Electroneutral transport

Figure 15.2 Ion transporters in human vaginal-ectocervical epithelial cells, sorted into the apical and basolateral domains of the plasma membrane. Black solid symbols depict electrogenic transporters hatched symbols depict electroneutral transporters. Squares are tight junctions. MCT—monocarboxylate transporter NBC— Na+/HCO co-transporter AE—acid (Cl /HCOj ) exchanger NHE (types 1 or 3)—Na+/H+ exchangers.

Figure 5.9 The sodium ion/glucose transporter and sodium ion/ amino acid transporter. The biochemistry of the two processes is identical. To maintain electroneutral transport K ion replaces Na ion, via NaVK ATPase. The broader arrow indicates overall effect (i.e. unidirectional) transport.

Activity of the H,K ATPase results In a primary secretion of 160 mM of HCl Into the secretory canaliculus. Because the H,K ATPase Is electroneutral. It Is necessary that the KCl permeability pathway assodated with the canaliculus transfer a minimum of 160 mmol of KCl for each liter of acidic fluid secreted. This Is true whether the KCl pathway consists of conductive or electroneutral transporters. It Is likely. In fact, that the KCl pathway allows transfer of a slight excess of KCl over the minimum required for the production of HCl. This Is suggested both by the observation that gastric secretions contain a low but significant concentration of KCl and by the likelihood that the H,K ATPase Is not fully efficient at recovering K from the canalicular fluid. In the absence of other mechanisms, the combined activity of the transporters at the apical pole of the parietal cell would lead to alkallnizatlon of the cell and depletion of cellular Cl" and K. The potential disturbances In electrolyte balance are prevented by the activity of transporters at the basolateral membrane. These Include an anion exchanger (AE, HCOj /Cl"), a sodlum/proton exchanger (NHE), and the Na,K ATPase. 

Electroneutral Transport. When the membrane was impregnated with oleic acicL it was possible to observe the decrease of pH in the receiving solution, due to H transport from the more acidic source to the more alkaline receiving solution. The rate of this process reached a maximum after a relatively small lag period ( 20 min) and H transport was stimulated by increasing the KCl concentration in the receiving solution. Addition of IM KCl to the source solution practically did not change the rate of transport. 

Acyclic oligoamides, 167-168,170/ Advancing front model, description, 116 Alkali metal cation facilitated transport through supported liquid membranes with fatty acids electrogenic processes, 79-81 electroneutral transport, 76-80 experimental description, 77 kinetics, 81-85... 

Goldshleger, R. Shahak, Y. Karlish, S. J. Electrogenic and electroneutral transport modes of renal Na/K ATPase reconstituted into prote-oliposomes. J. Membr. Biol. 1990, 113, 139-154. 

In the absence of die polyether, potassium fluoride is insoluble in benzene and unreactive toward alkyl halides. Similar enhancement of solubility and reactivity of other salts is observed in the presence of crown ethers The solubility and reactivity enhancement result because the ionic compound is dissociated to a tightly complexed cation and a naked anion. Figure 4.13 shows the tight coordination that can be achieved with a typical crown ether. The complexed cation, because it is surrounded by the nonpolar crown ether, has high solubility in the nonpolar media. To maintain electroneutrality, the anion is also transported into the solvent. The cation is shielded from interaction with the anion as a... 

Electroneutral NaCl transport in the distal convoluted tubule is inhibited by the class of thiazide diuretics (chlorothiazide, hydrochlorothiazide, metolazone, chlorthalidone, and others). Thiazides interfere with the Cl binding site of NCC, and cause a relatively small... 

The H,K-ATPase, expressed in the parietal cells of the stomach, transports H+ ion from cytoplasm to lumen in exchange for extracytoplasmic K+ ion in an electroneutral exchange using the energy of ATP hydrolysis. 

The exchange of sodium ions for protons occurs with a 1 1 stoichiometry, and is thus an electroneutral event. Transport is a passive, energetically downhill event, not... 

The above statements are valid for monomolecular layers only. In the case of polymer films with layer thickness into the p-range, as are usually produced by electropolymerization, account must also be taken of the fact that the charge transport is dependent on both the electron exchange reactions between neighbouring oxidized and reduced sites and the flux of counterions in keeping with the principle of electroneutrality Although the molecular mechanisms of these processes... 

