Membrane, transport processes

From a thermodynamic and kinetic perspective, there are only three types of membrane transport processes passive diffusion, faeilitated diffusion, and active transport. To be thoroughly appreciated, membrane transport phenomena must be considered in terms of thermodynamics. Some of the important kinetic considerations also will be discussed. 

The discrepancy arises because ATP is used to drive processes which are not directly related to growth, eg membrane transport processes, protein turnover. These are called the maintenance and dissipation demands for ATP. 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

Active transport. The definition of active transport has been a subject of discussion for a number of years. Here, active transport is defined as a membrane transport process with a source of energy other than the electrochemical potential gradient of the transported substance. This source of energy can be either a metabolic reaction (primary active transport) or an electrochemical potential gradient of a substance different from that which is actively transported (secondary active transport). 

Cereijido M, CA Robbins, DD Sabatini. (1987). Polarized epithelial membranes produced in vitro. In JF Hoffman, ed. Membrane Transport Processes. New York Raven Press, pp 443-456. 

JR Williamson, SK Joseph, KE Coll, AP Thomas, A Verhoeven, M Prentki. (1986). Hormone-induced inositol lipid breakdown and calcium-mediated cellular responses in liver. In G Poste, ST Crooke, eds. New Insights into Cell and Membrane Transport Processes. New York Plenum Press, pp 217-247. 

Although there are a number of mechanisms in cells for buffering or sequestering Ca2+ to prevent untoward or inappropriate rises in [Ca2+]i( in the long term, it is the activity of plasma membrane transport processes that determines the steady-state [Ca2+]j. This is because the plasma membrane acts as a Ca2+ buffer of essentially infinite capacity. This results from in vivo clamping of the extracellular concentration of Ca2+ by dietary and endocrine mechanisms. In in vitro experiments, this results from incubation volumes very much larger than the cell volume. 

Many drugs have been recognized to cross the intestinal epithelial cells via passive diffusion, thus their lipophilicity has been considered important. However, as described above, recent studies have demonstrated that a number of drug transporters including uptake and efflux systems determine the membrane transport process. In this chapter, we provide an overview of the basic characteristics of major drug transporters responsible not only for absorption but also for disposition and excretion in order to delineate the impact of drug transport proteins on pharmacokinetics. 

This process is commonly referred to as assimilatory nitrogen (nitrate or nitrite) reduction. The electrons for these reductions are supplied by half-cell oxidations involving NADPH/NADP" and NADH/NAD" (Table 7.11). All of these reactions and membrane transport processes are mediated by enzymes that are specific to the DIN species. Considerable variation exists among the phytoplankton species in their ability to produce the necessary enzymes. Since marine phytoplankton are often nitrogen limited, the quantity and type of DIN available in the water column can greatly influence overall phytoplankton abundance and species diversity. 

At the cellular level, plant secondary metabolites have five major effects on herbivores (a) alteration of DNA replication, RNA transcription, and protein synthesis (b) alteration of membrane transport processes (c) enzyme inhibition and activation (d) blocking of receptor sites for endogenous chemical transmitters and (e) affecting the conformation of other macromolecules (Robinson, 1979). 

The expected incorporation of mi/o-inositol into phospholipids has been observed in two31 126 and in kidney preparations in vitro,138 and some progress has been made toward understanding the enzymic mechanism involved.101 1 138,139 It appears that the inositol is neither phosphorylated nor converted to a coenzyme prior to incorporation. Hokin and Hokin140 have made the interesting observation that the turnover of myo-inositol in the monophosphoinositide of brain and pancreas slices is stimulated by acetylcholine. The effects of various drugs on this turnover have been studied by other workers. It has been postulated that the decomposition and resynthesis of inositol phosphatide is a part of the membrane-transport process.140... 

Drug-Drug Interactions Involving the Membrane Transport Process... 

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