Water transport, pathways

Figure 14 Ion transport pathways responsible for water flux across intestinal epithelia. Sodium absorption in villus tip cells (left) stimulates water absorption, while chloride channel exit in crypt cells (right) stimulates water secretion.

There are two pathways by which a drug molecule can cross the epithelial cell the transcellular pathway, which requires the drug to permeate the cell membranes, and the paracellular pathway, in which diffusion occurs through water-filled pores of the tight junctions between the cells. Both the passive and the active transport processes may contribute to the permeability of drugs via the transcellular pathway. These transport pathways are distinctly different, and the molecular properties that influence drug transport by these routes are also different (Fig. 

A number of differenf approaches have been used to try to overcome some of these disadvantages of existing membranes. One such approach is to try to prevent water loss from the proton transport pathways, thus maintaining proton conductivity above the boiling point of wafer. Typically, this is attempted by adding hydrophilic inorganic species into the membrane. Furthermore, these particles in themselves may also be capable of proton conduction. 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

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. 

Contaminated bed sediments exist at numerous locations in the United States and around the world. These result mainly from past indiscriminate pollution of our aquatic environments and consist of freshwater and marine bodies including streams, lakes, wetlands, and estuaries. The bed sediments contain many hydrophobic organic compounds and metal ions that in the course of time act as sources of pollutants of the overlying aqueous phase. There are a number of transport pathways by which pollutants are transferred to the aqueous phase from contaminated sediments. One of the lesser known, but potentially important, modes of transport of pollutants from bed sediments is by diffusion and advection of contaminants associated with colloidal-size dissolved macromolecules in pore water. These colloids are measured in the aqueous phase as dissolved organic compounds (DOCs). (These are defined operationally as particles with a diameter smaller than 0.45 micrometer.)... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

Looking at the barrier in more detail, we find that it can be described as composed of two main components. Interspersed between the corneocytes we find the hydrophobic (water-repellent) substance, the barrier lipids. The keratinized corneocytes containing fibrous and amorphous proteins represent a hydrophilic (water-attracting) component. Neutral lipids (fatty acids, cholesterol) and ceramides dominate the lipid phase, and it is mainly these lipids that are responsible for the control and limitation of water transport through the skin.14 Visualization of the penetration pathway through the skin by tracer methods has demonstrated that the extracellular pathway is likely to be the only route through the barrier for substances other than water.15 Water diffusion through the keratinocytes... 

Figure 3.2 Pathways of water transport on land. This illustrates the difficulty and complexity of measuring the movement of water on land. (Modified from Vorosmarty et al., 2000.)...

The system depends on an electron transport pathway that transfers electrons from NADPH through a flavoprotein (NADPH cytochrome P-450 reductase) to cytochrome P-450 that is the terminal oxidase of the chain (10). The xenobiotic first forms a complex with the oxidized form o cytochrome P-450 which is reduced by an electron passing down the chain from NADPH. The reduced cytochrome P-450/substrate complex then reacts with and activates molecular oxygen to an electrophilic oxene species (an electron deficient species similar to singlet oxygen) that is transferred to the substrate with the concommitant formation of water. Cytochrome P-450 thus acts primarily as an oxene transferase (2). Substrate binding is a relatively nonspecific, passive process that serves to bring the xenobiotic into close association with the active center and provide the opportunity for the oxene transfer to occur. 

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