It is characteristic for the actual diffusion in electrolyte solutions that the individual species are not transported independently. The diffusion of the faster ions forms an electric field that accelerates the diffusion of the slower ions, so that the electroneutrality condition is practically maintained in solution. Diffusion in a two-component solution is relatively simple (i.e. diffusion of a binary salt—see Section 2.5.4). In contrast, diffusion in a three-component electrolyte solution is quite complicated and requires the use of equations such as (2.1.2), taking into account that the flux of one electrically charged component affects the others. 

From the point of view of the stoichiometry of the transported ions during active transport, the electroneutral pump, where there is no net charge transfer or change in the membrane potential, must be distinguished from the electrogenic pump connected with charge transfer. 

The zero-order solution reproduces the transport equations for an electroneutral solution. At length scales where A is close to unity, space charges become significant, and the transport equations can be expanded in powers of A. The concentration and... 

FIGURE 31-7 Mitochondrial carriers. Ions and small molecules enter the intermembrane space, since the outer mitochondrial membrane is not a significant permeability barrier. However, the inner mitochondrial membrane is impermeable to ions except those for which there are specific carriers. Most of the carriers are reversible, as indicated by two-headed arrows. Compounds transported in one direction are indicated in red. The ATP/ADP translocase and the aspartate-glutamate carrier are both electrophoretic their transport is driven in the direction of the mitochondrial membrane potential, as indicated by red arrows. Glutamine is carried into the matrix by an electroneutral carrier. The unimpaired functioning of mitochondrial carriers is essential for normal metabolism. (Adapted with permission from reference [70].)... 

Cellular Cl- replenishment is maintained by a basolateral anion-exchanger (Cl /OH or Cr/HCOJ) or via the Na+/K+/2C1 co-transporter, whose activities are closely tied synergistically through action of Na+/K+-ATPase, K+ channels, and the IIC()7/Na+ co-transporter (with a 3 1 stoichiometry) that extrudes HCOJ [61]. Passive Cl- diffusion through the paracellular pathway can occur because of the greater mobility of Cl- than Na+ in the paracellular space ( <1.3) [13]. Electroneutrality is maintained by transcellular Na+ transport in the luminal to subluminal direction, accomplished by both the apical Na+/H+ exchanger (NHE-3) and a basolateral HCOJ/Na+ co-transporter (with a 3 1 stoichiometry). 

This means that the motions of the two species are coupled and show the same transport rate because of electroneutrality. The fastest species move ahead and generate an electrical field in such a way that the faster species are slowed down and the slower ones are accelerated. 

Earlier, Gavach et al. studied the superselectivity of Nafion 125 sulfonate membranes in contact with aqueous NaCl solutions using the methods of zero-current membrane potential, electrolyte desorption kinetics into pure water, co-ion and counterion selfdiffusion fluxes, co-ion fluxes under a constant current, and membrane electrical conductance. Superselectivity refers to a condition where anion transport is very small relative to cation transport. The exclusion of the anions in these systems is much greater than that as predicted by simple Donnan equilibrium theory that involves the equality of chemical potentials of cations and anions across the membrane—electrolyte interface as well as the principle of electroneutrality. The results showed the importance of membrane swelling there is a loss of superselectivity, in that there is a decrease in the counterion/co-ion mobility, with greater swelling. 

Nafion is a copolymer of poly(tetrafluoroethylene) and polysulfonyl fluoride vinyl ether. It has fixed anions, which are sulfonic acid sites, and consequently, by electroneutrality, the concentration of positive ions is fixed. Furthermore, the transference number of protons in this system is 1, which greatly simplifies the governing transport equations, as seen below. There can be different forms of Nafion in terms of the positive counterion (e.g., proton, sodium, etc.). Most models deal only with the proton or acid form of Nafion, which is the most common form used in polymer-electrolyte fuel cells due to its high proton conductivity. 

Figure 9.13 Examples of mitochondrial transport systems for anions. 0 The anb port system transfers malate into but oxo-glutarate out of the mitochondrion. The symport system transfers both pyruvate and protons into the mitochondrion across the inner membrane. Both transport processes are electroneutral.

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