Permeabilities of membranes

The physical characterisation of membrane structure is important if the correct membrane is to be selected for a given application. The pore structure of microfiltration membranes is relatively easy to characterise, SEM and AFM being the most convenient method and allowing three-dimensional structure of the membrane to be determined. Other techniques such as the bubble point, mercury intrusion or permeability methods use measurements of the permeability of membranes to fluids. Both the maximum pore size and the pore size distribution may be determined.1315 A parameter often quoted in manufacturer s literature is the nominal... 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

High concentrations of carotenoids may increase the permeability of membranes to toxins and radicals. 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

J Kopecek, J Vacik, D Lim. Permeability of membranes containing ionogenic groups. J Polym Sci A-l 9 2801-2815, 1971. 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

Kim, J., Kim, B. and Jung, B. 2002. Proton conductivities and methanol permeabilities of membranes made from partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymers. Journal of Membrane Science 207 129-137. 

Polymers are permeable to liquids having similar solubility parameters. Several different celts have been designed to measure the permeability of membranes to gases and vapors. 

Batrochotoxin. Toxin extracted from skin of Columbian arrow poison frog. Effect via irreversible increase in permeability of membranes to Naf ions... 

The toxin acts on nerves to block transmissions to the muscle by increasing the permeability of membranes to sodium ions. It is specifically antagonized by tetro-dotoxin (Fig. 30-16). Batrachotoxin alkaloids are present also in certain birds.3303 Some echinoderms make powerful steroid toxins such as holothuiin A (Fig. 22-12), a surface active agent that causes irreversible destruction of the excitability of neuromuscular tissues. The common toad produces in its skin steroid toxins such as bufotoxin (Fig. 22-12), which are sufficiently powerful to teach a dog to leave toads alone. 

On the permeability of membranes with parallel, but interconnected, pathways. Math. Bi-osci. (Bellman Memorial Issue) 77,5-16 (1985). 

The antibiotics are compounds secreted by microbes that enhance the permeability of membranes to cations. One class functions by binding a metal to give a liposoluble complex that can then pass across the membrane. Examples are valinomycin, a cyclic peptide that binds K+ selectively, and monensin which binds Na+. These too are oxygen-donor ligands, and will be discussed in the following section. They function as antibiotics because they allow the concentrations of a cation across membranes to become equalized, and so cause the collapse of the membrane potential. 

Itonaga, T., Kimura, K. and Watanabe, Y. (2004) Influence of suspension viscosity and colloidal particles on permeability of membrane used in membrane bioreactor (MBR). Water Science and Technology, 50, 301-309. 

An, permeability of membrane for CP, permeability of membrane for K+ Axa1 > permeability of membrane for Na+... 

Ultrasound cleaning has also been used to remove fouling from UF polysulphone and MF cellulose membranes used to treat peptone and milk aqueous solutions, respectively. The US employed had 28, 45 and 100 kHz frequency with 23 W/cm output power. With 28-kHz US, water was found to be effective for recovery from a deteriorating condition due to fouling US-enhanced permeability of membranes was also observed. It is worth noting... 

Regulates structure and permeability of membranes, root growth Enzyme catalyst Component of chlorophyll Activates enzymes... 

Lipid peroxidation Increased permeability of membrane Cancer... 

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