Ion permeation

In the resting (closed) state, the ion channel is occluded by a hydrophobic girdle that constitutes a barrier to ion permeation. Agonist binding in the extracellular domain promotes a conformational change that results in a rotational movement of the M2 helices lining the pore, widening the pore by A. This results... 

Electrophysiological studies (mainly using voltage-clamp and patch clamp) revealed the essential properties of the sodium channels kinetics of channel gating and selective ion permeation. Sodium channels are... 

Membranes exhibiting selectivity for ion permeation are termed electrochemical membranes. These membranes must be distinguished from simple liquid junctions that are often formed in porous diaphragms (see Section 2.5.3) where they only prevent mixing of the two solutions by convection and have no effect on the mobility of the transported ions. It will be seen in Sections 6.2 and 6.3 that the interior of some thick membranes has properties analogous to those of liquid junctions, but that the mobilities of the transported ions are changed. 

Plant cells have several internal compartments separated by plasma membranes. The main compartments controlling the ionic relations of the cell are the cytosol and vacuole. The vacuole occupies more than 90% of the cell volnme and so contains the bnlk of the ions. Many different ions permeate the membranes, bnt in general K+, Na+ and Cr have the greatest concentrations and permeabilities. Studies of ion relations in plant cells have led to the following conclnsions ... 

Recent evidence also suggests that polyvinylpyrrolidone itself may act as a corrosion inhibitor (24). As the temperature is increased, further crosslinking may result in the buildup of stress in the coating. If cohesive breakdown were to occur, a pathway for water and ion permeation becomes available. 

When ions permeate through cellulose acetate their transport pathways will tend to follow the regions where water is most concentrated. Thus they will meet and interact with the dissociated, fixed carboxylate ions. The concentrations of ions absorbed from salt solutions by swollen cellulose acetate are small for reasons connected with the low dielectric constant of the latter (2). The electro-chemical potentials of ions undergoing transport may therefore be influenced significantly by the presence of the fixed charges. Such influences are familiar with normal ion-exchange membranes. 

A metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons. Positive ions are the atomic cores the electrons are the valence electrons. For example, copper has a configuration (electronic structure)... 

EXTERNAL ION-PAIR RETURN HIDDEN RETURN Ion permeation across membrane, PERMEABILITY CONSTANT PERMEABILITY COEFFICIENT ION PUMPS... 

The permeation of cephalosporin antibiotics in an SLM system under reactive extraction condition can be considered a coupled transport phenomenon which can describe metal ion permeation behavior. Various steps are involved in the coupled transport in the SLM system ... 

A metal can be considered as a fixed lattice of positive ions permeated by a gas of free electrons. Positive ions are the atomic cores, while the electrons are the valence electrons. For example, copper has a configuration (electronic structure) ls22s22p63s23p63dl04sl (superscripts designate number of electrons in the orbit) with one valence electron (4s). The atomic core of Cu+ is the configuration given above, less the one valence electron 4s1. The free electrons form an electron gas in the metal and move nearly freely through the volume of the metal. Each metal atom contributes its valence electrons to the electron gas in the metal. Interactions between the free electrons and the metal ions makes a large contribution to the metallic bond. 

Clear understanding of the differences between channels and carriers only came two decades later with the realization that the turnover number, the rate of maximal ion permeation, for the fastest carrier was at least two orders of magnitude lower that of a typical channel (see Hille, 1984, and below). In retrospect, it is odd to find that the experiments underpinning the two conceptual pillars (channels and carriers) which nurtured membrane transport biology through the second half of the 20th... 

Ions permeating the membrane carry solvating water molecules in their hydration shell. Also, osmotic transport of water from the dilute to the concentrated chambers can occur. 

The physiological effects of ion channels are based on the regulation of ion fluxes (e.g., K+, Na+, Ca2+, Cl-) across membranes, which affect, for example, osmotic pressure, nerve signal transmission, and muscle contraction. Ion permeation is extremely fast (up to 107 ions s-1) and highly selective [1],... 

Chapman, M. L., VanDongen, H. M., and VanDongen, A. M. (1997). Activation-dependent subconductance levels in the drkl K channel suggest a subunit basis for ion permeation and gating. Biophys.J. 72, 708-719. 

Cortes, D., Cuello, L., and Perozo, E. (2001). Molecular architecture of full-length KcsA role of cytoplasmic domains in ion permeation and activation gating./ Gen. Physiol. 117, 165-180. 

Assuming that one can represent the structure of a metal as a lattice of metal ions permeated by a sea of electrons, then metals like lanthanum can be shown as (Ln +) (0 )3 however, metals based upon divalent ions (like Eu and Yb) would be (Ln +) (e )2. The ions with the (+3) charge have a smaller radius, as the higher charge draws in the electrons more closely, and the stronger attraction means that it takes more energy to boil them (similarly, they would be predicted to have higher conductivities). 

Figure 1.7 Bacterial Cl channel homolog stmcture. The dimeric protein is viewed from the side (the plane of the image is perpendicular to the membrane plane). The two monomers are separated by a line down the middle of the figure. The membrane is omitted. Each monomer contains a separate pore for Cr ion permeation. Each pore contains a selectivity filter and a gating domain which is believed to involve a pH-sensitive glutamate residue. The location of the ion transit pathways and the mechanism of transport through the pores are topics of current channel research (Yin et al, 2004).

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