Membrane, selectively permeable composition

In all cells, the plasma membrane acts as a permeability barrier that prevents the entry of unwanted materials from the extracellular milieu and the exit of needed metabolites. Specific membrane transport proteins In the plasma mem brane permit the passage of nutrients Into the cell and metabolic wastes out of It others function to maintain the proper Ionic composition and pH ( 7.2) of the c3d osol. The structure and function of proteins that make the plasma membrane selectively permeable to different molecules are discussed In Chapter 7. 

Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way. 

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 concept of the pH electrode has been extended to include other ions as well. Considerable research has gone into the development of these ion-selective electrodes over the years, especially in studying the composition of the membrane that separates the internal solution from the analyte solution. The internal solution must contain a constant concentration of the analyte ion, as with the pH electrode. Today we utilize electrodes with 1) glass membranes of varying compositions, 2) crystalline membranes, 3) liquid membranes, and 4) gas-permeable membranes. In each case, the interior of the electrode has a silver-silver chloride wire immersed in a solution of the analyte ion. 

The membrane of the glass electrode used for pH measurements is selectively permeable to hydrogen ions and from this basic concept a whole range of ion-selective electrodes have been developed. Varying the composition of the glass membrane can change the permeability of the glass and several cation-sensitive... 

Fig. 2. Comparison of the selectivities of neutral-carrier-modified solvent polymeric- [43] and bilayer membranes. The permeability ratios PJP (at equilibrium" (Ref. 18) as far as available) fulfilled for the glyceryl dioleate BLM s are taken from Figs. 10 and 11 in Ref. 18. Values on the SPM s were obtained using 0.1 M solutions of the aqueous chlorides and membranes of the composition 33.1 wt.% polyvinyl chloride, 66.2 wt.% dioctyl adipate, 0.7 wt.% carrier. For the macrotetrolides I2 NH( for valinomycin IZ K. ...

Equation (9.11) identifies the three factors that determine the performance of a pervaporation system. The first factor, pevAp, is the vapor-liquid equilibrium, determined mainly by the feed liquid composition and temperature the second is the membrane selectivity, G-men, an intrinsic permeability property of the membrane material and the third includes the feed and permeate vapor pressures, reflecting the effect of operating parameters on membrane performance. This equation is, in fact, the pervaporation equivalent of Equation (8.19) that describes gas separation in Chapter 8. 

Porous membranes with selective permeability to oiganic solvents have been prepared by the extraction of latex films prepared with moderate ratios of PVA—PVAc graft copolymer fractions. The extracted films are made up of a composite of spherical cells of PVA, PVAc microgel, and PVA—PVAc graft copolymers (113). 

Membrane Pervaporation Since 1987, membrane pervaporation has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate liquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selectively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed by the physicochemical composition of the membrane. Pervaporation differs from reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref 24). 

The most general case of catalyst-membrane systems are systems containing a conventional granulated catalyst and a membrane catalyst. Two varieties of such systems are possible (1) a pellet catalyst with a monolithic membrane or (2) a pellet catalyst with a porous (sometimes composite) membrane. The inorganic membrane reactors with or without selective permeability are discussed in Chapter 17 of this book. Examples of applications of systems of selective metal-containing membrane and granulated catalyst are presented in Table 5. 

Membranes play an important role in natural science and for many technical applications. Depending on their purpose, their shape can be very different. For instance, membranes include porous or non-porous films, either supported or non-supported, with two interfaces surrounded by a gas or by a liquid. Important properties of non-porous membranes are their permeability for certain compounds and their stability. In biological cells their main task is to stabilize the cell and to separate the cell plasma from the environment. Furthermore, different cells and cell compartments have to communicate with each other which requires selective permeability of the membranes. For industrial applications membranes are often used for separation of gases, liquids, or ions. Foams and emulsions for instance are macroscopic composite systems consisting of many membranes. They contain the continuous liquid phase surrounded by the dispersed gas phase (foams) or by another liquid (emulsions). Beside these application possibilities membranes give the opportunity to investigate many questions related to basic research, e.g. finite size effects. 

Biological membranes are sheetlike structures, typically from 60 to 100 A thick, that are composed of protein and lipid molecules held together by noncovalent interactions. Membranes are highly selective permeability barriers. They create closed compartments, which may be entire cells or organelles within a cell. Proteins in membranes regulate the molecular and ionic compositions of these compartments. Membranes also control the flow of information between cells. 

Molecularly imprinted membranes can be prepared either as thick films or as composites in the pores of base-membranes. In composite membranes, the selective properties of the imprinted material are combined with the properties of the base-membrane. Membranes can also be prepared by phase inversion polymerization. The selective nature of MIPs makes it possible to prepare membranes with selective permeability [113, 114],... 

In this chapter, the authors describe the composition, structural organization, and general functions of biological membranes. After outlining the common features of membranes, a new class of biomolecules, the lipids, are introduced in the context of their role as membrane components. The authors focus on the three main kinds of membrane lipids—the phospholipids, glycolipids, and cholesterol. The amphi-pathic nature of membrane lipids and their ability to organize into bilayers in water are then described. An important functional feature of membranes is their selective permeability to molecules, in particular the inability of ions and most polar molecules to cross membrane bilayers. This aspect of membrane function is discussed next and will be revisited when the mechanisms for transport of ions and polar molecules across membranes is discussed in Chapter 13. 

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