Transport in biological cells

Example 15.15 Transport in biological cells osmotic and pressure driven mass transport across a biological cell membrane... 

It was tempting to base the study of membrane transport in eukaryotic cells on similar simple principles. For this purpose, as well as for molecular biology as a whole, the yeast Saccharomyces cerevisiae appeared to be the best suited organism. From early times on, this yeast has occupied a privileged place for mankind. Due to... 

SECM is a powerful tool for studying structures and heterogeneous processes on the micrometer and nanometer scale [8], It can probe electron, ion, and molecule transfers, and other reactions at solid-liquid, liquid-liquid, and liquid-air interfaces [9]. This versatility allows for the investigation of a wide variety of processes, from metal corrosion to adsorption to membrane transport, as discussed below. Other physicochemical applications of this method include measurements of fast homogeneous kinetics in solution and electrocatalytic processes, and characterization of redox processes in biological cells. 

Prognosis of a compounds permeability should be made stressing limitations of the model. There is no bioavailability prognosis from in vitro data - a cellular assay can provide only permeability potential through a biological membrane. The membrane, in most cases CACO-2 cells, is very similar to what we observe in vivo in the small intestine and resembles many characteristics to in vivo enterocytes. CACO-2 cells can be used for prediction of different pathways across intestinal cells. Best correlation occurs for passive transcellular route of diffusion. Passive paracellular pathway is less permeable in CACO-2 and correlations are rather qualitative than quantitative for that pathway. CACO-2 cells are an accepted model for identification of compounds with permeability problems, for ranking of compounds and selection of best compounds within a series. Carrier-mediated transport can be studied as well using careful characterization of transporters in the cell batch or clone as a prerequisite for transporter studies. 

This representative example illustrates transport processes in biological cells using a highly simplified analysis. Biological cells also operate hydrogen and calcium pumps. Some of the concentrations also represent only approximate values. 

Outliers in these models, as well in biological assays, are due to several reasons which encompass issues related to transporters in the cells which could be efflux or uptake transporters. We have tried to address the problem using MI Fs to understand P-glycoprotein (PGP) efflux, especially as it pertains to substrates. 

Baker, S.A., Pazour, G.J., Witman, G.B. and Besharse, J.C. (2004) Photoreceptor and intraflagellar transport. In Photoreceptor Cell Biology and Inherited Retinal Degenerations. D.S. Williams ed. (Singapore World Scientific Publishing Co. Pte. Ltd), pp. 109-132. 

It is precisely because of the absence of a long half-lived radioisotope of Mg that there have been few studies of Mg transport in biological systems. Mg has a 21.3-h half-life and emits a 0.45-meV p particle as well as y radiation (33). It is not available from commercial sources, but it has been made on a fixed schedule of every two or three weeks from the Hot Laboratory Division, Brookhaven National Laboratory in the United States and was available on request from the Radiochemical Centre, Amersham, Great Britain. Jasper and Silver (4) summarized all known single-cell and subcellular studies on and biological mate-... 

Mechanisms and Metal Involvement in Neurodegenerative Diseases 2 Binding, Transport and Storage of Metal Ions in Biological Cells 3 2-Oxoglutarate-Dependent Oxygenases 4 Heme Peroxidases... 

Bettendorlf, L., and Wins, P., 1994. Mechanism of thiamine transport in neuroblastoma cells. Inhibition of a high affinity carrier by sodium channel activators and dependence of thiamine uptake on membrane potential and intracellular ATP. Journal of Biological Chemistry. 269 14379-14385. 

The transport of K+ ions through cell membranes by antibiotics (valinomycin) has been a very important example. Addition of K+ ions to the subphase of a valinomycin monolayer showed that the surface potential became positive. This clearly indicated the ion-specific binding of K+ to valinomycin. In biological cells the concentrations of different ions are sometimes 20 times different than in the solution outside the cell. The membrane peptides, such as valinomycin, create... 

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