Specific Membranes

An iodide-selective electrode can be used to determine dissolved chlorine. Iodide is added when the sample solution is prepared. Loss of iodide (by reaction with chlorine) is then measured by the electrode. 

Clay membranes are prepared by the carefully regulated heating of layers of specially selected clays. By regulation of the doping levels, the clay matrix carries the necessary ions which regulate the adsorption of the ions, and hence selectivities. Clay membranes are limited in use, because they are difficult to prepare, have a high electrical resistance, and are easily broken. 

Early clay membranes were thin plates formed from natural single crystals of chabazite and apophyllite (Marshall, 1939,1942,1944). These are zeolite (hydrous silicate) materials which possess a very open porous structure. Membranes of this sort are quite fragile and their fragility has led to the development of special clays (Marshall et al, 1948). 

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This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

This system utilizes specific membranes, between which the dmg reservoir is enclosed (Fig. 4). A tiny ehiptical disk, inserted into the cul-de-sac of the eye, releases pilocarpiae steadily. The dmg is deUvered through selected polymeric membranes. The dmg reservoir maintains a saturated solution between the membranes which acts osmoticaHy as the driving force for the dmg to diffuse through the rate-limiting membranes. 

Fleece-Back Sheet. A fleece-back sheet is a nonreinforced polymeric membrane that has had a nonwoven mat made of polyester, weighing 101.7—203.4 g/m, laminated to the back of the sheet. The prime use of the fleece-back sheet is in the fully adhered roofing systems. The fleece provides the chemical separator, which eliminates the need for an adhesive that is compatible with the specific membrane or a compatible substrate. 

Thermoplastics. There are five elastomeric membranes that are thermoplastic. Two materials, chlorinated polyethylene (CPE) and polyisobutylene (PIB), are relatively obscure. Thermoplastic materials can be either heat-fused or solvent-welded. In contrast to Hypalon and uncured EPDM, this abiHty to fuse the membranes together remains throughout the life of the material. However, cleaning of the membrane surface after exposure to weather is required. Correct cleaning procedures for specific membranes are available from the individual manufacturer. 

While recent attention has been largely on proteins, it should be borne in mind that membrane fusion ultimately involves the merger of phospholipid bilayers. However, little is known about the specific membrane lipid requirements. When membranes fuse, energetically unfavorable transition states are generated that may require specific lipids and lipid domains for stabilization. Although there is some evidence for a specific influence of lipids on exocytosis, it is still unclear whether specific lipid metabolites are needed or even generated at the site of membrane merger. 

A = approximate area of the bilayer lipid membrane G = membrane conductance Gj = specific membrane conductance Cm = membrane capacitance C, = specific membrane capacitance. 

Bhatnagar, A., Srivastava, S.K. and Szabo, G. (1990). Oxidative stress alters specific membrane currents in isolated cardiac myocytes. Circ. Res. 67, 535-549. 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

Joarmon S, Pin C (2001) Ultra-trace determination of Ra in thermal waters by high sensitivity quadrapole ICP-mass spectrometry following selective extraction and concentration using radium-specific membrane disks. J Anal At Spectrom 16 32-37... 

The design and capacity of an RO unit is dependent upon the type of chemicals in the plating solution and the dragout solution rate. Certain chemicals require specific membranes. For instance, polyamide membranes work best on zinc chloride and nickel baths, and polyether/amide membranes are suggested for chromic acid and acid copper solutions. The flow rate across the membrane is very important. It should be set at a rate to obtain maximum product recovery. RO systems have a 95% recovery rate with some materials and with optimum membrane selection.22... 

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

Receptors carbohydrates may also serve as specific membrane receptors for extracellular substances such as hormones. 

Thornber, J.P., Trosper, T.L., Strouse, C.E. Bacteriochlorophyll in vivo relationship of spectral forms to specific membrane components. In The Photosynthetic Bacteria (Clayton, R.K., Sistrom, W.R., eds.). New York Plenum Press 1978, pp. 133-160... 

Other systems like electroporation have no lipids that might help in membrane sealing or fusion for direct transfer of the nucleic acid across membranes they have to generate transient pores, a process where efficiency is usually directly correlated with membrane destruction and cytotoxicity. Alternatively, like for the majority of polymer-based polyplexes, cellular uptake proceeds by clathrin- or caveolin-dependent and related endocytic pathways [152-156]. The polyplexes end up inside endosomes, and the membrane disruption happens in intracellular vesicles. It is noteworthy that several observed uptake processes may not be functional in delivery of bioactive material. Subsequent intracellular obstacles may render a specific pathway into a dead end [151, 154, 156]. With time, endosomal vesicles become slightly acidic (pH 5-6) and finally fuse with and mature into lysosomes. Therefore, polyplexes have to escape into the cytosol to avoid the nucleic acid-degrading lysosomal environment, and to deliver the therapeutic nucleic acid to the active site. Either the carrier polymer or a conjugated endosomolytic domain has to mediate this process [157], which involves local lipid membrane perturbation. Such a lipid membrane interaction could be a toxic event if occurring at the cell surface or mitochondrial membrane. Thus, polymers that show an endosome-specific membrane activity are favorable. 

Most transport vesicles bud off as coated vesicles, with a unique set of proteins decorating their cytosolic surface. The coat has two major known functions. First, it concentrates and selects specific membrane proteins in a discrete portion of donor organelle membrane that will serve as origin to the transport vesicle. Second, the assembly of coat proteins into curved structures delineates the area of the forming transport vesicle. The size and curvature is a function of the coat composition. Thus, vesicles with similar vesicle coat have closely similar size and shape [3]. 

Another major protein component of clathrin-coated vesicles, generally known as adaptins, bind and link clathrin coats to the membrane [5], Multiple types of adaptins have been described, each type binding a unique set of cargo receptors and associated to a specific membrane organelle. Different sets of adaptins participate in forming the coat assembly complexes for the Golgi (API) and the plasma membrane (AP2). Sequential assembly and... 

Retrieval of membrane components in the secretory pathway through receptor-mediated endocytosis (RME) is a clathrin-coat-dependent process [5]. The clathrin coat provides stability to the vesicle core and allows uptake of specific membrane proteins for reuse or degradation. RME shows a remarkable degree of specificity, allowing cells to internalize with astonishing efficiency only those selected molecules independent of their extracellular concentration. 

Specific membrane components must be delivered to their sites of utilization and not left at inappropriate sites [3]. Synaptic vesicles and other materials needed for neurotransmitter release should go to presynaptic terminals because they serve no function in an axon or cell body. The problem is compounded because many presynaptic terminals are not at the end of an axon. Often, numerous terminals occur sequentially along a single axon, making en passant contacts with multiple targets. Thus, synaptic vesicles cannot merely move to the end of axonal MTs. Targeting of synaptic vesicles thus becomes a more complex problem. Similar complexities arise with membrane proteins destined for the axolemma or a nodal membrane. 

ElectroCell System AB [99], EL-TECH[269], ICI [271,272], and de Nora [129,273] are now developing electrohydrolysis of sodium sulfate for commercial applications. In the electrohydrolysis process sodium sulfate is fed as anolyte to an electrochemical cell divided by a cation specific membrane. Protons are generated in the anolyte, hydroxyl ions at the cathode. Sodium ions cross the membrane to produce a catholyte solution of sodium hydroxide. The net reaction is ... 

Scheme 15. Kinetic trapping model for the enrichment of isoprenylated proteins with a free acylation site in specific membranes...

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