Ionic transport, across membranes

Parlin, R. B. Eyring, H. Ionic Transport Across Membranes Clarke, H. T. Nachmansohn, D., eds., Academic Press 1954,... 

Ionic transport across membranes is of fundamental importance in physiology. To model such processes, lipid bilayers without [526] and with the addition of complexing compounds, such as crown ethers [527, 528], valinomycin [508, 513], or gramicidin [502, 504, 529-531], were also studied. 

Ion transport across membranes can be evaluated by using mucosal and serosal electrodes to read transepithelial current (I) and potential difference OP). With these parameters, equivalent circuit analysis can be utilized to account for the relative contributions of transcellular and paracellular pathways. Ionic flux (J) is defined by the Nernst-Planck equation,... 

Successive H-bond urea self-assembly of 4 and sol-gel transcription steps yield preferential conduction pathways within the hybrid membrane materials. Crystallographic, microscopic and transport data confirm the formation of self-organized molecular channels transcribed in solid dense thin-layer membranes. The ionic transport across the organized domains illustrates the power of the supramolecular approach for the design of continual hydrophilic transport devices in hybrid membrane materials by self-organization (Figure 10.8) [42-44]. 

I. Rubinstein, Effects of deviation from local electroneutrality upon electrodiffusional ionic transport across a cation-selective membrane, Reactive Polymers, 2 (1984), pp. 117-131. 

Transdermal iontophoresis involves the application of an electric field across the skin to facilitate (primarily) ionic transport across the membrane. Iontophoresis, it is important to point out, is differentiated from electroporation [14], another electrical approach to enhance transdermal transport, by the low fields employed. Whereas iontophoresis has achieved commercialization, there is (to our knowledge) no active development in progress of a transdermal delivery system employing electroporation. 

Sodium, potassium and chloride are the primary dietary ions that influence the electrolytic balance and acid-base status, and the proper dietary balance of sodium, potassium and chloride is necessary for growth, bone development, eggshell quality and AA utilization. Potassium is the third most abundant mineral in the body after calcium and phosphorus, and is the most abundant mineral in muscle tissue. It is involved in electrolyte balance and neuromuscular function. The content of potassium in poultry diets is usually adequate. Chloride is present in gastric juice and chlorine is part of the HC1 molecule which assists in the breakdown of feed in the proventriculus. Sodium is essential for nerve membrane stimulation and ionic transport across cell membranes. Signs of sodium, potassium or chloride deficiency include reduced appetite, poor growth, dehydration and increased mortality. 

Thus, under equilibrium conditions, the emf of the double electrode-pair system is determined solely by electric potential differences developed at the two liquid junctions that involve KC1 salt bridges. The two Ej may differ because of the effect of soil colloids. Thus the fact that this emf can develop is known as the suspension effect.40 Only ionic transport processes across the liquid junctions need be taken into account in order to evaluate E. Ionic transport processes across the semipermeable membrane between the suspension and the solution are not germane. Moreover, since neither Ej nor Ej can be calculated by strictly thermodynamic methods, the interpretation of E must be made in terms of specific models of ionic transport across salt bridges contacting suspensions and solutions. Thus the relation between E and the behavior of ions in soil suspensions is not direct. 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

In real cells, multiple transmembrane pumps and channels maintain and regulate the transmembrane potential. Furthermore, those processes are at best only in a quasi-steady state, not truly at equilibrium. Thus, electrophoresis of an ionic solute across a membrane may be a passive equilibrative diffusion process in itself, but is effectively an active and concentra-tive process when the cell is considered as a whole. Other factors that influence transport across membranes include pH gradients, differences in binding, and coupled reactions that convert the transported substrate into another chemical form. In each case, transport is governed by the concentration of free and permeable substrate available in each compartment. The effect of pH on transport will depend on whether the permeant species is the protonated form (e.g., acids) or the unprotonated form (e.g., bases), on the pfQ of the compound, and on the pH in each compartment. The effects can be predicted with reference to the Henderson-Hasselbach equation (Equation 14.2), which states that the ratio of acid and base forms changes by a factor of 10 for each unit change in either pH or pfCt ... 

