Carrier transport membrane

In this chapter, the structure and the characteristics of various functional membranes such as gas permeation, dialysis, reverse osmosis, ultrafiltration, pervaporation, evapomeation, carrier transport membranes from chitin, and chitosan derivatives were described. 

Facilitated transport membranes have been attracting attention since they have veiy high selectivity, compared with conventional polymer membranes (7). This high selectivity is attributable to carriers which can react reversibly with permeant specif There are two types of facilitated (carrier) transport membranes. One is the mobile carrier membrane in which the carrier can diffuse in the membrane, and the other is the fixed carrier membrane in ich the carrier cannot move. 

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... 

Ubiquinone, known also as coenzyme Q, plays a crucial role as a respiratory chain electron carrier transport in inner mitochondrial membranes. It exerts this function through its reversible reduction to semiquinone or to fully hydrogenated ubiquinol, accepting two protons and two electrons. Because it is a small lipophilic molecule, it is freely diffusable within the inner mitochondrial membrane. Ubiquinones also act as important lipophilic endogenous antioxidants and have other functions of great importance for cellular metabolism. ... 

Novel chiral. separations using enzymes and chiral surfactants as carriers have been realized using facilitated transport membranes. Japanese workers have reported the synthesis of a novel norbornadiene polymeric membrane with optically active pendent groups that show enantio.selectivity, which has shown promi.se in the. separation of propronalol. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

There seems to be an opportunity to extend the electrochemical process to direct membrane transport that is, with electrodes plated on either side of a facilitated-transport membrane similar to that of Johnson [24]. The shuttling action of the carrier (Fig. 9) could then be brought about by electrochemical reduction and oxidation instead of pressure difference. 

SlROTNAK, F. M. AND B. TOLNER. Carrier-mediated membrane transport of folates in mammalian cells. Annu. Rev. Nutr. 1999, 39, 91-122. 

Rt Total number of sensitive sites (carriers, transporters, receptors, etc.) at the membrane surface ... 

Keywords Colon Controlled release Sustained release Rat Single-pass perfusion Recirculation Closed loop Carrier-mediated transport Passive transport Membrane permeability P-glycoprotein Paracellular pathway Transcellular pathway... 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

Net carrier transport can be examined whenever a transportable substrate is present on both sides of a membrane. Again for the case of a single class of carriers, the inward transport rate and outward transport rates show rate-saturation behavior ... 

In spite of the overwhelming importance of the channel mechanism for the transport of alkali and alkaline earth metal ions in biological systems, only carrier transport has been studied extensively by chemists. Studies on ion channel mimics of simple structures have long been limited to antibiotic families of gramicidin, amphotericin B, and others. Several pioneers have reported successful preparation of non-peptide artificial channels. However, their claims have been based on kinetic characteristics observed for the release of metal ions through liposomal membrane and lacked the very critical proofs of channel formation. Such a situation was... 

Comparison of rates in the gel and liquid state of the membrane. In the case of channel mechanism, ions can pass through a structured pathway which is more or less similar to the bulk aqueous phase, while carriers encapsulate ions and transfer in the membrane. Therefore, the former is insensitive to gel-liquid phase transition of the membrane, while the gel state inhibits the carrier transport. 

A series of ion-selective membrane electrodes based on neutral carrier solvent polymeric membranes has been designed for the potentiometric determination of ion activities (for reviews see Refs. 52, 65). Systems with analytically relevant selectivities for Li+, Na+, K+, NHJ, Ca2+, and Ba2+, are available. In agreement with the treatment given in Sections III and IV, the ions preferred in potentiometric studies may be transported preferentially through the same membranes in electrodialytic experiments. So far, selective carrier transports have been realized for Li+, Na+, K+, and Ca2+. 

Cd in natural water using a liquid membrane system with 2-acetylpyr-idine benzoylhydrazone as carrier. Transport processes across the membrane were optimised... 

After a LCFA enters a cell, it is converted to the CoA derivative by long-chain fatty acyl CoA synthetase (thiokinase) in the cytosol (see p. 174). Because 0-oxidation occurs in the mitochondrial matrix, the fatty acid must be transported across the mitochon drial inner membrane. Therefore, a specialized carrier transports the long-chain acyl group from the cytosol into the mitochondrial matrix. This carrier is carnitine, and the transport process is called the carnitine shuttle (Figure 16.16). 

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

The nasal epithelium possesses selective absorption characteristics similar to those of a semipermeable membrane, i.e., it allows a rapid passage of some compounds while preventing the passage of others. The process of transportation across the nasal mucosa involves either passive diffusion, via paracellular or transcellular mechanisms, or occurs via active processes mediated by membrane-bound carriers or membrane-derived vesicles involving endo- or transcytosis. 

Carrier-mediated membrane transport proteins on the RPE selectively transport nutrients, metabolites, and xenobiotics between the choriocapillaris and the cells of the distal retina, and include amino acid [33 35], peptide [36], dicarboxylate, glucose [37], monocarboxylic acid [38,39], nucleoside[40], and organic anion and organic cation [41] transporters. Membrane barriers such as the efflux pumps, including multidrug resistance protein (P-gp), and multidrug resistance-associated protein (MRP) pumps have also been identified on the RPE. Exploitation of these transport systems may be the key to circumventing the outer BRB. 

The barrier to paracellular diffusion potentially isolates the brain from many essential polar nutrients such as glucose and amino acids that are required for metabolism and, therefore, the BBB endothelium must contain a number of specific solute carriers (transporters) to supply the CNS with its requirements for these substances. The formation of tight junctions essentially confers on the BBB the properties of a continuous cell membrane, both in terms of the diffusional characteristics imposed by the lipid bilayer, and the directionality and properties of the specific transport proteins, and solute carriers (SLC) that are present in the cell membrane. Examples of BBB solute carriers (SLC transporters) are listed in Table 27.2. 

In facilitated transport (also known as carrier-mediated membrane transport), a substance combines with a specific carrier protein on the membrane, and the resultant protein-sub-stance complex diffuses to the other side of the membrane, where it dissociates to release the substance. The absorption of glucose from the intestines into the blood, for example, requires facilitated transport of glucose across the cellular membranes of the epitheleal lining of the intestines. Many amino adds cross cellular membranes by fadlitated transport. 

Schematic diagram of a generalized noradrenergic junction (not to scale). Tyrosine is transported into the noradrenergic ending or varicosity by a sodium-dependent carrier (A). Tyrosine is converted to dopamine (see Figure 6-5 for details), which is transported into the vesicle by a carrier (B) that can be blocked by reserpine. The same carrier transports norepinephrine (NE) and several other amines into these granules. Dopamine is converted to NE in the vesicle by dopamine-B-hydroxylase. Release of transmitter occurs when an action potential opens voltage-sensitive calcium channels and increases intracellular calcium. Fusion of vesicles with the surface membrane results in expulsion of norepinephrine, cotransmitters, and dopamine-13-hydroxylase.

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