Mechanisms of Transport

In general, there are four main ways by which small molecules cross biological lipid membranes  

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The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

Simplified environmental fate estimation procedures are based on the predominant mechanisms of transport within each medium, and they generally disregard intermedia transfer or transformation processes. In general, they produce conservative estimates (i.e., reasonable upper bounds) for final ambient concentrations and the extent of hazardous substance migration. However, caution should be taken to avoid using inappropriate analytical methods that underestimate or overlook significant pathways that affect human health. 

Fig. 18. Kinetic scheme for the Valinomycin carrier mechanism of transport. Reproduced with permission from Ref. 87)...

The term intracellular transport comprises both the correct targeting and the mechanism of transport of... 

Rama, Moore WS (1984) Mechanism of transport of U-Th series radioisotopes from sohds into ground water. Geochim Cosmochim Acta 48 395-399... 

Curtiss, C. E and Bird, R. B.i Statistical Mechanics of Transport Phenomena Polymeric Liquid Mixtures. Vol. 125, pp. 1-102,... 

The yellow coloration in the Monarch as well as the larva of three other species of butterfly from South Florida is exclusively due to the specific accumulation of exceptionally high levels of lutein producing a pigmented epidermis. This active accumulation, reminiscent of the specific accumulation that occurs in the primate macula, indicates that butterfly larva is an excellent animal model for the study of carotenoid transport and binding. As such, elucidation of the mechanism of transport and binding of lutein in the epidermis and other tissues of these butterfly larvae may provide insight into xanthophyll uptake within the human eye (Bhosale et al. 2004). 

The combined diffusivity is, of course, defined as the ratio of the molar flux to the concentration gradient, irrespective of the mechanism of transport. The above equation was derived by separate groups working independently (8-10). It is important to recognize that the molar fluxes (Ni) are defined with respect to a fixed catalyst pellet rather than to a plane of no net transport. Only when there is equimolar counterdiffusion, do the two types of flux definitions become equivalent. For a more detailed discussion of this point, the interested readers should consult Bird, Stewart, and Lightfoot (11). When there is equimolal counterdiffusion NB = —NA and... 

Liang, E., J. Proudeoot, and M. Yazdanian. Mechanisms of transport and structure-permeability relationship of sulfasalazine and its analogs in Caco-2 cell monolayers, Pharm. Res. 2000, 17, 1168-1174... 

Hu, M., et al. Mechanisms of transport of quinapril in Caco-2 cell mono-layers comparison with cephalexin. Pharm. Res. 1995, 12, 1120-1125. 

The applicability range of any model should be limited to molecules having a similar mechanism of transport. Therefore, we have selected from the literature only those compounds with well-characterized Caco-2 cell permeability and excluded compounds with a high efflux ratio. Known P-gp substrates and actively transported compounds were also excluded from the list. 

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