Lithium ( Li) (1=1, 312). A review with 117 references was given on the applications of Li NMR spectroscopy and imaging in biology and experimental medicine. The interest derives primarily from the clinical use of Li salts to treat mania and manic-depressive illness. One area of investigation is ionic transport across the cellular membrane and compartmentation, so as to elucidate the mechanisms of therapeutic action and toxicity in clinical practice. The second is the development of a noninvasive, in vivo analytical tool to measure brain Li concentrations in humans, both as an adjunct to treatment and as a mechanistic probe. 

Other phenomena are interesting from the noise point of view. They related to ion transport across membranes/ " equilibrium and nonequilibrium kinetic systems/ nerve membrane noise, " and membrane current fluctuations from ionic channels (Na channels and K channels in axons) in stationary or nonstationary states.Some of these studies have been described in extended reviews. 

Ionic concentration gradients across membranes serve a variety of functions in cellular activity such as maintainance of resting potential, transmission of nerve impulse, driving cotransport, and secretion or activation of enzymes, hormones and other substances. Thus, studies of the kinetics and mechanism of ion transport across membranes are essential for the understanding of many physiological processes. 

In this report, the application of NMR spectroscopy for studying ionic flows across membranes is presented. NMR has two inherent advantages over the other methods used to study transport processes (mainly the isotope tracing techniques). 1) It is applicable for... 

In any case, we have here a system, and an extremely sensitive tool for examining it, to model enhancement of ion pairing by a low-dielectric microenvironment This should help to understand, not only phase-transfer catalysis, but also ionic transport across lipophilic membranes, neurotransmission which is mediated by Q A salts such as acetylcholine, and enzymatic action in which it is a fundamental factor. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

The possibility of active transport of substances across membranes had first been pointed out in the middle of the nineteenth century by the physiologist Emil Heinrich du Bois-Reymond, a German of Swiss descent. The ability to accomplish active transport of ions and uncharged molecules in the direction of increasing electrochemical potentials is one of the most important features of cell membrane function. The law of independent ionic migration as a rule is violated in active transport. 

The energy needed to transport ions across the membrane is obtained by the cell in chemical reactions occurring in it that is, the oxidation of organic substances with oxygen (for more details, see Section 30.2). Every second about 10 to 10 ions are transported across 1 m of membrane area. This process requires 20 to 30% of all energy generated by the cell. It has been calculated that the total power of the ionic pumps in the cells of the brain is about 1 watt. 

The assumption that Junction potentials are absent is also valid, since our usual practice during a potentiometric titration is to stir the solution vigorously. Such stirring causes rapid mixing, thus obviating any worries about poor mass transport. In addition, note that junction potentials are caused by differing rates of ionic movement across a membrane. 

As detailed in chapter 17, biological membranes are basically lipid—think fat or oil—in nature with some attached proteins. As such, these thin sheets of phospholipids and proteins are nearly impermeable to charged particles such as sodium, potassium, or chloride ions. While the isolation of the cell interior from the exterior ionic environment is critical in many ways, it is also true that controlled permeability to ions may be critical. In fact, it is the near-impermeability of biological membranes to ions that permits control of ion transport across them by certain, specific proteins. 

Equality (1.20) is of primary importance because of the following reason. It is customary in most ionic transport theories to use the local electroneutrality (LEN) approximation, that is, to set formally e = 0 in (1.9c). This reduces the order of the system (1.9), (l.lld) and makes overdetermined the boundary value problems (b.v.p.s) which were well posed for (1.9). In particular, in terms of LEN approximation, the continuity of Ci and ip is not preserved at the interfaces of discontinuity of N, such as those at the ion-exchange membrane/solution contact or at the contact of two ion-exchange membranes or ion-exchangers, etc. Physically this amounts to replacing the thin internal (boundary) layers, associated with N discontinuities, by jumps. On the other hand, according to (1-20) at local equilibrium the electrochemical potential of a species remains continuous across the interface. (Discontinuity of Cj, ip follows from continuity of p2 and preservation of the LEN condition (1.13) on both sides of the interface.)... 

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