Selective barrier

PERMANENT GASES Table 3 lists the permeabilities of oxygen [7782-44-7] nitrogen [7727-37-9] and carbon dioxide [124-38-9] for selected barrier and nonbarrier polymers at 20°C and 75% rh. The effect of temperature and humidity are discussed later. For many polymers the permeabihties of nitrogen, oxygen, and carbon dioxide are in the ratio 1 4 14. 

The normal arterial wall consists of the intima, media, and adventitia, as illustrated in Fig. 4—3A. The endothelium is located in the intima and consists of a layer of endothelial cells that line the lumen of the artery and form a selective barrier between the vessel wall and blood contents. The internal elastic lamina separates the intima and media, where vascular smooth muscle cells are found. The vascular adventitia comprises the artery s outer layer. Atherosclerotic lesions form in the subendothelial space between the endothelial cells and internal elastic lamina. 

Since the use of combined oral contraceptives may decrease the use of selected barrier contraceptive methods that do protect against STDs (e.g., latex condoms), one of the most common risks associated with the use of oral contraceptives is the risk of acquiring an STD.8... 

Guth, D. and van Engelhardt, W. (1989). Is gastro-intestinal mucus an ion-selective barrier In Mucus and Related Topics, eds. Chantler, E. and Ratcliffe, N. A., The Company of Biologists, Cambridge, pp. 117-121. 

TMP). See also Trimethylolpropane butyraldehyde derivative, 4 461, 467 Ethyl 2-methylbutyrate, permeation in selected barrier polymers, 3 389t... 

At the same time, all cells are unified at the molecular level, as emphasized in the first chapter. There are other commonalities as well. All cells have a plasma membrane that surrounds and encloses them. The fundamental function of the plasma membrane is to act as a selective barrier between the cell interior and the external environment. In the Eubacteria and Archaea, single cell organisms all, the plasma membrane is the sole membrane of the cell. In contrast, in the Eukarya, there are a variety... 

All cells are surrounded by a thin lipid membrane. This is a selective barrier, allowing some substances to pass across it and excluding others in order to maintain a relatively constant internal enviromnent. Some of the different proteins that are embedded in the cell membrane transport compounds and ions across the membrane, whereas others act as receptors that respond to factors in the external enviromnent and initiate responses within the cell, and stiU others provide a mechanism for cells to interact and communicate with each other. 

Cellular membranes function as selective barriers and integral membrane protein scaffolds. Membranes allow the compartmentalization of cells, and individual organelles within cells, and are critical in energy transduction and cell signaling. In vivo membranes contain hundreds to thousands of lipid types, making characterization of particular lipid-lipid interactions challenging. 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

Binds to sterols in fungus cell membrane, resulting in an inability to function as a selective barrier and a subsequent loss of essential cellular components... 

Results of this study confirm the expected improved recoveries of trace organics with membranes more selective and more highly cross-linked than the classical cellulose acetate membrane. Improved recoveries were predicted from literature data reported for similar membrane types. In light of these results, cellulose acetate should no longer be considered for applications such as these. Further improvements in recovery can be expected as developmental membranes with more highly selective barriers are brought into commercial use. Each new membrane type considered for use on disinfected waters should be evaluated for sensitivity to common disinfectants (oxidants). Both decreased selectivity and potentially troublesome chemical breakdown products should be considerations under these conditions. Although the cellulose acetate and FT-30 composite membranes did not prove to be particularly sensitive to chlorine, many commercially available... 

Biological membranes are always pictured as being very selective barriers separating different biochemical reaction compartments. This high performance transport specificity solely depends on the presence of membrane proteins embedded in the lipid matrix. On the other hand, most membrane proteins cease to function in the absence of lipids. In order to introduce biological transport abilities into artificial membrane systems protein-lipid interactions are of vital interest. The question is how the activity of membrane proteins is affected if they are placed into a polymeric environment. 

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