Electrical gradients

RJ Naftalin, S Tripathi. Passive water flows driven across isolated rabbit ileum by osmotic, hydrostatic and electrical gradients. I Physiol 360 27-50, 1985. 

Reabsorption of CT ions is a passive process CP ions are reabsorbed according to the electrical gradient created by the reabsorption of Na+ ions. 

Recall that the reabsorption of Na+ ions is accompanied by reabsorption of Cl- ions, which diffuse down their electrical gradient, and by reabsorption of water, which diffuses down its osmotic gradient. The net result is an expansion of plasma volume and consequently an increase in blood pressure. Therefore, the regulation of sodium reabsorption is important in the long-term regulation of blood pressure. As such, aldosterone and ANP, as well as the factors involved in their release, are discussed further in subsequent sections. 

Chloride reabsorption. Chloride ions are reabsorbed passively according to the electrical gradient established by the active reabsorption of sodium. Chloride ions move from the tubular lumen back into the plasma by two pathways ... 

This requires the participation of a protein transporter. Molecules move spontaneously toward lower concentration (chemical gradient) and opposite charge (electrical gradient). Moving in the opposite direction requires the input of energy. 

The process operates at current densities of about 1 kA/m2 and unit cell voltage of 1.5 V. The specific energy consumption is about 2 kWh/kg NaOH. Under the influence of the electric gradient the H + and OH ions emerge on opposite faces of the membrane. Bipolar membrane electrodialysis is being developed by several companies, e.g. WSI Technologies Inc. [270] and Aquatech Systems [129,275,276], Typical product specification ranges for the ICI electrodialysis process is summarized in Table 19. 

The combination of these events may create both chemical and electrical gradients across the cell membrane, which must be overcome by energy expenditure if the solutes are to be moved against these electrochemical gradients. The absolute rate of flux of a solute will also depend on the surface area of the cell membrane and the particular types of lipids and proteins that constitute the cell membrane in a particular cell type. 

The methods of solute transfer across the serosal/basolateral membrane can include ion channels and antiporters similar to those described earlier. In the case of serosally located cation channels, these primarily work because the intracellular electrolyte concentration is high enough to overcome the electrical gradient (e.g. some K+ channels). For anion channels, the negative charge inside the cell compared with the blood will help drive (repel) anions from the cell (e.g. CL efflux on voltage-sensitive channels in the intestine [58]). In the case of antiporters, the operation is fundamentally the same as that used in the mucosal membrane, except that the driving force is derived from an ion... 

The movement of solute across a semipermeable membrane depends upon the chemical concentration gradient and the electrical gradient. Movement occurs down the concentration gradient until a significant opposing electrical potential has developed. This prevents further movement of ions and the Gibbs-Donnan equilibrium is reached. This is electrochemical equilibrium and the potential difference across the cell is the equilibrium potential. It can be calculated using the Nemst equation. 

The electrochemical soil decontamination process is designed to treat organic compounds and heavy metals. It utilizes induced electrical currents to establish chemical, hydraulic, and electrical gradients designed to extract contaminants for soils. Treatment may be accomplished in situ or on site in lined cells. 

There is a conceptual model of hydrated ions that includes the primary hydration shell as discussed above, secondary hydration sphere consists of water molecules that are hydrogen bonded to those in the primary shell and experience some electrostatic attraction from the central ion. This secondary shell merges with the bulk liquid water. A diagram of the model is shown in Figure 2.3. X-ray diffraction measurements and NMR spectroscopy have revealed only two different environments for water molecules in solution of ions. These are associated with the primary hydration shell and water molecules in the bulk solution. Both methods are subject to deficiencies, because of the generally very rapid exchange of water molecules between various positions around ions and in the bulk liquid. Evidence from studies of the electrical conductivities of ions shows that when ions move under the influence of an electrical gradient they tow with them as many as 40 water molecules, in dilute solutions. 

FIG. 3.1 Donnan equilibrium and regulation of ionic concentrations resulting from the presence of a semipermeable membrane (a) conditions in the absence of polyelectrolyte molecules and (b) ionic concentrations and electrical gradients in the presence of the polyelectrolyte. The membrane is permeable to water and K+ and Cl , but not to the polyelectrolyte Pz. ... 

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