Charge movement across membranes

Plasma membrane channels. The most common mechanism for the movement of into smooth muscle cells Ifom the extracellular space is the electrodiffusion of Ca " ions through highly selective channels. This movement can be significant in two quite different ways. First, Ca ions carry two positive charges and, in fact, most of the inward charge movement across the plasma membrane of smooth muscle myocytes is carried by Ca. Most smooth muscle action potentials are known to be Ca " action potentials. And second, the concentration of intracellular free calcium, the second messenger, is increased by inward calcium movement. 

Benz, R. (1988). Structural requirement for the rapid movement of charged molecules across membranes. Experiments with tetraphenylborate analogues, Biophys. J., 54, 25-33. 

I wish to respond to Professor Ubbelohde s question regarding what thinness, per se, of biological membranes could be important. For ion movements across membranes as mediated catalytically by carriers or channels, the thinness permits local deviations from the electroneutrality constraint that, for example, enables neutral molecules to carry cations across the membrane as charged species, leaving their counterions behind in the aqueous solutions. This is not possible when the thickness of the system becomes large. 

Iontophoresis is a novel drag delivery system that involves the use of electric current to drive charged ions across membranes (see Section 8.6.2). The technique generates an electrical potential gradient that facilitates the movement of solute ions. Iontophoresis has a long history and the earliest documented use dates back to 1740. 

Certain molecules freely diffuse across membranes, but the movement of others is restricted because of size, charge, or solubility. 

The electrical signals are carried by the movement of charged ions across the cell membrane. This makes use of the potential energy stored across the cell membrane in the form of ionic gradients. Concentration gradients for the principal ions across a typical nerve cell membrane are indicated in Fig. 2.1(a). The cell interior has a high concentration of K+ ions and a low concentration of Na+, Cl and Ca + ions relative to the exterior. 

A liquid junction potential E-f forms when the two half-cells of a cell contain different electrolyte solutions. The magnitude of Ej depends on the concentrations (strictly, the activities) of the constituent ions in the cell, the charges of each moving ion, and on the relative rates of ionic movement across the membrane. We record a constant value of j because equilibrium forms within a few milliseconds of the two half-cells adjoining across the membrane. 

Unlike enzymes or G protein-coupled receptors, the consequence of ion channel activation does not involve the depletion of a substrate or the generation of a product. Rather, when an ion channel opens (or activates), it permits the passive movement of Na+, Ca2+, K+, or CF ions down their electrochemical gradients. By regulating the flow of ionic charges across membranes, activated ion... 

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 practice, it is not feasible to test the derived equations experimentally by varying all the forces and fluxes independently. Usually some simplification is gained by allowing the system to develop to a specific steady state. A useful steady state for illuminated bacteriorhodopsin liposomes is that of electroneutral total flow, i.e., the condition in which the net movement across the membrane of all chemical species adds up to no charge movement. It can be derived and shown that this condition is attained within seconds, considering the membrane resistance and electrical capacity in the usual salt media [28], Electroneutral total flow is mathematically expressed as ... 

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