Mobilities carrier transport

With the Monte Carlo method, the sample is taken to be a cubic lattice consisting of 70 x 70 x 70 sites with intersite distance of 0.6 nm. By applying a periodic boundary condition, an effective sample size up to 8000 sites (equivalent to 4.8-p.m long) can be generated in the field direction (37,39). Carrier transport is simulated by a random walk in the test system under the action of a bias field. The simulation results successfully explain many of the experimental findings, notably the field and temperature dependence of hole mobilities (37,39). 

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. 

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))...

Traditionally, charge-carrier transport in pure and doped a-Se is considered within the framework of the multiple-trapping model [17], and the density-of-state distribution in this material was determined from the temperature dependence of the drift mobility and from xerographic residual measurements [18] and posttransient photocurrent analysis. 

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. 

The specific application of a material generally determines the particular structure desired. For example, hydrogenated amorphous silicon is used for solar cells and some specialized electronic devices (10). Because of their higher carrier mobility (see Carrier Transport, Generation, and Recombination), single-crystalline elemental or compound semiconductors are used in the majority of electronic devices. Polycrystalline metal films and highly doped polycrystalline films of silicon are used for conductors and resistors in device applications. 

Danesi, P.R. Horwitz, E.P. Rickert, P.G. Rate and mechanism of facilitated americium(III) transport through a supported liquid membrane containing a bifunctional organophosphorous mobile carrier, J. Phys. Chem. 87 (1983) 4708-4715. 

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