Cell Exchange diffusion

Organelles within cells have their own ion-concentrating mechanisms. Thus, mitochondria can concentrate K+, Ca2+, Mg2+, and other divalent metal ions as well as dicarboxylic acids (Chapter 18). The entrance and exit of many substances from mitochondria appear to occur by exchange diffusion, i.e., by secondary active transport. Such ion exchange processes may also occur in other membranes. 

Figure 4.33. Equivalent circuit of a catalyst layer [8]. (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RB, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

For convenience, we have been discussing facilitated diffusion into a cell, but the same principles apply for exit and for fluxes at the organelle level. Let us assume that a transporter for K+ exists in the membrane of a certain cell and that it is used as a shuttle for facilitated diffusion. Not only does the carrier lead to an enhanced net flux density toward the side with the lower chemical potential, but also both the unidirectional fluxes and i ut can be increased over the values predicted for ordinary diffusion. This increase in the unidirectional fluxes by a carrier is often called exchange diffusion. In such a case, the molecules are interacting with a membrane component, namely, the carrier hence the Ussing-Teorell equation [Eq. 3.25 = c /(ctjeljFEM/RT)] is not obeyed because it does not consider... 

Once inside the cell, exchange to stronger ligands may take place, thereby preventing back diffusion and forming a kinetic trap for the metal (Langston and Bryan 1984). 

Many substrates cross cell membranes by processes other than passive diffusion. When the transport is carrier-mediated, e.g., facilitated diffusion, active transport, and exchange diffusion, the carrier modifies the conductance of the membrane and may either increase or decrease the flux of the substrate across the membrane. A common characteristic of all carrier-... 

The chlorine evolution reaction and the hydrogen evolution reaction at the anode and the cathode, respectively, in a chlor-alkali cell are controlled by the electrochemical and/or chemical steps rather than by mass transfer. However, the transport phenomena across the separator, either a porous diaphragm or an ion-exchange membrane, are governed by the solution flow near the surface. The disproportionation reaction of hypochlorites in a chlorate cell is diffusion-controlled process. Consequently, knowledge... 

The PC 12 cell line has also been used to probe the mechanisms of action of various neurologically active drugs. For instance, Sulzer et al. have investigated the effects of amphetamine on exocytosis from PC 12 cells [52], Two distinct mechanisms of action have been proposed (1) exchange diffusion [53] and (2) vesicle depletion [54,55], The exchange-diffusion model involves the attachment of extracellular amphetamine to the dopamine transporter, and the subsequent transport of amphetamine into the cell while dopamine is simultaneously transported out of the cell. This model appears to predominate at low concentrations of applied amphetamine [52]. The second mechanism of action, apparently operational at high doses, involves the depletion of catecholamine from intracellular storage vesicles followed by reverse transport out of the cell [52]. The ampero-metric detection of individual exocytosis events at PC 12 cells provides a unique method to probe this second mechanism. 

Interference withcaldiantraniport Some eviderKe indicates that ionophores may alter calcium transport by changing the sodium component of the s iunv-calcium exchange diffusion carrier in cell membranes, le ing tofiie accumula> tion of calcium in the cells and mitochondria and pOKiUy causii cell death. 

In this process there is no net exchange of ions between the cell and its surroundings. The membrane component that complexes with the ion discharges that ion into one phase, and then takes up from that phase another ion of the same species. This exchange-diffusion type of mechanism has been experimentally observed for sugar transfer reactions, for amino acids and for inorganic ions. 

As noted above, cw-DDP enters cells by diffusion where it is converted to an active form. This is due to the lower intracellular chloride concentration, which promotes ligand exchange of chloride for water and thus formation of the active aquated complex. Thus, the platinum-containing complex should be neutral to enter the cell and labile chloride groups need to be present to form the active species widiin the cell. The antineoplastic activity of cw-DDP appears to be related to its interaction with DNA nucleotides, as a monoaquo species. The monohydrated complex reacts with the DNA nucleotide, forming intra/interstrand crosslinks. Of the four nucleic acid bases, cw-DDP has been shown to preferentially associate with guanine. The most common are intrastrand crosslinks between adjacent guanines. ... 

Heinz, E., and Walsh, P. M., 1958, Exchange diffusion, transport and intracellular level of amino acids in Ehrlich carcinoma cells, /. Biol. Chem. 233 1488. 

Macromixing is estabflshed by the mean convective flow pattern. The flow is divided into different circulation loops or zones created by the mean flow field. The material is exchanged between zones, increasing homogeneity. Micromixing, on the other hand, occurs by turbulent diffusion. Each circulation zone is further divided into a series of back-mixed or plug flow cells between which complete intermingling of molecules takes place. 

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