Membranes, natural permeability

Finkelstein, A. and Cass, A. (1967). Effect of cholesterol on the water permeability of thin lipid membranes, Nature, 216, 717-718. 

If a system contains two types of species, but the membrane is permeable only to species number 1, the natural variables for the system are T, K //, and N2, where N2 is the number of molecules of type 2 in the system. The thermodynamic potential for this system containing two species is represented by U[T, //,]. The corresponding ensemble is referred to as a semigrand ensemble, and the semigrand partition function can be represented by P(71 K /q, N2). The thermodynamic potential of the system is related to the partition function by... 

Some of the variables that are important for the subsequent discussion are recalled here. The membrane properties are related to the mass transport of the different chemical species through the membrane itself or its separating layer (for an asymmetric or multilayer membrane). Permeability and selectivity were defined for the mass transport by permeation both depend on the membrane nature and morphology that impose the specific transport mechanism driving the permeation of which it is characteristic. Table 13.2 reports the permeability coefficient, selectivity and permeating driving force of some permeation mechanisms. 

The membrane potential in chloroplasts is almost entirely composed of the (ZpH component. The thylakoid membrane is permeable to Mg2+ and CP ions, so electrical neutrality is maintained. This differs from mitochondrial ATP synthesis where both a pH and an electrochemical potential exist. Because the chloroplast gradient is primarily SZj H in nature, three protons must move across the membrane to synthesize an ATP, rather than the two that move during mitochondrial synthesis of a single ATP. The ATP and NADPH from synthesis are both formed on the stromal side of the thylakoid membrane. They are available for the fixation of C02, which occurs in the stroma. See Figure 3-3. 

It is often necessary to use a divided cell, in which a membrane, non-permeable to substrate and product but permeable to ions (usually cation exchange membranes are used), is inserted in the cell in order to protect substrate and/or product from reacting at the auxiliary electrode. Complications due to this arrangement are entirely of a practical nature. 

The moisture content of heartwood in softwood trees is reduced to a level much lower than that of normal sap-wood (2, 24), During the moisture reduction period, the membranes of bordered pits in sapwood fibers have a strong tendency to become aspirated. This situation, together with that of pit membrane incrustation, greatly reduces the natural permeability of heartwood tissue to liquids and gases. 

The studies of elementary films formed in inverse emulsions and stabilized by different synthetic and natural surfactants revealed that the membrane electric conductivity experiences a sharp increase upon the addition of some biologically active surfactants. For instance, membrane conductivity may increase by five orders of magnitude when trace amounts of valinomycin antibiotic are introduced into the outer aqueous medium of lipid membrane. At the same time the membrane becomes permeable to potassium and hydrogen ions but impermeable to sodium ions. A sharp decrease in electric resistance of synthetic membranes is observed when proteins and enzymes with suitable substrates are introduced into them. By studying the properties of such membranes one may model important biological processes, e.g. the transfer of neural impulses. 

The study of gas transport in membranes has been actively pursued for over 100 years. This extensive research resulted in the development of good theories on single gas transport in polymers and other membranes. The practical use of membranes to separate gas mixtures is, however, much more recent. One well-known application has been the separation of uranium isotopes for nuclear weapon production. With few exceptions, no new, large scale applications were introduced until the late 1970 s when polymer membranes were developed of sufficient permeability and selectivity to enable their economical industrial use. Since this development is so recent, gas separations by membranes are still less well-known and their use less widespread than other membrane applications such as reverse osmosis, ultrafiltration and microfiltration. In excellent reviews on gas transport in polymers as recent as 1983, no mention was made of the important developments of the last few years. For this reason, this chapter will concentrate on the more recent aspects of gas separation by membranes. Naturally, many of the examples cited will be from our own experience, but the general underlying principles are applicable to many membrane based gas separating systems. 

The scaled surface area and its variation with d> are of crucial importance in the definition and evaluation of the osmotic pressure , H, of a foam or emulsion. We introduced the concept in Ref 37, where it was referred to as the compressive pressure , P. It has turned out to be an extremely finitful concept (22,27,38). The term osmotic was chosen, with some hesitation, because of the operational similarity with the more familiar usage in solutions. In foams and emulsions, the role of the solute molecules is played by the drops or bubbles that of the solvent by the continuous phase, although it must be remembered that the nature of the interaetions is entirely different. Thus, the osmotic pressure is denned as the pressure that needs to be applied to a semipermeable, freely movable membrane, separating a fluid/fluid dispersion from its continuous phase, to prevent the latter from entering the former and to reduce thereby the augmented surface free energy (Fig. 4). The membrane is permeable to all the components of the continuous phase but not to the drops or bubbles. As we wish to postpone diseussion of compressibility effects in foams until latter, we assume that the total volume (and therefore the volume of the dispersed phase) is held constant. 

Furukawa K, Matsuzaki-Kobayashi M, Hasegawa T, et al. Plasma membrane ion permeability induced by mutant alpha-synuclein contributes to the degeneration of neural cells. / Neurochem. 2006 97(4) 1071-1077. Lashuel HA, Hartley D, Petre BM, Walz T, Lansbury Jr PT. Neurodegenerative disease amyloid pores from pathogenic mutations. Nature. 2002 418(6895) 291. 

Larsen, B. Increased permeability to albumin induced with protamine in modified gelatin membranes. Nature (Lond.) 215, 641—642 (1967). 

The permeability provides a quantitative measure of the easiness with which a certain species penetrates the membrane. The permeability depends on the nature of the gas, on the polymeric matrix, on temperature, and on the pressure values, Pl and Ph, on both sides of the membrane. For simple gases the solubility increases as the gas diameter increases, since the gas becomes easier to condense. The gas diffusivity through a membrane is inversely proportional to its molecular size, because large molecules interact more with the polymer chains. 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Most of the naturally-occurring pyrazine hydroxamic acids appear to be derived from valine, leucine and isoleucine, and biosynthetic studies by MacDonald and coworkers (61JBC(236)512, 62JBC(237)1977, 65JBC(240)1692) indicate that these amino acids are incorporated. However, it would seem that the logical intermediates, viz. the 2,5-dioxopiperazines such as (111) and (112), are not always incorporated. This does not rule out their intermediacy, as there may be problems such as low solubility or membrane permeability which prevent their efficient incorporation. An exception to these results was reported for pulcherrimic acid (113) (65BJ(96)533), which has been shown to be derived from cyclo-L-leu-L-leu which serves as an efficient precursor. 

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

Another common impurity of natural gas is nitrogen. Since nitrogen has essentially no calorific value, it lowers the heating value of gas, Gas purchasers may set a minimum limit of heating value (normally approximately 9.50 Biu/scl). In some cases it may be necessary to remove the nitrogen to satisfy this requirement. This is done in very low temperaliire plants or with permeable membranes. These proces [Pg.4]

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. 

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