Transporters versus channels

Using the above equations, isotherms of the fraction of expanded channels versus liquid pressure can be generated as shown in Figure 5.5. From the curves, the temperature dependence of the saturation is not strong since the transition still occurs over a small liquid-pressure range. All of the curves show that, at a liquid pressure of 1 bar, the channels are completely expanded and filled with liquid in agreement with experimental observations. If the liquid pressure falls below about 0.15 bar, then the liquid water phase ceases to exist at all temperatures and the transport of water is solely by the vapor-equilibrated transport mode, which also agrees with the physical model. If the liquid pressure is above around 0.6 bar, then X is 22 (only the liquid-equilibrated transport mode). 

As may be noted, water vapor speeds up device response to hydrogen. Figure lib gives V-p (where V-pn is the turn on voltage for transport down the channel created by the stored charge) versus time for a MOSFET based on the Pd/Si02/Si structure. These data are for an exposure to 180 ppm H2 in air at 150°C. In this case, both response and recovery behavior are shown. Figure 12 clearly shows that Pd/SiC>2/... 

Apart from the redox-active units, the design strategy was devised to test the efficacy of 18 versus 21 crowns for ion transport and the value of decyl chains or anthrylmethyl units as anchors for the prospective channels. The anthryl units also offered an opportunity to detect cation complexation and establish host-guest stoichiometry by the extremely sensitive technique of fluorescence spectroscopy. 

Abundance of channels may contribute theoretically to a general permeability or leakiness of the ER membrane, which has been demonstrated in case of the ribosome-bound translocon complex, which cotranslationally imports nascent peptides into the RER lumen (51). The average diameter of the translocon tunnel is approximately 20 A, which is wide enough to allow the transport of Ca + (52) and small water-soluble molecules (4-6). Therefore, the presence of translocon complexes might be responsible for the higher permeability of the RER membrane versus the SER membrane. 

Vitamin D-mediated transcellular calcium transport also involves entry and extmsion. Calcium influx from the intestine or bone is propelled by a steep electrochemical gradient mediated by the superfamily of Transient Receptor Potential (TRP) ion channels noted above. TRP channel ion selectivity and mode of activation are extremely variable. Some are activated or regulated by ligands such as amino acid amines or small peptides, others by physical stimuli (e.g., heat), and still others by as yet unknown mechanisms. All TRP channels are Ca2+ and Na+ selective, but the selectivity for Ca2+ versus Na+ (Ca/Na ratio) varies enormously from >100 1, to <0.05 1. This variability has contributed to a confusing TRP channel nomenclature. 

Figure 6. Channel length at the same point in time versus Damkohler number. In the transport-controlled regime, the channel length is independent of the reaction rate. As the Damkohler number decreases from 0.1 to 0.01, the length of channels increase (see text for discussion). At Damkohler numbers below about 0.01, dissolution becomes pervasive and the channel does not propagate.

Figure 1.15. Density of sodium channels in frog skin epithelium versus the level of sodium transport which can be maintained by the tissue. Sodium channel density was determined by measuring / Clamiloride binding. Sodium transport was measured as the damping current required to maintain the transepithelial potential at zero, with a low sodium solution fl.l mMj bathing the mucosal surface. The six epitheHa represented by the extreme right-hand point had been deprived of sodium for 1 week. The values for 11 untreated skins and for 4 sham-treated skins are shown at the right. Mean values and standard errors are illustrated. Redrawn from Cuthbert and Shum //97/...

Relevant articles are E. Gouaux and R. MacKinnon (2005) Science, vol. 310, p. 1461 - Principles of selective ion transport in channels and pumps S. Varma, D. Sabo and S.B. Rempe (2007) /. Mol. Biol., vol. 376, p. 13 — K /Na selectivity in K Channels and valinomycin over-coordination versus cavity-size constraints . 

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