Selectivity/selective permeability

Ultra filtration. This process removes macromolecules, microorganisms, particulate matter, and pyrogens using a thin, selectively permeable membrane. Ultrafiltration caimot remove ions from water and is generally employed as a polishing process. 

Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, permeable sohd phase with which it is associated. The separation usually involves selective dissolution, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid and it may be incorporated within, chemically combined with, adsorbed upon, or held mechanically in the pore structure of the insoluble material. The insoluble sohd may be massive and porous more often it is particulate, and the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated. 

Process Descriptions Selectively permeable membranes have an increasingly wide range of uses and configurations as the need for... 

Membrane Proeesses These processes use a selectively permeable membrane to separate pollutants from water. Most of the mem-... 

Boundary layer effects Membranes (selective permeability for ions, gases etc.), ion exchangers, controlled release of pharmaceuticals. 

Cell membrane The cell membrane is composed of about 45% lipid and 55% protein. The lipids form a bilayer that is a continuous nonpolar hydrophobic phase in which the proteins are embedded. The cell membrane is a highly selective permeability barrier that controls the entry of most substances into the cell. Important enzymes in the generation of cellular energy are located in the membrane. 

This selective permeability is ascribed to the presence on the membrane of a negative charge, which is attributed to carboxyl groups attached to the polymer chains. Paint films can, therefore, be regarded as very large anions. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

An important selection of materials to packaging, particularly food, is based on the permeability of the materials to oxygen, water vapor, and, in the case of packaging bananas, to ethylene gas that is used to artificially ripen the bananas. Selective permeability provides chemical separations, one of the most interesting of which is the use of PTFE materials to separate the hexafluorides of the different isotopes of uranium. 

There are a number of industrial gas separation systems that use the selective permeability of plastics to separate the constituents. In design problems relating to such applications, the designer must consider the environmental conditions to determine whether the materials having the desired properties will withstand the temperatures and physical and chemical stresses of the application. Frequently the application will call for elevated temperatures and pressures. In the case of uranium separation, the extreme corrosivity of the fluorine compounds precluded the use of any material but PTFE. The PTFE... 

A large membrane pack of this type will act like an artificial gill, permitting a swimmer to breathe like a fish and remain submerged for much longer periods of time than are possible with scuba equipment. Speculative fiction has man returning to live in the seas, and this type of application may make it possible. Their application in spacecraft is obvious as a part of a continuously recycled air support system. The oxygen permeability of silicone materials is just one example of the selective permeability of plastics. 

The open channel has in most cases a selective permeability, allowing a restricted class of ions to flow,for example Na+, K+, Ca++ or Cl- and, accordingly, these channels are called Na+-channels, K+-channels, Ca -channels and Cr-channels. In contrast, cation-permeable channels with little selectivity reject all anions but discriminate little among small cations. Little is known about the structures and functions of these non-selective cation channels [1], and so far only one of them, the nicotinic acetylcholine receptor (nAChR, see Nicotinic Receptors), has been characterized in depth [2, 3]. The nAChR is a ligand-gated channel (see below) that does not select well among cations the channel is even permeable to choline, glycine ethylester and tris buffer cations. A number of other plasma... 

Mitochondria have an outer membrane that is permeable to most metabohtes, an inner membrane that is selectively permeable, and a matrix within (Figure 12-1). The outer membrane is characterized by the presence of various enzymes, including acyl-CoA synthetase and glycerolphosphate acyltransferase. Adenylyl kinase and creatine kinase are found in the intermembrane space. The phospholipid cardiolipin is concentrated in the inner membrane together with the enzymes of the respiratory chain. 

Membranes are highly viscous, plastic structures. Plasma membranes form closed compartments around cellular protoplasm to separate one cell from another and thus permit cellular individuality. The plasma membrane has selective permeabilities and acts as a barrier, thereby maintaining differences in composition between the inside and outside of the cell. The selective permeabilities are provided mainly by channels and pumps for ions and substrates. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, and there are special areas of membrane strucmre—the gap junctions— through which adjacent cells exchange material. In addition, the plasma membrane plays key roles in cellcell interactions and in transmembrane signaling. 

The membrane acts as a selective permeability barrier between the cytoplasm and the eell environment the wall acts only as a sieve to exclude molecules larger than about 1 nm. Certain enzymes, and especially the electron transport chain, that are located in the membrane are responsible for an elaborate achve transport system which uhlizes the electrochemical potenhal of the proton to power it. 

Membrane Types Key membrane properties include their size rating, selectivity, permeability, mechanical robustness (to allow module fabrication and withstand operating conditions), chemical robustness (to fabrication materials, process fluids, cleaners, and sanitizers), low extractibles, low fouling characteristics, high capacity, low cost, and consistency. 

The lipid bilayer arrangement of the plasma membrane renders it selectively permeable. Uncharged or nonpolar molecules, such as oxygen, carbon dioxide, and fatty acids, are lipid soluble and may permeate through the membrane quite readily. Charged or polar molecules, such as glucose, proteins, and ions, are water soluble and impermeable, unable to cross the membrane unassisted. These substances require protein channels or carrier molecules to enter or leave the cell. 

The cell wall is a more rigid structure and is responsible for the form of the bacterial body. It behaves as a selectively permeable membrane and apparendy plays a fundamental role in the life activities of the cell. 

All cell membranes contain transmembrane proteins that form ion channels. These ion channels are usually selectively permeable to particular ions. Some channels, such as GABA-gated ion channels, are permeable to Cl ions and are inhibitory in nature because they make the inside of the nerve or muscle cells more negative as the Cl ions enter. Some ion channels are permeable to the cations Na and K, and an example of this type is the nicotinic acetylcholine-gated channel. Nicotinic channels have an excitatory effect when they open because Na ions enter and K ions leave through these channels. The cell becomes more positive inside and depolarizes. If the cell is a muscle cell, calcium accumulates in the cytoplasm and it contracts. We have found that all over the surface of Ascaris muscle there are GABA receptors (Martin, 1980) as well as nicotinic acetylcholine channels (Martin, 1982 Robertson and Martin, 1993). 

VaporSep A family of separation processes, based on membranes which are selectively permeable to organic vapors. Developed by Membrane Technology Research, CA, in the... 

Fluorine-containing polymers exhibit unique chemical and physical properties and high performance that are not observed with other organic polymers. They possess high thermal stability, high chemical stability, a low coefficient of friction, low adhesion, water and oil repellency, low refractive index, and outstanding electric insulation. In addition, there have recently been new expectations of selective permeability, piezoelectricity, and biocompatibility. 

Poly(organosiloxane)s have a number of outstanding properties such as thermal and oxidative stability, water and chemical resistance, electric insulating capacity, selective permeability to gases, and biocompatibility. 

Quite generally, the interphase between an organism and its environment encompasses the elements outlined in Figure 1 of Chapter 1. The scheme shows that the cell membrane, with its hydrophobic lipid bilayer core, has the most prominent function in separating the external aqueous medium from the interior of the cell. The limited and selective permeabilities of the cell membrane towards components of the medium - nutrients as well as toxic species - play a governing role in the transport of material from the medium towards the surface of the organism. 

Overall, bacteria attacked by the myeloperoxidase system undergo a loss of selective permeability prior to death. 

Membranes of PVA/PAcr.Ac blends evidence a selective permeability against different components of a liquid mixture. So, they may be used for the ethanol dehydration by pervaporation technique. 

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