Membranes affecting permeability

Modification of the membranes affects the properties. Cross-linking improves mechanical properties and chemical resistivity. Fixed-charge membranes are formed by incorporating polyelectrolytes into polymer solution and cross-linking after the membrane is precipitated (6), or by substituting ionic species onto the polymer chain (eg, sulfonation). Polymer grafting alters surface properties (7). Enzymes are added to react with permeable species (8—11) and reduce fouling (12,13). 

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

There is one major caveat of using the tissue culture transport experiment to study P-gp efflux that cannot be overlooked—P-gp efflux is not directly determined in this experiment. Rather, the effects of P-gp-mediated efflux activity and changes to this activity are inferred from the resulting overall transport data. Particularly with regards to substrate identification, there is the potential for false negatives. For a compound to be affected by P-gp-mediated efflux, it must reach P-gp s binding site that is within the cell. Compounds with poor membrane (transcellular) permeability are not likely to be identified as substrates (395,397). Conversely, compounds with very high passive membrane permeability can saturate P-gp efflux at low micromolar concentrations and are often not identified as substrates... 

MWCO), usually defined as the molar mass at which the membrane rejects 90% of solute molecules. However, as in microfiltration, the molecular shape can affect permeability through the membrane pores. For example, a membrane with a nominal cut-off of 100 kDa, which does not allow globular molecules with a molar mass of 100 kDa to flow through, may allow fibrous molecules with higher molar masses to flow across the pores. As in microfiltration, the membrane pore size is not uniform, with a normal distribution around an average value. 

Possible transport mechanisms in a fluid system through the membrane pores are multiple. They vary to a great extent with the membrane pore size and, to a less extent, with chemical interaction between the transported species and the membrane material. Under the driving force of a pressure gra nt, permeants (whether in the form of solvents, solutes or gases) can transport across a membrane by one or more of the mechanisms to be discussed below. The degree by which they affect permeability and permselectivity depends on the operating conditions, membrane characteristics and membrane-permeating species interactions in the application environment. 

The cell membrane serves as a protective barrier in renal cells. It is the initial site which p-lactams encounter in their journey to the cellular environment from the blood or tubular fluid, p-lactams may disrupt the functional organization of the membrane through peroxidation of membrane lipids, which, in turn, leads to the inability of membrane to serve as an osmotic barrier and causes the cytosol contents to leak. As a result of the cephalosporins disruptive effect on cell membrane, increased leakage of the cytosolic enzyme lactate dehydrogenase (LDH) occurs. The increased LDH concentration was from the cytosol of the renal cortex [49,71] or from isolated proximal and distal tubular cells [39] or in the urine of experimental animals [39]. The results of these studies indicate that plasma membrane became permeable to large molecules such as LDH. After cephalosporin treatment, cephaloridine caused the greatest decrease of LDH concentration in cytosol [49]. Whereas, cephaloridine induced a greater release of LDH from proximal tubular cells than cepha-lothin and cephalexin, distal cells were not affected by any of these cephalosporins [38,39]. 

While water content of the stratum corneum affects permeability of the tissue, hydration also impacts various physical properties of the membrane such as tensile strength and elasticity, modihes the microenvironment for microorganisms on the tissue surface, alters the thermal conductivity of the tissue and also affects skin appearance. Further, increasing hydration also alters the thickness of the stratum corneum as shown in Figure 4, again taken from data provided by Blank et al. (1984). 

Intracellular ionized calcium acts as a second messenger, coupling the action of a hormone or electrical impulse (the first messenger) on the outside of the cell to intracellular events, such as hormone or protein secretion, protein kinase activity, or muscle contraction. The effect of Ca on intracellular processes is often mediated by a small calcium-binding protein, such as troponin C in muscle (Chapter 21) or calmodulin in many other cells (Chapters 15 and 30). Synthesis of these calciumbinding proteins is not directly affected by vitamin D or any of its metabolites. Many stimuli that affect permeability to calcium also activate membrane-bound adenylate cyclase and increase the intracellular concentration of cAMP (Chapter 30). 

Related to thelenotoside B and asterosaponin L. Starfish neurotoxins, steroidal glycosides, affect membrane ion permeability and inhibit Na+/K+ ATPase, depolarize membranes, block cholinergic neuromuscular transmission. Act like ouabain. 

Toxins that act directly on cell membranes, called cytolytic toxins, disturb and ultimately kill the target cells. Produced by many organisms (e.g., bacteria, fungi, plants, fish, and snakes), cytolytic toxins may cause damage in several ways. For example, streptolysin O (67,000 D), produced by the bacterium Streptococcus pyogenes, causes pores to form in the target cell membranes. Affected cells are rapidly lysed because the cell membrane is much more permeable to ions such as Na+. Streptolysin O is believed to cause some of the damage in rheumatic fever. 

Modulation of the cholesterol content of membranes affects passive permeability to water and other molecules. For example, permeability to Na and is known to increase at the boundary between domains of rigid and fluid phospholipid within a bilayer which may result from cholesterol removal [72,73]. 

Sometimes even membrane transport and mechanical properties are affected by the harsh chemical treatments required in the immobilization procedure. Figure 7.35 shows how immobilization of urease by diazotization on heterogeneous polysulfone flat membranes reduces the membrane hydraulic permeability by 50%.74 It also shows how the higher permeability membranes are more affected by the immobilization procedure. After the enzyme is immobilized, it is wise to check the integrity of the membrane (e.g., the permeability to the species of interest and the strength).71 74... 

Eqn. 5 provides a very clear theoretical basis for the data of Fig. 1 (and similar data on other systems, as we shall see). The measured permeability coefficients for a set of solutes should parallel the measured partition coefficients, if the model solvent corresponds exactly in its solvent properties to the permeability barrier of the cell membrane. In addition, the molecular size of the solute is very likely to be an important factor as it will affect the diffusion coefficients within the membrane barrier phase. Data such as those of Fig. 1 will convince us that we have in our chosen solvent a good model for the solvent properties of the membrane s permeability barrier. We can now calculate values of PLx/K for the various solutes, and obtain estimated values of the intramembrane diffusion coefficient, and are in a position to study what variables influence this parameter. Fig. 3 is such a study in which data from Fig. 1 are plotted as the calculated values of f>n,c,n/A.t (calculated as P/K) against the molecular weight of the permeating solute. The log/log plot of the data has a slope of — 1.22, which means that one can express the dependence of diffusion coefficient on molecular weight (A/) in the form where... 

Plasticizers may also change properties of a polymer surface from l drophobic to hydrophilic which affects permeability of Itydrophilic penetrants. Decreased hydropho-bidty is recpiired for PVC material to increase its receptivity of water-based coatings. Combinations of plasticizers and metal salts were used to induce hydrophilic properties. ° Rubber print rolls also require to have hydrophilic properties to hold film of water-based ink and this was achieved by use of a special plasticizer. Increase in hpophillicity of plasticizer causes linear increase in water penetration coefficient and water flux across membranes obtained from PVC plasticized with various plasticizers. Compatibility of a polymer and a plasticizer was found to affect staining resistance. Compatible plasticizers were less likely to dissolve stains and help them to penetrate into material. 

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