Selectively transport

Liquid Membranes. A number of reviews summarize the considerable research effort ia the 1970s and 1980s on Hquid membranes containing carriers to faciUtate selective transport of gases or ions (58,59). Although stiU being explored ia a number of laboratories, the mote recent development of much mote selective conventional polymer membranes has diminished interest ia processes using Hquid membranes. 

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... 

Fig. 5-6. Chiral separation by MIP membranes a combination of sieving and selective transport [44],...

Rose, A. B. (1979). Energy Intensity and Related Parameters of Selected Transportation Modes Passenger Movement. Oak Ridge, TN Oak Ridge National Lalaoratoiy. 

Monensin, which is one of the natural antibiotics, selectively transports Na+ across an artificial liquid membrane (organic solvent) from the basic aqueous phase (IN) to the acidic aqueous phase (OUT), driven by the proton gradient8). (Fig. 1, 2)... 

In mimicking this type of function, noncyclic artificial carboxylic ionophores having two terminal groups of hydroxyl and carboxylic acid moieties were synthesized and the selective transport of alkali metal cations were examined by Yamazaki et al. 9 10). Noncyclic polyethers take on a pseudo-cyclic structure when coordinating cations and so it is possible to achieve the desired selectivity for specific cations by adjusting the length of the polyether chain 2). However, they were not able to observe any relationship between the selectivity and the structure of the host molecules in an active transport system using ionophores 1-3 10). (Table 1)... 

Table i. Active and selective transport of sodium, potassium and cesium ions with synthetic ionophores ... 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

Carboxylic ionophores selectively transport cations by using intramolecular complexation in the uptake process of cations (basic region). A new ion transport system has been developed which incorporates a structural device which assists in the release process by using intramolecular complexation of an [18]crown-6 ring and a primary ammonium ion 48>. The experimental conditions are shown in Fig. 7. All these com-... 

This review surveys the types of host molecules that are applicable to the active transport system. It need scarcely be said that these results, which are based on selective transport in passive transport systems (see the Chaps. 3 and 5), strongly supports this consideration. From this point of view, a systematic investigation into the passive transport system as that by Izatt et al. is noted as one of the best approaches for clarifying the question of membrane transport77). 

In the biological field, much attention has been directed toward the transport phenomena through membrane. Although the function of some natural ionophores has been known, the investigation of active and selective transport of ions using the artificial ionophores in the simple model systems may be important to simulate the biological systems and clarify the transport behaviour of natural membranes. 

With the adequacy of lipid bilayer membranes as models for the basic structural motif and hence for the ion transport barrier of biological membranes, studies of channel and carrier ion transport mechanisms across such membranes become of central relevance to transport across cell membranes. The fundamental principles derived from these studies, however, have generality beyond the specific model systems. As noted above and as will be treated below, it is found that selective transport... 

The nuclear pore complex, located in the nuclear envelope, contains more than 50 proteins. It allows diffusion of small proteins between cytoplasm and nucleoplasm. Larger molecules (>50kD) are selectively transported by an energy-dependent mechanism. 

Some investigations have tested the ability of reversed micelles to act as efficient carriers of molecular species. Solutions of water-containing AOT-reversed micelles have been employed for the selective transport and the efficient separation of the two amino acids tryptophane and j9-iodophenylalanine [160]. 

It is well known that the selective transport of ions through a mitochondrial inner membrane is attained when the oxygen supplied by the respiration oxidizes glycolysis products in mitochondria with the aid of such substances as flavin mononucleotide (FMN), fi-nicotinamide adenine dinucleotide (NADH), and quinone (Q) derivatives [1-3]. The energy that enables ion transport has been attributed to that supplied by electron transport through the membrane due to a redox reaction occurring at the aqueous-membrane interface accompanied by respiration [1-5],... 

Robbins JA (1988) A model for particle-selective transport of tracers in sediments with conveyor-belt deposit feeders. Journal of Geophysical Research 91 8542-8558 Robinson LF, Belshaw NS, Henderson GM (in press) U and Th isotopes in seawater and modem carbonates from the Bahamas. Geochim Cosmochim Acta Rosenthal Y, Boyle EA, Labeyrie L, Oppo D (1995a) Glacial enrichments of authigenic Cd and U in subantarctic sediments A climatic control on the elements oceanic budget Paleoceanography 10(3) 395-413... 

Embedded within the brain are four ventricles or chambers that form a continuous fluid-filled system. In the roof of each of these ventricles is a network of capillaries referred to as the choroid plexus. It is from the choroid plexuses of the two lateral ventricles (one in each cerebral hemisphere) that cerebrospinal fluid (CSF) is primarily derived. Due to the presence of the blood-brain barrier, the selective transport processes of the choroid plexus determine the composition of the CSF. Therefore, the composition of the CSF is markedly different from the composition of the plasma. However, the CSF is in equilibrium with the interstitial fluid of the brain and contributes to the maintenance of a consistent chemical environment for neurons, which serves to optimize their function. 

Boroxine ring-containing polymers have found extensive use in the development of polymeric electrolyte materials used in ion-selective transport membranes. Matsumi and Ohno cover this area in Chapter 6 of this book. 

Sandy parent materials are also abundant in areas where sand accumulates after selective transportation of weathering material by wind or water. 

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