Glucose membrane transport

To demonstrate the ability of this AFM-tip-integrated biosensor to study glucose membrane transport, a porous... 

In addition to secretory cells, many non-secretory cells are capable of regulating exocytotic fusion of transport vesicles that are derived from endosomal precursors. For instance, vesicles enriched in plasma membrane transport proteins are incorporated in a regulated manner in order to alter metabolite fluxes. Examples include the glucose transporter GLUT-4 in muscle and fat tissues, a key element in the control of... 

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. 

Acyl coenzyme As are introduced into mitochondria following coenzyme A esterification in the cytoplasm. Mitochondrial entry depends upon a double membrane transport involving carnitine acyltransferases II and I. Excess acetyl CoA is used for KB synthesis. KBs are transported in the blood and ultimately metabolized via the Krebs cycle. KBs are necessary to provide energy to the brain during fasting, a true alternative substrate to glucose. 

Widdas s quantitative model of the simple carrier was able to explain a number of earlier observations and to make predictions about what would be observed in more complex experiments on membrane transport. Thus it was a highly productive scientific insight. One of the earlier, apparently anomalous, results that the theory explained was the dramatic fall of membrane permeability found for solutes which were rapidly transported as solute concentration was increased. For example, in the human red blood cell, Wilbrandt and colleagues had previously measured a permeability constant for glucose which was 1000 times higher in dilute solutions of glucose than it was in a concentrated solution. This phenomenon, subsequently called saturation kinetics, is formally equivalent to the fall, as substrate concentration increases, in the proportion of substrate converted to product by a limited amount of an enzyme. 

Widdas, W.F. (1988). Old and new concepts of the membrane transport for glucose in cells. Biochim. Biophys. Acta 947,385-404. 

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. 

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. 

The analysis demonstrates that glucose uptake is very dynamic and complex. In the simple context, glucose is transported over the cell membrane, phosphorylated, and then oxidized, converted to lactate or fat, or stored as glycogen as shown in Fig. 6.6a. But these processes take place at very different rates in the different organs and they depend strongly on the metabolic state of the organism. The real challenge to the... 

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