Cell membranes alkali metal transport

Catoptromers —. see Enantiomers Cell division calcium, 6.595 Cell membranes alkali metal transport, 3, 54 Cells labelled... 

Alkali metal transport in biochemistry is a vital process in maintenance of cell membrane potentials of use, for example, in nerve signal transduction and is at the core of some of the early work on artificial ionophores that mimic natural ion carriers such as valinomycin. Ionophore mediated ion transport is much slower than transport through cation and anion ion channel proteins, however. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

K+ channels selectively transport K+ across membranes, hyperpolarize cells, set membrane potentials and control the duration of action potentials, among a myriad of other functions. They use diverse forms of gating, but they all have very similar ion permeabilities. All K+ channels show a selectivity sequence of K+ Rb+ > Cs+, whereas the transport of the smallest alkali metal ions Na+ and Li+ is very slow—typically the permeability for K+ is at least 104 that of Na+. The determination of the X-ray structure of the K+-ion channel has allowed us to understand how it selectively filters completely dehydrated K+ ions, but not the smaller Na+ ions. Not only does this molecular filter select the ions to be transported, but also the electrostatic repulsion between K+ ions, which pass through this molecular filter in Indian file, provides the force to drive the K+ ions rapidly through the channel at a rate of 107-108 per second. (Reviewed in Doyle et al., 1998 MacKinnon, 2004.)... 

Monensin belongs to the family of polyether ionophores. These compounds consist of a series of tetrahydrofuran and tetrahydro-pyran rings and have a carboxyl group that forms neutral salts with alkali metal cations. Their three-dimensional structure presents a lipophilic hydrocarbon exterior with the cation encircled in the oxygen-rich interior. They probably act by transporting cations through the lipid bi-layer of cell membranes, thereby preventing the concentration of potassium by the cells. Evidence for this is... 

Complexes of alkali metals and alkaline-earth metals with carbohydrates have been reviewed in this Series,134 and the interaction of alkaline-earth metals with maltose has been described.135 Standard procedures for the preparation of adducts of D-glucose and maltose with the hydroxides of barium, calcium, and strontium have been established. The medium most suitable for the preparation of the adduct was found to be 80% methanol. It is of interest that the composition of the adducts, from D-glucose, maltose, sucrose, and a,a-trehalose was the same, namely, 1 1, in all cases. The value of such complex-forming reactions in the recovery of metals from industrial wastes has been recognized. Metal hydroxide-sugar complexes may also play an important biological role in the transport of metal hydroxides across cell membranes. 

The linear peptide gramicidin A (15 residues) dimerizes in biological and synthetic membranes in a head-to-head manner30 to give channels of about 5 A in diameter and 32 A in length. These channels are specific for alkali metal cations, and show high transport rates, e.g. 107 Na+ ions s l, a value close to the ion fluxes found for the Na+ channel of nerve cells. Each structural unit appears to contain two channels, each of which contains two binding sites for cations. 

A recent study (12) has shown that Nafion is also suitable for use in water electrolyzers with alkaline solution as the supporting electrolyte. The major charge carrier is the alkali metal ion because of the negligible IT " ion concentration in alkaline solution and the Off ion rejection capability of the cation exchange membrane. The current efficiency of the cell is related to the inhibition of the transport of gaseous products across the separator. Thus, the ionic groups of the membrane are not important, in this case, because alkaline solution is the major electrolyte, and the migration of any ionic species across the membrane would not affect the current efficiency of the cell (33). 

Current research is centred on making compact cells of high efficiency. They are described in terms of the electrolyte that is used. The principle types are alkali fuel cells, described above, with aqueous KOH as electrolyte, MCFCs (molten carbonate fuel cells), with a molten alkali metal or alkaline earth carbonate electrolyte, PAFCs (phosphoric acid fuel cells), PEMs (proton exchange membranes), using a solid polymer electrolyte that conducts ions, and SOFCs, (solid oxide fuel cells), with solid electrolytes that allow oxide ion, 0 , transport The... 

Bulk Liquid Membranes (BLM). This is the simplest type of liquid membrane (2-8) and is utilized for fundamental studies of certain aspects of liquid membrane transport processes. In one such process, a beaker-in-a-beaker cell (Figure 1) consists of inner and outer compartments which contain the aqueous feed (F) and strip (S) solutions, respectively. The inner beaker contains the stripping solution and is surroimded by the feed solution. Both aqueous solutions contact the upper organic layer, which is the liquid membrane. Mass transfer takes place from the feed solution through the liquid membrane and into the strip solution. Bartsch et aL studied the transport of alkali metal cations across bulk liquid membranes in which a crown ether carboxylic acid in the organic layer served as the carrier (2,3). 

DILLON R A and SHRivER D F (2001), lon transport and vibrational spectra of branched polymer and dendrimer electrolytes , Chem Mater, 13 1369-1373 FENTON D E, PARK J M and WRIGHT p V (1973), Complexes of alkali metal ions with poly (ethylene oxide) . Polymer, 14 589-589 FEUILLADE G and PERCHE p (1975), lon-condnctivc macromolecular gels and membrane for solid lithium cells , J Appl Electrochem, 5 63-69 FONTENELLA J J, WINTERGILL M C, CALAME J P and ANDEEN C G (1983), Electrical relaxation in pure and alkah metal thiocyanate complexed poly(ethylene oxide) . Solid State Ionics, 8 333—339... 

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