Permeation of Ions

The effect of alkali metal ions, notably potassium, seems to be on the latter process. The shape of the Pulver-Vefzar curves (71) suggests this possibility. Farmer and Jones (19) note that the maximum stimulation by potassium occurs when fermcuitation has proceeded only to a slight extent.  

It appears that potassium and other alkali metal salts act on some intermediates or enzymes by momentaiy or more lasting combination with them. Phosphorylation seems to occur as the first step after the permeation (57). A hexose potassimn phosphate might conceivably occur and be important. In this event this ionic combination, traditionally rapid, might follow the course of pho.sphorylation, which in turn could be the second of two. successive unimolecular proccs.s . . 

However, the relation between the potassium concentration and that of the hexose potassium phosphate must depend on the dissociation of the second hydrogen, whose constant appears to be normally 2 X 10 . If this were true and if such a compound occurs, then the amount of potassium taken in should be about one-half the number of glucose molecules phosphorylated at the pH of the cytoplasm, if its pH were about 6.8, that of many types of cells tested. It is probably more acidic than this, and less potassium would be used as a partner of this acid. Rothstein and Haege calculated that one-third the glucose not stored was equimolecular with the potassium taken in. Obviously unknowm factors at least complicate this hypothesis and forbid its quantitative application. 

The theoretical bases for the study of the relation between permeability and enz3Tne reactions were first studied in some detail. It seems that enzyme reactions can be followed in accordance with the laws of mass action, provided that such variables as those introduced by pH, inhibition, and autocatalysts are known and taken into account. Aside from this not always simple requirement, most of the difficulty in this inquiry lies in the complex metabolic chains and networks. It is often true that it is impossible to trace with confidence the relation between permeation and the measured quantity, a relation mediated through many steps. Burton, in two papers, has presented a good analysis of reaction chains, and has offered equations which may prove helpful when it becomes possible to delineate the metabolic steps. 

Weichherz was apparently the first to utilize the concept that permeation can be treated as a reversible unimolecular reaction. However his equations assume a direct one-step relation between permeation into yeast cells by glucose and the liberation of carbon dioxide. But it seems that several steps intervene, four or probably several more. The sigmoid curves observed by Nord and others for this end result seem to result from phos-phorylated processes occurring when glucose penetrates the yeast cell. 

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A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

Other resolutions of the Poisson Nernst Planck equations (i.e. using various simplifying assumptions) have been proposed that couple the adsorption, desorption and permeation of ions through a membrane (e.g. [273,274]) as might be observed for a carrier-mediated transport. For example, for a symmetrical membrane (identical electrolyte on both sides of the membrane) and variation in the electrical potential profile given by i//m, /int can be estimated from ... 

It is well known that inward K+-channels at the plasma membrane are organized as tetramers, with the assembly of the four identical subunits making up a single pore for the permeation of K+ [58]. By contrast many other cation channels are composed of single polypeptides in which a set of domains corresponding to the subunit of the inward K+-channels is repeated four times to form the channel structures [58, 59]. It is therefore reasonable to suppose that cAMP-sensitive ion channels for the permeation of ions other than K+ exist and function in plant plasma membranes. The activation of these unidentified channels in response to... 

J. Jagur-Grodzinski, S. Marian and D. Vofsi, The Mechanism of a Selective Permeation of Ions Through Solvent Polymer Membranes , Sep. Sci. 8, 33 (1973). 

According to the third catalyst-membrane coupling possibility, represented in Fig. 5c, the surface of the membrane is deposited with some catalytic material. This setup is typical of solid-electrolyte membranes, where the catalyst is also playing the role of the electrode, necessary to drive the permeation of ions throughout the membrane at a desired rate. Problems may arise here concerning the fact that the catalyst per unit membrane surface is limited to some extent, and that several catalytic materials (e.g., metal oxides) are poor electricity conductors [26]. 

A. Katchalsky and P.F. Curran, Nonequilibrium Thermodynamics in Biophysics, Harvard University Press, Cambridge, MA, 1965, Ch. 12 S.R. de Groot and P. Mazur, Non-equilibrium Thermodynamics, North-Holland Publishers, Amsterdam, 1962 H. Kimizuka, Membrane Permeation of Ions, Basic Theory of Irreversible Thermodynamics, Kyoritsu Shuppan, Tokyo, 1988. 

In the electrodialysis of a mixed salt solution, preferential permeation of specific ions is interesting in academic studies and important in practical application. The preferential permeation of ions is defined as the permselectivity between two ions or the transport number of A ions relative to B ions. Details are given in Chapter 5. 

The cation rejection depends strongly on the electrolyte concentration. The calcium rejection increases with calcium concentration and in the presence of 20 mM NaCl. Sodium rejection also increases with concentration, but decreases in the presence of calcium, even reaching negative rejections when the calcium concentration increases to 2.5 mM. Macoun (1998) attributes negative rejection to a faster permeation of ions than water under certain conditions of enhanced driving force, and Hagmeyer (1999) states that the multivalent ion may pump the monovalent ion across the membrane. The final flux depends on the electrolyte concentration. Calcium is more effective in reducing flux than sodium chloride. 

Transport of a solute from one side (1) of the membrane to the other side (2) may be considered as the net result of adsorption and desorption at both sides of the membrane, combined with a permeation step. Permeation of ion i involves the passage of a Gibbs energy barrier that is determined by the profile of d(p, + z,F /)/dx 0. In our model, the barrier is the highest in the apolar center of the lipid bilayer, as shown in Figure 19.5. From the laws of conservation for the permeating ion i it follows that... 

Permeation of Ions Through a Model Biological Channel Effect of Periodic Boundary Conditions and Cell Size. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (elec-tron/hole) conductivity. Naturally, in aity material there are in principle nonzero electronie and ionic conductivities (Oei, Oj). It is customary to limit the use of the name MIEC to those materials in which Oj, and Oei do not differ by more than 2 orders of magnitude. It is also customary to use the term MIEC if Oj and 0 1 are not too low (Oj, > 10 Q- cm ). Obviously, these are not strict rales. There are processes where the minority carriers play an important role despite the fact that exceeds those limits and Oj, < 10 Q cm For example, the small electronic conductivity in a pmely ionic conductor (Oj 0 1), i.e., in a solid electrolyte (SE), is a necessary condition for ion permeation through the SE and therefore, e g., shortens the lifetime of a battery based on this SE. On the other hand, a small ionic conductivity in an electronic conductor is a necessary condition for permeation of ions through the electronic conductor, which may be of advantage in some applications (e.g., as electrode material). 

The molecular mechanism of ion transfer across liquid-liquid interfaces is remarkably similar to the mechanism of unassisted ion transport across membranes, recently studied by molecular dynamics simulations. The study revealed that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. As can be seen in Figure 14, these defects are quite similar to water fingers observed during... 

Identify the two most important determinants of gas permeability through CP membranes. Described their fanction in detail. Do the same or different determinants come into play for the case of permeation of ions Of organic molecules ... 

Complete ion removal is also offered by reverse osmosis, in principle, although the permeation of ions through even the tightest membrane is not zero, so there will be a finite, if very low, metal salt content in the purified water. If extremely low salt contents are required, then the most cost-effective method is probably reverse osmosis followed by a deionization process. 

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