Membranes early

Green fluorescent protein (GFP) and related fluorescent proteins can be used to label practically any protein or subcellular compartment of living cells (49). Transfection of cells with plasmids that encode appropriately targeted fluorescent fusion proteins has been used to define the plasma membrane, early endosomes, late endosomes, caveolae, the golgi complex, the ER, and other subcellular locations. Several fluorescent small molecules are also available for labehng specific cellular organelles, including endosomes and lysosomes, for analysis by fluorescence microscopy. 

A characteristic of virtually all methanogen hydrogenases is their very large native size (although they are soluble enzymes), typically 600-1000 kDa. This is often explained as a result of hydrophobic interactions with other proteins, and as an indication that it is a loosely-bound membrane protein. H2ase is often associated with membranes early during its purification [271,280]. These properties have been investigated in several ways by electron microscopy. 

Phosphatidylserine, which normally is on the inner leaflet of the plasma membrane, early during apoptosis becomes exposed on the outside cell surface (13). Because the anticoagulant protein annexin V binds with high affinity to phosphatidylserine, fluorochrome-conjugated annexin V can serve as a marker of apoptotic cells (21). During progression of apoptosis, the ability to bind annexin V precedes the loss of the plasma membrane s ability to exclude cationic dyes such as PI. 

A field in which NMR has contributed extremely valuable information is biological membranes. Early studies concentrated on development of labeling methods, spectrometer techniques, and elucidation of the properties of lipids in model membranes (2, > 16). The experience gained with the models is now actively under application to intact biological membranes. 

Types of membranes. Early membranes were limited in their use because of low-selectivities in separating two gases and quite low permeation fluxes. This low-flux problem was due to the fact that the membranes had to be relatively thick (1 mil or 1/1000 of an inch or greater) in order to avoid tiny holes which reduced the separation by allowing viscous or Knudsen flow of the feed. Development of silicone polymers (1 mil thickness) increased the permeability by factors of 10 to 20 or so. 

As an example for organic liquids and proton transport therein we will discuss liquid imidazole. As the above-mentioned phosphonic acid, immobilised imidazole can be used in applications, such as fuel-cell membranes. Early studies have shown the different time scales that are involved in proton-transfer mechanisms. Recently, Chen and co-workers have used a multi-state empirical valence-bond (MS-EVB) model to atomistically simulate proton transport in liquid imidazole. The system consists of 216 imidazole molecule and one excess proton. 

Although the structural hypothesis of the molecular conformation of biological membranes was introduced over five decades ago (Gorter and Grendel, 1925), we still do not know the real structure of the cell membrane. Early research into membrane structure dates back to 1895, when Overton proposed the existence of lipid components in biological membranes. On the basis of experiments with human red cells, Gorter and Grendel (1925) concluded that the lipid was spread over the red cell surface in a bimolecular layer, with hydrophobic tails directed toward the center of the lipid leaflet and polar, hydrophilic heads on the surface. 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

The depressed prices of most metals in world markets in the 1980s and early 1990s have slowed the development of new metal extraction processes, although the search for improved extractants continues. There is a growing interest in the use of extraction for recovery of metals from effluent streams, for example the wastes from pickling plants and electroplating (qv) plants (276). Recovery of metals from Hquid effluent has been reviewed (277), and an AM-MAR concept for metal waste recovery has recentiy been reported (278). Possible appHcations exist in this area for Hquid membrane extraction (88) as weU as conventional extraction. Other schemes proposed for effluent treatment are a wetted fiber extraction process (279) and the use of two-phase aqueous extraction (280). 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Membrane modules have found extensive commercial appHcation in areas where medium purity hydrogen is required, as in ammonia purge streams (191). The first polymer membrane system was developed by Du Pont in the early 1970s. The membranes are typically made of aromatic polyaramide, polyimide, polysulfone, and cellulose acetate supported as spiral-wound hoUow-ftber modules (see Hollow-FIBERMEMBRANEs). 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Systematic studies of membrane phenomena can be traced to the eighteenth century philosopher scientists. For example, Abbn Nolet coined the word osmosis to describe permeation of water through a diaphragm in 1748. Through the nineteenth and early twentieth centuries, membranes had no industrial or commercial uses but were used as laboratory tools to develop physical/chemical theories. 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Protected-Membrane Roofs. Primitive roofs coveted with earth and sod over sloping wood decks shingled with bark were early examples of protected-membrane roofs (PMRs). Grass and earth provided iasulation and protected the shingled deck from inclement weather. 

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