Ionic flux, mechanisms

The membrane model and mechanism of ionic flux gating... 

THE MEMBRANE MODEL AND MECHANISM OF IONIC FLUX GATING... 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

THE NATURE OF THE PERMEABILITY BARRIER AND THE BASIC MECHANISM of ion permeation are understood only in the most general sense even though the first measurements of ionic flux across lipid bilayer membranes were conducted 25 years ago. Establishing a permeation mechanism is difficult because the fluid lipid bilayer is described in terms of average motions of... 

In writing equation (8.18) we assumed that only cations migrate. However, in many oxides, anions also contribute to the ionic flux. Sometimes, they even dominate the growth mechanism. Table 8.8 shows the transport numbers of cations, t+, and anions, t, for a number of oxides. The transport number indicates the fraction of the ionic current carried by a given species. For binary oxides that contain only one type of cations and anions 1+ + = 1. 

Ion channels interact strongly with their environment. From a microscopic viewpoint, these proteins cross the lipid bilayer that forms the cell membrane, and are exposed to the electrochemically different environments found inside and outside the cell. They are designed to react in a highly specialized way to specific stimuli - mechanical, chemical, or electrical - and to express their function by regulating the ionic flux across the cell membrane. For this reason, any simulative approach meant to model ion channels must accovmt in some way for the combined behavior of the protein channel, the membrane, and the aqueous solution containing the ionic species of interest. Additionally, a way to represent a specific stimulus must be devised, in order to model the transient behavior of the channels as a function of the external perturbation. 

Studies with taurine-deficient cats have provided a new approach to the study of the cell biology of photoreceptor cells. The mechanism by which retinal taurine-deficiency leads to photoreceptor cell death remains to be defined. The close correlation of retinal taurine deficiency with peak-to-peak ERG amplitudes in taurine-deficient cats suggests that taurine deficiency may have some effect on the ionic fluxes of Na" " and K involved in the generation of the ERG. Since the generation of the ERG depends on hyperpolarization of photoreceptor cells and depolarization of Mtiller cells, it is possible th t taur. ne deficiency has led to abnormal ionic concentrations Na and K ) in photoreceptor and MUller cells. This possibility is currently under investigation. 

For electrolytic solutions, migration of charged species in an electric field constitutes an additional mechanism of mass transfer. Thus the flux of an ionic species Nj in (g mol)/(cm s) in dilute solutions can be expressed as... 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

Numerous and disparate copper criteria are proposed for protecting the health of agricultural crops, aquatic life, terrestrial invertebrates, poultry, laboratory white rats, and humans (Table 3.8) however, no copper criteria are now available for protection of avian and mammalian wildlife, and this needs to be rectified. Several of the proposed criteria do not adequately protect sensitive species of plants and animals and need to be reexamined. Other research areas that merit additional effort include biomarkers of early copper stress copper interactions with interrelated trace elements in cases of deficiency and excess copper status effects on disease resistance, cancer, mutagenicity, and birth defects mechanisms of copper tolerance or acclimatization and chemical speciation of copper, including measurement of flux rates of ionic copper from metallic copper. 

No carrier is completely specific for a given trace metal metals of similar ionic radii and coordination geometry are also susceptible to being adsorbed at the same site. The binding of a competing metal to an uptake site will inhibit adsorption as a function of the respective concentrations and equilibrium constants (or kinetic rate constants, see below) of the metals. Indeed, this is one of the possible mechanisms by which toxic trace metals may enter cells using transport systems meant for nutrient metals. The reduced flux of a nutrient metal or the displacement of a nutrient metal from a metabolic site can often explain biological effects [92]. 

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