Mobile carriers

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Carriers and channels may be distinguished on the basis of their temperature dependence. Channels are comparatively insensitive to membrane phase transitions and show only a slight dependence of transport rate on temperature. Mobile carriers, on the other hand, function efficiently above a membrane phase transition, but only poorly below it. Consequently, mobile carrier systems often show dramatic increases in transport rate as the system is heated through its phase transition. Figure 10.39 displays the structures of several of these interesting molecules. As might be anticipated from the variety of structures represented here, these molecules associate with membranes and facilitate transport by different means. 

FIGURE 10.38 Schematic drawings of mobile carrier and channel ionophores. 

Other mobile carrier ionophores include monensin and nonaetin (Figure 10.39). The unifying feature in all these structures is an inward orientation of polar groups (to coordinate the central ion) and outward orientation of non-... 

Fig. 1. The relative selectivity of two mobile carriers. In both parts of the figure, the circles and squares represent the sodium and potassium ions transported, respectively. (Cited from Ref. 8))...

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

Nucleotide triphosphates Mobile carrier of phosphate and energy... 

It is easiest to start with the configurational entropy, Sc- Suppose that the number of defects, which is equal to the number of mobile (localized) holes or electrons, is nd and moreover that only one type of mobile carrier, either holes or electrons, is present. The configurational entropy Sc is given by using the Boltzmann formula ... 

A compound that binds to an ion in a manner which greatly facihtates the bound ion s permeabihty across a membrane. Naturally occurring ionophores include both mobile carriers (e.g., valinomycin and nigericin) and channel formers (e.g., gramicidin A). 

Figure 4.15 — (A) Tubular flow-through electrode 1 Perspex body 2 conducting epoxy cylinder 3 mobile carrier PVC membrane 4 electric cable 5 channel (1.2 mm ID) 6 holders 7 screws 8 0-rings. (B) Schematic diagram of a system for on-line monitoring of ammonia ISE tubular flow-through ammonium ion-selective electrode R reference electrode W waste. (Reproduced from [137] with permission of the Royal Society of Chemistry).

The detection principle of field-effect sensors with catalytic metal contacts is based on tbe change of the electric charge at the insulator surface caused by dissociation of the gas molecules by the catalytic material. Adsorbed gas molecules and reaction products form a polarized layer at the metal-insulator interface (Figure 2.1). This gives rise to an electric field in the insulator, which causes the concentration of mobile carriers in the semiconductor underneath the insulator to change. 

In an oxygen atmosphere CO sometimes gives a direct gas response for a porous metal film. This indicates that the CO molecule may be detected when adsorbed at a site where the dipole moment of CO is able to influence the mobile carriers in the semiconductor. 

The applied negative substrate bias depletes part of the n"-doped epilayer on top of the buried channel from mobile carriers. The conducting channel of the JFET device will then in fact extend also into this depleted epilayer. Then we can define the intrinsic gate of the device as the area in the epilayer, on top of the conducting channel, where the electrons have the highest energy. 

Doping a p-type semiconductor generates fixed acceptor sites with a density Na, and an equal number of mobile carriers with an opposite charge h+, whose distribution is controlled by the local value of the potential T>(x), following the Boltzmann function so that the mobile charge distribution is given by ... 

This Eq. (13) is derived from the fundamental electrochemical kinetics at metal electrodes and mainly accounts for the mechanism of the transformation, but the Gerischer Eq. (10) introduces an additional term accounting for the density of available mobile carriers at the electrode surface. 

The doping level of the silicon substrate is a determining parameter for the density of mobile carriers at the interface, even when the electrode is in the accumulation regime. Zhang [6] determined the cathodic current/voltage graphs for an n-type sample doped 10 and 10 ... 

Lithium ferrite itself (x = 0.5) has a high Curie temperature and can be fabricated so as to give a square hysteresis loop satisfactory for digital-computer memory cores. In this application, the dielectric losses connected with the presence of mobile charge carriers can cause a dramatic loss in core quality. The mobile carriers may be introduced by... 

Electroded TOF experiments have already been described previously [29]. They were carried out under small-signal conditions to determine hole and electron drift mobility. Carriers were generated by illuminating the sample through the... 

It is found that in a-Sb cSei- c alloys, electrons (the mobile carrier species) are depleted (n-type system) during dark decay, leaving behind a deeply trapped positive space charge. Note that the same situation prevails in alkali-doped a-Se [16]. 

It strikes me that in biological membranes, at least in eucaryotic cells, the transport mode almost universally chosen is the channel, or pore, mode, and not the mobile carrier mode. Surely there must be reasons for this, and it would seem appropriate to me if either Professor Simon or Professor Eisenman could start this discussion with a description of the respective merits of the two transport modes, with respect to selectivity, efficiency, and other parameters. 

Professor Eisenman, there is a large body of results indicating the existence of channel systems. One could mention the Ca2+ ATPase of sarcoplasmic reticulum, the FF transporting ATPase of the inner mitochondrial membrane, the purple protein system of halobacteria, the Na and K+ channels of the axonal membranes. Apart from the classical type of evidence provided, for example, by the noise fluctuation technique, we now even begin to see direct electron microscopic evidence for the existence of transport-related openings in biological membranes. On the other hand, solid evidence for the existence of mobile carriers in eucaryotic cell membranes is very scarce, if not outright absent. 

---