The permeability of natural membranes

It is evident that the distribution of an agent is highly dependent on its ability to penetrate semi-permeable membranes. The plasma membrane is discussed in Section 5.4. In the classical conception of this membrane, its behaviour was static, something like a dialysis sac. More recent work discovered its latent dynamic properties, such as phase-reversal and processes akin to enzymic activity (e.g. permease activity, and transport in response to the metabolism of glucose). In the following pages, membranes will be discussed according to their observed functions and classified as four main types. 

This type of membrane ensures simple diffusion, namely the transport velocity is directly proportional to the concentration difference across the membrane. Thus, when an equilibrium is reached, the internal and external concentrations of the drug are the same. The transport velocity is affected by molecular weight, lipid solubility, and charge, but not greatly by temperature. 

This seems to be by far the commonest type of membrane. It hinders the passage of ions, and permits that of neutral molecules. Through this type of membrane, those molecules that have high oil/water partition coefficients (and hence are quite lipophilic) diffuse fastest. The half-time for equilibrium can vary 

Permeability of living cells (Mol/s//im Vmolar cone, difference) XlO  

Curcuma (flowering plant) (Collander, 1937) B, Gregarina (protozoon) (Adcock, 1940) C, Arabaciatggs (marine animal) (Stewart and Jacobs, 1936) Z), Ox erythrocytes (Jacobs a/., 1935) E, Ohara (green alga) (Collander, 1937). 

The influence of chemical constitution on partition coefficients is discussed in Section 3.2, and also the effects of varying the organic phase. 

The Type i membrane appears to be about 5 nm thick and to consist mainly of lipoidal material mixed with some protein. The presence of this type is diagnosed if substances of similar molecular weight and molecular diameter are found to penetrate at a rate proportional to their partition coefficients. It is interesting to note that if the partition coefficient is too high, the substance enters the membrane freely but cannot leave it (it would take 60 kcal/mol to release a lipophile from the membrane into water). 

For a discussion on membrane permeability and equilibria see Wilbrandt (1959)3 for the kinetics of diffusion see Laidler and Shuler (1949)5 and Zwolinski, Eyring and Reese (1949). 

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H Davson, JF Danielli. The Permeability of Natural Membranes. 2nd ed. New York Cambridge University Press, 1952. 

Danielli, J. F. in The permeability of natural membranes, by Davson and DanieLLI (Cambridge 1943), chap. XXI and appendix A. 

In 1943, Davson and Danielli introduced, in their seminal book The Permeability of Natural Membranes, the idea that solute permeability was not a generalized property of the plasma membrane but rather was associated with discrete and... 

Lamellar, and partly micellar, mosaics of lipids and proteins form a thin, semi-permeable membrane around the exterior of every cell, and also around each organelle in the cell (see Section 5.4). The permeability of natural membranes was discussed in Section 3.2. The interaction of these membranes with diuretics, cardiac glycosides, and other ionophoric effects will now be considered. 

From his early youth, under his father s influence, K. H. Meyer had retained a keen interest in biological problems, as was evident from his study of the phenomena of narcosis, which he pursued during his stay in industry. As a natural consequence, he extended his thoughts to biological problems, and evolved a quantitative theory of muscular contraction (in collaboration with Picken), based on analogies with the elasticity of rubber. With J. F. Sievers, the permeability of synthetic membranes was investigated, and a mathematical treatment of the phenomenon was advanced which was later applied to living membranes. 

Graham, who was one of the first to consider the permeabilities of natural rubber films to a wide range of gases, found responses such as that seen in Fig. 2a. The description he formulated in 1866 of the so-called "solution-diffusion" mechanism still prevails today (30). He postulated that a penetrant leaves the external phase in contact with the membrane by dissolving in the upstream face of the film and then undergoes molecular diffusion to the downstream face where it evaporates into the external phase again. Mathematically, one can state the solution-diffusion model in terms of permeability, solubility and diffusivity coefficients, as shown in Eq(2). 

Apparently, the idea that it could be interesting to let a solvent and a solution communicate through a semi-permeable surface, came from biology, and more precisely from the study of natural membranes and from dialysis practices. 

The modified membrane surfaces exhibit more hydrophilic and negative charged features after the treatment. Grafting with MA and AAG decreases the permeability of natural organic matter (NOM) to less than half of the untreated membrane. The modification reduces fouling by foulants such as NOM. In a similar study, best results, with respect to protein retention and protein solution flux, were obtained by grafting NVP, 2-acrylam-ido-2-methyl-l-propane sulfonic acid, and AA onto a 50 kDalton PES. ... 

The resistance of this class of polymers was reported in detail because they have multiple applications in the various economical branches [7, 83-85]. The natural ageing was also intensively studied because of the outdoor usage as cables, gaskets, membranes in various equipments. Because the permeability of EPDM membranes depends on their oxidation states, the prediction of oxygen diffusion is not enturely possible during or after weathering treatments [86]. 

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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