Cation-coupled transport

Fig. 12. Electron-cation coupled transport a redox-driven electron-cation symport consisting of an electron carrier (nickel complex) and a selective cation carrier (macrocyclic polyether). RED, potassium dithionite OX, Na3[Fe(CNft)].

If the cations of variable valency (e.g., Fe2+/Fe3 + ) are present in not too low concentrations, the crystals will be semiconductors. In non-equilibrium vermiculites, the internal electric field is then strongly influenced by their electronic conductivity, as explained in Section 4.4.2. If we start with an equilibrium crystal and change either pH, ae, aor a, (where i designates any other component), coupled transport processes are induced. The coupling is enforced firstly by the condition of electroneutrality, secondly by the site conservation requirements in the T-O-T blocks (Fig. 15-3), and thirdly by the available free volume in the (van der Waals) interlayer. It is in this interlayer that the cations and the molecules are the more mobile species. However, local ion exchange between the interlayer and the relatively rigid T-O-T blocks is also possible. 

The demonstration by Crane (1960, 1965) that Na+ ions were essential for the translocation of monosaccharides by segments of the intestine brought in a new era of understanding of the central role of ion coupled transport, particularly in higher organisms. While Na+ is clearly the predominant cation involved in cation driven solute accumulation in mammalian systems, current work has provided examples of H+ driven solute transport in intestine and kidney (Jessen et al., 1989 Ganapathy and Leibach, 1986). Conversely, in yeast and bacteria, H+ driven mechanisms are in the majority (Seaston et al., 1973 Hirata et al., 1973), but examples of Na+-cou-pled fluxes exist, e.g., proline transport (Dibrov, 1991). 

Currently, patch clamping (Jauch and Lauger, 1986 Bimir et al., 1991 Parent et al., 1992a,b) has all but replaced lipophilic dyes and lipophilic cations as the method of choice in measuring membrane potential. The latter studies also address the site of action of the membrane potential in coupled transport and show that the... 

Some transporters may function with either cation (Jessen et al., 1989). Cation coupled ATP-ases have also been shown to be capable of substituting H+ for Na+, depending on pH (Polvani and Blostein, 1988 Polvani et al., 1989). Such observations are consistent with the idea that the selectivity for H+ or Na+ reflects adaptation of a common, basic mechanism which has evolved subtle differences to exploit the variations in the ambient conditions. 

A few cations are transported vectorially into and out of cells, leading to transmembrane concentration gradients. This type of transport, called active transport, is important for H+, Na+, K+, Ca2+ and (possibly) Mg2+. Specific enzymes are found in membranes which couple the hydrolysis of ATP to ion transport, and thereby achieve active pumping21. The activities of humans can seriously affect this type of transport. 

As examples of coupled counter-transport (see Figure 13.2d) and coupled cotransport (see Figure 13.2e) the transport of titanium(lV) from low acidity (pH = 1) and high acidity ([H+] = 7 M) feed solutions, respectively using the HUM system [1,2] may be presented. The di-(2-ethyUiexyl) phosphoric acid (DEHPA) carrier reacts with Ti(IV) ion to form complexes on the feed side (see Equations 13.25 and 13.26) and reversible reactions take place on the strip side (see Equations 13.27 and 13.28). Energy for the titanium uphill transport is gained from the coupled transport of protons in the direction opposite to titanium transport from the strip to the feed solutions. In the second case (high-feed acidity), Cl anion cotransported with Ti(IV) cation in the same direction. In both cases fluxes of titanium, protons, and chlorine anion are stoichiometrically coupled. 

Finally, if the metal carrier is a basic extractant (i.e., a long-chain amine), L, the difference in Dm between feed and strip is generally obtained by a concentration gradient of the counterion, X as well as H which is accompanying the metal cation into the membrane and the chemical reaction that is responsible for the coupled transport can be schematized as... 

The conditions of Donnan equilibrium derived in Equation 34.6 can be understood in a slightly different way. For the coupled transport described previously, the heterogeneous exchange of cation A in the aqueous phase and cation B in the membrane phase is represented by the following equation ... 

---