Active transport of ions

Active Transport of Ions Using Synthetic Ionophores... 

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

Kostyuk, P. G. Electrical events during active transport of ions through biological membranes, in Topic in Bioelectrochemistry and Bioenergetics, Vol. 2, (ed.) Milazzo, G., New York, Wiley 1978... 

Okahara, M., and Nakatsuji, Y. Active Transport of Ions Using Synthetic Ionophores Derived from Cyclic and Noncyclic Polyoxyethylene Compounds. 128, 37-59 (1985). 

Active Transport of Ions Using Synthetic Ionophores Derived from Macrocyclic Polyethers and the Related Compounds... 

Active transport of ions using synthetic ionophores derived from macrocyclic polyethers and related compounds. M. Okahara and Y. Nakatsuji, Top. Curr. Chem., 1985,128,37 (77). 

The possibility of active transport of substances across membranes had first been pointed out in the middle of the nineteenth century by the physiologist Emil Heinrich du Bois-Reymond, a German of Swiss descent. The ability to accomplish active transport of ions and uncharged molecules in the direction of increasing electrochemical potentials is one of the most important features of cell membrane function. The law of independent ionic migration as a rule is violated in active transport. 

Constitutive activation of Gs-proteins by cholera toxin is the cause of the devastating effect of the cholera bacterimn. Vibrio cholerae, on the water content of the intestine. Due to the lack of deactivation of the Gs-protein, adenylyl cyclase next in the reaction sequence is constantly activated, so that the level of cAMP in the cells of the intestinal epithelium is greatly increased. This, in turn, leads to increased active transport of ions and an excessive efflux of water and Na takes place in the intestine. 

This simple experiment was important in that it clearly established the key notion that cellular extrusion of sodium ions by the sodium pump was coupled to metabolism. Because in this and subsequent experiments of the same sort the electrochemical gradient for sodium was known precisely, and since the fluxes of sodium (and later potassium) both into and out of the cell could be measured independently, this study also laid the groundwork for a theoretical definition of active transport, a theory worked out independently by Ussing in the flux ratio equation for transepithelial active transport of ions (see below). 

Praag, D., Farber, S.J., Minkin, E. and Naftali, P. (1987). Production of eicosanoids by killifish gills and opercular epithelia and their effect in active transport of ions. General and Comparative Endocrinology 67,50-57. 

The most dramatic illustration of a mass-specific illusion is the comparative heat dissipation of the human erythrocyte and platelet. In mammals, both of these cell types are anucleate and discoid in shape, but the longest dimension of the former is four times that of the latter. Yet heat production of a human erythrocyte was shown to be 10 fW, a sixth that of a human platelet (61 fW see Table 1). The relatively high metabolic activity of platelets is probably due to the need to maintain a considerable phosphagen (phosphocreatine) pool for actomyosin contraction at stimulation and clot retraction. Phosphocreatine is synthesized from creatine using ATP and acts as a demand on the ATP cycle to drive the coupled catabolic half-cycle. On the other hand, ATP requirements of the erythrocyte are relatively small, being mostly confined to active transport of ions at the plasma membrane. 

The simplest of these functions is that of a permeability barrier that limits free diffusion of solutes between the cytoplasm and external environment. Although such barriers are essential for cellular life to exist, there must also be a mechanism by which selective permeation allows specific solutes to cross the membrane. In contemporary cells, such processes are carried out by transmembrane proteins that act as channels and transporters. Examples include the proteins that facilitate the transport of glucose and amino acids into the cell, channels that allow potassium and sodium ions to permeate the membrane, and active transport of ions by enzymes that use ATP as an energy source. 

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