Transport 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. 

The various proposed components of the permease system are based upon the response of the transport system to genetic or environmental changes. The complex nature postulated for the intact permease system is necessary to account for the various observed phenomena such as facilitated diffusion, active concentration, facilitated efflux, exchange diffusion, and counter transport of one compound driven by the downhill efflux of a second (2). 

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... 

The term exchange diffusion has another usage in the literature—namely, to describe the carrier-mediated movement of some solute in one direction across a membrane in exchangefor a different solute being transported in the opposite direction. Again, the Ussing-Teorell equation is not obeyed. 

Wodzki R and Sionkowski G. Exchange diffusion transport of ions in Uquid membranes. Part IV. Thermodynamic network analysis of nonstationary transport of divalent ions. Pol J Chem, 1995 69 407 22. 

Wodzki R. Exchange diffusion transport of ions through liquid membranes. Part II. Stationary process mediated by a multisided carrier. PolJChem, 1991 65(9-10) 1715-1729. 

The second mechanism is the one more relevant to the action of amphetamine and related agents. This mechanism is illustrated in Fig. 4.1. Amphetamine, and other small molecular weight compounds with similar structures, are substrates at the monoamine uptake carriers and are transported into the neuron. The uptake carrier has an extracellular and intracellular face, and after transporting a substrate (amphetamine, etc.) into the neuron, the intracellular carrier face can bind to dopamine and transport it back to the extracellular face. This exchange diffusion mechanism is calcium independent, and is capable of robustly increasing synaptic transmitter levels. This process is often described as a "reversal" of the normal uptake carrier process. 

The same steps as discussed above for the case of isotope exchange (diffusion in liquid film, surface reaction, intraparticle diffusion) were considered in a kinetic model [771] of metal ion adsorption from solution. This model was presented in a book with diskettes (FORTRAN program, rate controlled by reaction, by transport or mixed control). 

Full coupling of transport between the involved phases. This corresponds to a situation where there is a very fast cross-phase electron and/or mass transport exchange. Here, the apparent diffusion coefficient measured (e.g., via chronoamperometric experiments) should satisfy ... 

Absorption from the intestines back into the blood occurs by three mechanisms active transport, diffusion, and solvent drag. Active transport and diffusion are the mechanisms of sodium transport. Because of the high luminal sodium concentration (142 mEq/L), sodium diffuses from the sodium-rich gut into epithelial cells, where it is actively pumped into the blood and exchanged with chloride to maintain an isoelectric condition across the epithelial membrane. 

PCBs in water are transported by diffusion and currents. PCBs in surface water essentially exist in three phases dissolved, particulate, and colloid associated (Baker and Eisenreich 1990). The heavier and less soluble congeners in the water column are more likely to be associated with particulates and colloids, and do not freely exchange into the vapor phase. However, the more water soluble, lower chlorinated (and ori/io-rich) congeners are predominantly in the dissolved state in the water column and can readily partition into the vapor phase. In New Bedford Harbor, Massachusetts, Burgess et al. (1996) reported that the ratio of colloid associated PCBs to freely dissolved PCBs increased from 1.2 to 8.0 (di-CBs to octa-CBs, respectively) as the degree of chlorination increased. However, at this site, the majority of the PCBs were associated with the particulate phase regardless of solubility or chlorination. 

Isotope exchange diffusion profiles can also be measured ex situ by SIMS, and can, in principle, reveal surface kinetics in addition to bulk transport... 

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-... 

Hydrogen sulfide and methane fluxes were measured at ambient conditions for 200 um perfluorosulfonic acid cation exchange membranes containing monoposltlve EDA counterions as carriers. Facilitation factors up to 26.4 and separation factors for H2S/CH up to 1200 were observed. The HjS transport Is diffusion limited. The data are well represented by a simplified reaction equilibrium model. Model predictions Indicate that H S facilitated transport would be diffusion limited even at a membrane thickness of 1 um. 

Since carboxylic ionophores transport ions by an electrically silent exchange diffusion mechanism, it is the anionic form of the ionophore which interacts with cations at membrane interfaces Therefore, the lasalocid species most germane for ion complexation within membranes is the free anion Deprotonation of the C25 carboxyl with base results in substantial changes in the absorption and CD spectra from that of the protonated form stabilized by 0.5 equivalents of HCl (cf Fig. 5). The 245 nm absorption band observed for protonated lasalocid intensifies and shifts to 240 nm upon deprotonation while the 317 nm absorption band shifts to 310 nm with little change in intensity. In the CD spectrum, peak I shifts hypso-chromically to 240 nm and intensifies upon deprotonation while peak II shifts bathochromically and diminishes in intensity. 

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