Potassium ions description

We may now assemble the foregoing information into a molecular description of a few biological processes in which the interaction between water and metal ions plays an important role. First some problems related to signal transfer in nerve cells are discussed. This is followed by some comments on the mechanism operating at nerve synapses in which, in addition to the sodium and potassium ions, a specific transmitter substance and calcium ions take part. 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

Due to the presence of low-temperature desorption peak a new desorption site was included to phenomenological model of TPD experiments previously used for the description of the Cu-Na-FER samples [5], The fit of experimental TPD curves was performed in order to obtain adsorption energies and populations for individual site types sites denoted A (A1 pair), B (sites in P channel (A1 at T1 or T2)), C (sites in the M channel and intersection (A1 at T3 or T4)) [3] and D (newly introduced site). The new four-site model was able to reproduce experimental TPD curves (Figure 1). The desorption energy of site D is cu. 82 kJ.mol"1. This value is rather close to desorption energy of 84 kJ.mol"1 found for the site B , however, the desorption entropy obtained for sites B and D are rather different -70 J.K. mol 1 and -130 J.K. mol"1 for sites B and D , respectively. We propose that the desorption site D can be attributed to so-called heterogeneous dual-cation site, where the CO molecule is bonded between monovalent copper ion and potassium cation. The sum of the calculated populations of sites B and D (Figure 2) fits well previously published population of B site for the Cu-Na-FER zeolite [3], Because the population of C type sites was... 

Diehn and Kint115) and Mikolajczyk and Diehn71) demonstrated specific inhibition of photoaccumulation and of the step-down photophobic response of Euglena gracilis, respectively, by potassium iodide. Iodide ions are known to de-excite flavins, but do not affect carotenoids87). For a description of the analogous experiments with corn coleoptiles, see Sect. 3.2 of the contribution by Schmidt in this volume. 

In a recent review numerous examples were given of membrane ultrastructural textures consistent with the conformation discussed here [64]. Another obvious case of a conformation will be mentioned. The brain astrocytes are rich in potassium channels, which appear to play an important role in the regulation of the ion concentrations in the brain. Freeze-fracture electron micrographs of the outer astrocyte membrane contain patches of a periodic structure [65]. These ordered assemblies are thought to be potassium channels. In our membrane description these channels serve to plug the "holes" of a C D bilayer, whereas the rest of the membrane is in the conformation. 

Consider a typical eukaryotic cell, for instance, a muscle cell. By weight, the cell is about 75% water. However, this estimate fails to convey the truly aqueous nature of the cell a far more realistic description is in terms of mole ratios. Because of the low molecular weight of water, the nominal 75% water translates into a very large number of moles of water relative to the number of moles of other cell constituents. Thus, the aqueous nature of the cell is better illustrated by noting that for every 20,000 water molecules there are only about 75 lipid molecules, 100 sodium, potassium, and chloride ions (with at most a few hundred other small molecules or ions), and only one or two protein molecules. By sheer numbers water molecules totally dominate, and in this perspective life is merely some complex biochemistry in an extensive matrix of water, stabilized by a few lipids and macromolecules. The two dominating factors in cell biology are thus, simply, water molecules and interfaces. 

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