Transport across membranes simple diffusion

Fig 29. A simple equivalent circuit for the artificial permeable membrane. Physical meaning of the elements C, membrane capacitance (dielectric charge displaceme-ment) R, membrane resistance (ion transport across membrane) f pt, Phase transfer resistance (ion transport across interface) Zw, Warburg impedance (diffusion through aqueous phase) Ctt, adsorption capacitance (ion adsorption at membrane side of interface) Cwa, aqueous adsorption capacitance (ion adsorption at water side of interface). From ref. 109. 

Despite the known renal disposition and nephrotoxicity of vancomycin, elucidation of its renal transport pathway remains incompletely defined. Early research by Sokol in rabbits demonstrated that vancomycin enters the proximal tubule cell across the basal lateral membrane via the organic acid transport system [186]. In addition, Sokol showed that vancomycin remains sequestered and concentrates inside the PTC. Vancomycin was found to enter the tubular lumen only by the much slower transport pathway of simple diffusion a secretory pathway was not found. This data would suggest that this renal transport pathway is a potential mechanism of vancomycin nephrotoxicity. However, there is not yet data that associates the PTC sequestration of vancomycin with nephrotoxicity or... 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

Separations using LEMs can be effected in two different ways (5). The first of these (called TYPE I transport) is a simple diffusion process in which the solute partitions into the membrane phase from the exterior phase, diffuses across the membrane to the dispersed interior phase droplets, and partitions into the interior phase. A reaction takes place in the internal phase which converts the solute into a species which is incapable of partitioning back into the membrane phase. TYPE I transport is limited to uncharged solutes only, since only uncharged solutes will be able to favorably partition into the membrane phase. 

To ingest and excrete ions (such as Na+, K+, Ca +, and CE) from and to the surrounding environment, cells must pass these ions through a membrane. In addition, eukaryotic cells are compartmentalized by intracellular membranes that must also be traversed by these ions. There are two types of transport across the cell membranes mediated and unmediated. Unmediated transport is via simple diffusion, whereas mediated transport occurs through the action of specific carriers. Mediated transport can further... 

Abstract In previous chapters we have employed the formalism and techniques introduced in the book to study different biological systems by conceptualizing them as chemical reactions. In all cases this conceptualization was more or less evident. However, the formalism is more versatile as it can be applied to systems that apparently have nothing to do with chemical reactions. In the present chapter we tackle a few of those systems, all of which are related to diffusion. Not only we exemplify in this chapter how to employ the formalism here introduced to study systems with no obvious connection with chemical reactions, but we also derive some classical results regarding ion transport across membranes. As in previous chapters we start with a very simple example, and gradually make it more complex to end with a more realistic model. 

Under certain conditions, the transfer of various molecules across the membrane is relatively easy. The membrane must contain a suitable transport mediator , and the process is then termed facilitated membrane transport . Transport mediators permit the transported hydrophilic substance to overcome the hydrophobic regions in the membrane. For example, the transport of glucose into the red blood cells has an activation energy of only 16 kJ mol-1—close to simple diffusion. 

LBPs are likely to have conventional roles in the energy metabolism and transport of lipids in nematodes for membrane construction, etc. Many parasitic helminths have deficiencies in the synthesis of some lipids and so their lipid acquisition, transport and storage mechanisms clearly need to be specialized and therefore pertinent to the host-parasite relationship (Barrett, 1981). From a practical point of view, lipid transporter proteins may also be important in the delivery of anthelmintic drugs to their target most anthelmintics are hydrophobic and if they do not distribute to their site of action within the parasites by simple diffusion across and along membranes, then the parasite s own carrier proteins may be involved. 

Models of lipid bilayers have been employed widely to investigate diffusion properties across membranes through assisted and non-assisted mechanisms. Simple monovalent ions, e.g., Na+, K+, and Cl, have been shown to play a crucial role in intercellular communication. In order to enter the cell, the ion must preliminarily permeate the membrane that acts as an impervious wall towards the cytoplasm. Passive transport of Na+ and Cl ions across membranes has been investigated using a model lipid bilayer that undergoes severe deformations upon translocation of the ions across the aqueous interface [126]. This process is accompanied by thinning defects in the membrane and the formation of water fingers that ensure appropriate hydration of the ion as it permeates the hydrophobic environment. 

The cellular mechanism of action of hydrocortisone, a glucocorticoid, is also related to proteins but not by the enhancement of cAMP production. Hydrocortisone is transported by simple diffusion across the membrane of the cell into the cytoplasm and binds to a specific receptor The steroid-receptor complex is activated and enters the nucleus, where it regulates transcription of specific gene sequences into ribonucleic acid (RNA). Eventually, messenger RNA (mRNA) is translated to form specific proteins in the cytoplasm that are involved in the steroid-induced cellular response. 

Substances can be transported across epithelial membranes by simple passive diffusion, carrier-mediated diffusion, and active transport, in addition to other specialized mechanisms, including endocytosis. 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

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... 

Any volatile material, irrespective of its route of administration, has the potential for pulmonary excretion. Certainly, gases and other volatile substances that enter the body primarily through the respiratory tract can be expected to be excreted by this route. No specialized transport systems are involved in the loss of substances in expired air simple diffusion across cell membranes is predominant. The rate of loss of gases is not constant it depends on the rate of respiration and pulmonary blood flow. 

Passive transport requires no overt energy expenditure because the substances moving across the membrane are going from an area of higher concentration to one of lower concentration. The two types of passive transport are simple diffusion (osmosis) and facilitated diffusion. Molecules which may undergo simple diffusion can be found in Table 4. They are... 

In general, the specific constituents of milk are synthesized from small molecules absorbed from the blood. These precursors are absorbed across the basal membrane but very little is known about the mechanism by which they are transported across the membrane. Since the membrane is rich in lipids, and precursors are mostly polar with poor solubility in lipid, it is unlikely that the precursors enter the cell by simple diffusion. It is likely, in common with other tissues, that there are specialized carrier systems to transport small molecules across the membrane such carriers are probably proteins. 

This mechanism is important for compounds that lack sufficient lipid solubility to move rapidly across the membrane by simple diffusion. A membrane-associated protein is usually involved, specificity, competitive inhibition, and the saturation phenomenon and their kinetics are best described by Michaelis-Menton enzyme kinetic models. Membrane penetration by this mechanism is more rapid than simple diffusion and, in the case of active transport, may proceed beyond the point where concentrations are equal on both... 

Passive transport The passive transport of molecules across a membrane does not require an input of metabolic energy. The rate of transport (diffusion) is proportional to the concentration gradient of the molecule across the membrane. There are two types of passive transport simple diffusion and facilitated diffusion. 

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