Permeability of the membrane

Here the permeability of the membrane to the solute is defined in terms of reflection coefficients aQ and for osmosis and filtration respectively. When (To = 1, then perfect semi-permeabihty results. in Eq. (4) is the diffusive permeabihty of the membrane, while (Cj) is the average composition of the solute in the membrane. 

Amphotericin B, is a polyene antibiotic, used in the therapy of systemic fungal infections. Its mode of action exploits differences in membrane composition between the pathogen and the human host. Ergosterol, the predominant sterol of fungi, plants, and some protozoan parasites, interacts with Amphotericin B, resulting in an increased ion permeability of the membrane. Humans contain cholesterol, which has a low affinity for amphotericin B. 

The coefficient Lw can be determined from the mechanical permeability of the membrane (cf. Eq. 6.2.2) in the presence of a pressure difference and absence of a solute concentration difference (Ajt = 0) and of an electric potential difference A (electrodes Wx and W2 are connected in short, that is A = 0). The electrical conductivity of the membrane gM in the absence of a pressure and solute concentration difference gives the coefficient L ... 

Ion transport is characterized by conditional rate constants k +, k a+ and k - which can be identified with the permeabilities of the membrane for these ions. 

Permeability of an FML is evaluated using the Water Vapor Transmission test.28 A sample of the membrane is placed on top of a small aluminum cup containing a small amount of water. The cup is then placed in a controlled humidity and temperature chamber. The humidity in the chamber is typically 20% relative humidity, while the humidity in the cup is 100%. Thus, a concentration gradient is set up across the membrane. Moisture diffuses through the membrane, and with time the liquid level in the cup is reduced. The rate at which moisture is moving through the membrane is measured. From that rate, the permeability of the membrane is calculated with the simple diffusion equation (Fick s first law). It is important to remember that even if a liner is installed correctly with no holes, penetrations, punctures, or defects, liquid will still diffuse through the membrane. 

Figure 4 Release rate for KC1 plotted versus the fluid permeability of the membrane. (O) CA-398-10 + 50% sorbitol ( ) CA-320S 398-30 (50 50) + 50% sorbitol ( , , ) Same as ( ) but contains 5, 10, 25% diethylphthalate respectively. (From Ref 16.)...

The rate of diffusion of a substance is influenced by several factors (see Table 2.1). It is proporhonal to the concentration gradient the permeability of the membrane and the surface area of the membrane. For example, as the permeability of the membrane increases, the rate of diffusion increases. It is inversely proporhonal to the molecular weight of the substance and the thickness of the membrane. Larger molecules diffuse more slowly. 

Figure 5.1 Mechanism of action at a chemical synapse. The arrival of an action potential at the axon terminal causes voltage-gated Ca++ channels to open. The resulting increase in concentration of Ca++ ions in the intracellular fluid facilitates exocytosis of the neurotransmitter into the synaptic cleft. Binding of the neurotransmitter to its specific receptor on the postsynaptic neuron alters the permeability of the membrane to one or more ions, thus causing a change in the membrane potential and generation of a graded potential in this neuron.

At an inhibitory synapse, binding of the neurotransmitter to its receptor increases permeability of the membrane to K+ ions or to Ch ions through chemical messenger-gated channels. As a result, K+ ions may leave the cell down their concentration gradient carrying (+) charges outward or Cl ions... 

Figure 3.6 compares iso-pH permeabilities of ketoprofen at various pH values in a 2% DOPC-dodecane model (open circles) and the 20% soy lecithin with SLS in the acceptor compartment (filled circles, data in Table 3.5). In the presence of the latter negatively charged lipids (with the make-up similar to that of BBM in Table 3.1), ketoprofen is intrinsically more permeable, by a factor of 17. The UWL limit, indicated by the solid curves in low-pH solutions, and consistent with the permeability Pu 19.8 x 10-6 cm s 1 (log Pu —4.7), masks the true intrinsic permeability of the membranes, P0. However, it is possible to deduce the membrane permeability if the pKa is known. In Fig. 3.6, the bending in the dashed (calculated) curves at pH 4 corresponds to the pKa of the molecule. Due to the UWL, the point of bending is shifted to higher pH values in the solid (measured) curves. The difference between the apparent pKa (pK 5.3 for DOPC and 6.3 for soy) and the true pKa (4.12) is the same as the difference between log P0 and log Pu [23],... 

The answer is c. (Hardman, pp 1143-1144.) Bacitracin, cycloserine, cephalothin, and vancomycin inhibit cell-wall synthesis and produce bacteria that are susceptible to environmental conditions. Polymyxins disrupt the structural integrity of the cytoplasmic membranes by acting as cationic detergents. On contact with the drug, the permeability of the membrane changes. Polymyxin is often applied in a mixture with bacitracin and/or neomycin for synergistic effects. 

The major route for bioaccumulation of hydrophobic organic compounds in aquatic animals is passive diffusion over cell membranes. In fish, the gill epithelia are the predominant port of entry, with less than 40% of uptake across the skin [181]. Since permeability of the membrane is a direct function of the membrane-water partition coefficient and the diffusion coefficient across the membrane interior [182], the bioconcentration factor (logBCF) can be directly correlated with log K0Vl. or log Km%v for compounds with intermediate hydro-phobicity [183,184],... 

From the mercury porosimetry data, porosity can be calculated. A higher porosity means a more open pore structure, thus generally providing a higher permeability of the membrane. Porous inorganic membranes typically show a porosity of 20 to 60% in the separative layer. The porous support layers may have higher porosities. 

The electrolyte is, however, dissolved in one medium only, the water and the source of the membrane potential is attributable to the permeability of the membrane to one ion only and to no other cause. 

Included among drugs that alter the permeability of the membrane are phosphatidylserine, S-adenosylmethionine, and ganglioside extracts. Phosphatidylserine, produced from purified extracts of bovine brain cortex, alters the permeability and functionality of the neuronal membrane. A study... 

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