Transmembrane transport carrier mediated

Several other conditions can provoke this reverse pump type of release. One is when the transmembrane ionic gradient is reversed. Experimentally this is achieved by reducing extracellular Na+. Because the neuronal uptake of monoamines from the synapse by the transporter requires co-transport of Na+ and Cl , reversing the ionic gradient (so that the Na+ concentration is lower outside, than inside, the terminals) will drive the transporter in the wrong direction. Such carrier-mediated release could explain the massive Ca +-independent release of noradrenaline during ischaemia which increases intracellular Na+ concentration and reduces intracellular K+. 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

Figure 3. Schematic diagram of an apparatus for measuring transmembrane oxidation-reduction in a planar bilayer membrane. The mechanism described is simple carrier-mediated electron transport. D = aqueous electron donor A = aqueous electron acceptor ...

Carrier-Mediated Intestinal Transport. Various carrier mediated systems (transporters) are present at the intestinal brush border and basolateral membrane for theabsorption of specific ions and nutrients essential for the body. Many drugs are absorbed by these carriers because of the structural similarity to natural substrates. An intestinal transmembrane protein, P-Glycoprotein (F-Gp) appears to reduce apparent intestinal epithelial cell permeability from lumen to... 

This study shows that K. pneumoniae when growing under aerobic or semiaerobic conditions, transports NO3" into the cells. Therefore assimilatory or aerobic NOa" reduction probably occurs inside the cells. The observation that NO2" is transiently excreted into the mediiun from which it then disappears suggests the presence of both a NO2" export system (possibly a consequence of NO3" reduction by a membrane-associated enzyme) and a NO2" uptake system. The presence of a NO2" uptake capability may simply reflect the permeability of membranes to weak acids in the presence of a transmembrane pH gradient (19-23) or may indicate the presence of carrier-mediated transport systems. If the N02 uptake is carrier-mediated, the transient nature of the accumulation of N02 outside the cells suggests that uptake and/or reduction may be activated shortly after the cells are exposed to N03 . It may also be possible that simultaneous transport of nitrite across the cell membrane and reduction are mediated by one enzyme system. While we have not attempted to demonstrate a membrane-associated nitrite reductase, Jones and Garland (3) and Coleman et al. (22) have found such activities in other enteric bacteria. 

FIGURE 19.4 Types of ion transport across a biomembrane. (a) Diffusion without transport mediator carrier-mediated transport using (b) the traveling or (c) the hopping mode (d) transport through a transmembrane channel. 

Although there is a natural tendency toward equilibrium of the solute concentration on both sides of the membrane, such an equilibrium is rare in a living system, and selective permeability of the plasma membrane therefore assures the required distribution of metabolically important material inside and outside the cell. Kinetic studies of solute transport often permit characterization of the type of transmembrane movement involved (Neame and Richards, 1972). As outlined by Csaky (1965), a given substance can cross the cell membrane in several different ways free diffusion, diffusion through pores, pinocytosis, and carrier-mediated transport. 

Figure 1. Schematic representations of significant biological functions displayed by host-guest complexation in homogeneous solutions or at membrane surfaces, (a) Separation (e.g., antibody-antigen complex formation), (b) Transformation (e.g., enzymatic reaction), (c) Translocation (e.g., carrier- or channel-mediated transport), (d) Transduction (e.g., receptor-mediated transmembrane signaling).

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