Diffusion across membranes, asymmetrical

In this section, we want to describe the fundamentals of diffusion across membranes and the actual physical construction of the membrane. We will extend these basic ideas to specific types of separations in latter sections. The fundamentals of diffusion across membranes include the effects of partition coefficients, concentration units, and resistances in series. The physical construction of membranes includes both the membranes themselves and the modules in which the membranes are used. The membranes themselves may be symmetric or asymmetric the modules include hollow fibers, spiral-wound elements, and plate-and-frame assemblies. 

As shown in Fig. lb, when the DSL of the asymmetric membrane is placed against the draw solution (i.e., normal mode), water can fireely pass through the PSL and then diffuse across the DSL into the draw side. Meanwhile, solutes can also enter the PSL induced by the convective flow but is blocked by the DSL, which gives rise to the enrichment of solutes within the PSL of the asymmetric membrane. This phenomenon is the so-called con-centrative ICP. Therefore, forFO in normal mode, there exist mainly two kinds of CPs, dilutive ECP and concentrative ICP. 

Diffusion across thin membranes can sometimes produce chemical and physical separations at low cost. These low costs have spurred rapid development of membrane separations, especially during the last 20 years. This rapid development has sought both high fluxes and high selectivities. It has included the separation of gases, of sea water, and of azeotropic mixtures. It has used hollow fibers and spiral wound modules it has centered on asymmetric membranes with selective layers as thin as 10 nm. This rapid development is a sharp contrast to other diffusion-based separations like absorption, where the basic ideas have been well established for 50 years. 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

The equilibrium (also known as the Donnan effect) established across a semipermeable membrane or the equivalent of such a membrane (such as a solid ion-exchanger) across which one or more charged substances, often a protein, cannot diffuse. Diffusible anions and cations are distributed on the two sides of the membrane, such that the sum of concentrations (in dilute solutions) of diffusible and nondiffusible anions on either side of the membrane equals the sum of concentrations of diffusible and nondiffusible cations. Thus, the diffusible ions will be asymmetrically distributed across the membrane and a Donnan potential develops. 

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

A membrane can essentially be defined as a barrier that separates two phases and selectively restricts the transport of various chemicals. It can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, and can carry a positive or negative charge, or be neutral or bipolar. Transport across a membrane can take place by convection or by diffusion of individual molecules, or it can be induced by an electric field or concentration, pressure or temperature gradient. The membrane thickness can vary from as little as 100 p.m to several millimeters. 

Coupled transport systems frequently exhibit an asymmetric localization within plasma membranes. In enterocytes, the Na + -dependent glucose transporter and the a -dependent amino acid uptake systems are localized in apical (luminal) membrane, whereas the Na + / K+-ATPase is localized within the basolateral (blood-sided) membrane. Thus, the secondary active Na + - or H + -dependent transport systems are key elements for nutrient absorption, whereas subsequent transport across the basolateral membrane frequently follows the facilitated diffusion. 

Transport of molecules across the cell membrane occurs by passive and facilitated diffusion and active transport (Stein, 1986 Finkelstein, 1987). Passive transport is governed by a mass-transfer coefficient, surface area for exchange, transmembrane concentration difference, and a partition coefficient. The partition coefficient can be modified by charge, pH, temperature, and presence of other drugs. Facilitated transport may be most simply described by Michaelis-Menten kinetics. Depending upon the carrier system, symmetric or asymmetric models may be used. 

Tubular asymmetric y-alumina ceramic membranes have been prepared. The flow pattern of N2 across these membranes are mainly Knudsen diffusion. 

The membranes of cells are generally asymmetric, in that the lipids and proteins that inhabit the membrane are not evenly distributed across both the leaflets of the bilayer. To maintain this necessary membrane asymmetry, transverse diffusion of phospholipids (flip-flop. Figure 6a) in cellular membranes is accelerated by translocase enzymes like the flippases. These enzymes overcome the energy barrier for the passage of polar headgroups through the apolar center of the membrane and maintain asymmetry by the consumption of adenosine triphosphate (ATP). ... 

A number of cellular and molecular factors can iifflu-ence transport of molecules across the BBB. For most solutes and macromolecules, permeability is largely dependent on their lipophilicity. Hydrophilic solutes and macromolecules are believed to cross the barrier through specific carrier mechanisms or facilitated diffusion (Aschner, 1998). Some of these carriers are symmetrically distributed both on the luminal and abluminal membranes of the endothelial cells, whereas others have an asymmetric distribution. For example, the carriers for the essential neutral amino acids, which are required in the brain for neurotransmitter synthesis, are localized on both luminal and abluminal membranes. In contrast, the carrier for the amino add glycine appears to be located only on the abluminal membrane. The function of this asymmetric distribution is to remove glycine from the CNS and to keep its concentration low in the brain. The polar distribution of proteins maintains amino acid homeostasis in the brain. The existence of two facilitative... 

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