Permeation through cell layers

The movement of water through membranes and cell layers is critical for cell and tissue function. In its simplest formulation, the transmembrane volumetric fluid flux, V, is given by  

An alternative definition of solvent and solute flow is based on the irreversible thermodynamics and was initially derived by Kedem and Katchalsky (see [23] for a recent review). is the solvent flow  

General Concepts. The capillary endothelium is the major site of exchange for nutrients, hormones, and proteins between blood and tissues. When the solute is a macromolecule, transport occurs through pores in the endothelium. Capillaries in the general circulation are highly permeable to water and glucose, and nearly impermeable to proteins larger than albumin (Table 5.8). Capillary permeability is qualitatively different from cell 

C/lARGE = 5lARGE + LARGe(1 LARGE) p5 ( rsMALL,/ O LARGE, ) 

Capillaries in different tissues vary tremendously in their permeability characteristics, as described in the sections that follow. 

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Figure 31 Scheme for the protein-binding, diffusional, and partitioning processes and barriers that are encountered by a highly lipophilic and membrane-interactive drug (D) as it permeates through a cell within a continuous monolayer, h and h, thicknesses of the aqueous boundary layers. kd and ka, dissociation and association binding constants, respectively. P, protein molecule. Permeability coefficients Effective, Pe aqueous boundary layer, PABL and PW apical membrane, Pap basolateral membrane, Pbl. 

The precise mode of interaction between lipid vesicles and skin remains unclear. There is considerable doubt about the ability of whole vesicles to permeate intact stratum corneum. The majority of evidence suggests that vesicles can penetrate the outer cell layers of the stratum corneum where desmosomal linkages have become disrupted and presumably, the keratinocytes are less tightly bound and surrounded by a mixture of intercellular lipid and sebum. However, continuing diffusion of vesicles through the approximately 60 nm intercellular space of the deeper layers of the stratum corneum seems unlikely. Current thinking suggests... 

By studying the penetration of horseradish peroxidase across buccal mucosa, it was demonstrated that the permeation barrier is located in the upper one third to one quarter of the epithelium. After topical application, the horseradish peroxidase only permeated through the first 1-3 cell layers. However, when injected subepithelially it was found to permeate through as deep as the connective tissue and np as far as the MCGs zone. This is well correlated with the presence of the lipid-rich domain of the epithelium in its upper region. " The lipid composition in the buccal epithelium has a higher content of phospholipids, cholesterol esters, and glycosylceramides, while... 

Figure 3.3 sketches the profiles of the methanol and oxygen concentrations across the cell. To simplify the analysis, c and are assumed to be constant across the respective active layer (Figure 3.3). However, c and depend on current density and on the rate of methanol permeation through the membrane. Note that the superscripts a and c denote the anodic and cathodic values respectively.

As the fuel cell starts generating current, hydrogen concentration in the catalyst layer decreases, which reduces the driving force for hydrogen permeation through the membrane. That is one of the reasons these losses are mainly negligible at operating currents [1]... 

There is considerable methanol permeation through Nafion, which affects the fuel cell performance in a DMFC. Using doped PBI the same proton conductivity as Nafion can be maintained while virtually eliminating the crossover of methanol. PBI is doped with a conducting solid, usually phosphoric acid, to make it suitable for DMFC applications [55, 56]. In another attempt PBI is modified by sulfonation to make it an intrinsic proton conductor and is deposited onto a layer of Nafion membrane. This gives a composite polymer electrolyte that is a reasonable proton conductor and reduces the crossover of methanol [57]. 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

The skin is composed of an outer layer called the stratum corneum. This layer consists of dead, dried cells that are resistant to permeation by toxicants. Absorption also occurs through... 

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