled together ... 

Diffusion means a local non-convective flux of matter under the action of a chemical or - in the case of charged particles - an electrochemical potential gradient. By expressing the flux of particles of type i per unit of the concentration gradient dc /dx (i. e. by forming the quotient of the measurable parameters ji and dCijdx), we arrive at a definition of the partial chemical diffusion coefficient of the particles of type r. 

The Nernst-Planck equations can be used for modeling mass transfer within a single-phase dense ceramic membrane with neither external diffusion and surface exchange effects nor occluded porosity in the dense layers (Figure 14.1c) [25]. The flux of each charged species i (i.e., vacancies or other charged species), y,-, can be modeled as a function of the electrochemical potential gradient, V/i ... 

Remark The ionic species that undergo the electrochemical reactions move under the influence of several transport phenomena ionic drift under an electric field (a transport phenomenon also called conduction or migration), drift under a chemical-potential gradient (diffusion transport), and/or a convection phenomenon. The origin of the transport of electroactive species plays a very important part in the principles of the electrochemical methods of analysis. 

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