Flux across liposome membranes

Proton flux across lipid bilayers can be measured by a variety of techniques. For example, a buffered pH gradient can be established across liposome membranes, and the rate of decay of the gradient can be monitored by any of several methods. Early measurements were carried out by monitoring pH shifts in the external medium with a glass electrode (1) later measurements used pH-sensitive dyes such as pyranine, carboxyfluorescein, and 9-aminoacridine (4-6). Cafiso and Hubbell (7) used spin labels very effectively, and Perkins and Cafiso (8) conducted an extensive series of measurements with this system. 

The transport of molecules across biological cell membranes and biomimetic membranes, including planar bilayer lipid membranes (BLMs) and giant liposomes, has been studied by SECM. The approaches used in those studies are conceptually similar to generation-collection and feedback SECM experiments. In the former mode, an amperometric tip is used to measure concentration profiles and monitor fluxes of molecules crossing the membrane. In a feedback-type experiment, the tip process depletes the concentration of the transferred species on one side of the membrane and in this way induces its transfer across the membrane. 

In practice, it is not feasible to test the derived equations experimentally by varying all the forces and fluxes independently. Usually some simplification is gained by allowing the system to develop to a specific steady state. A useful steady state for illuminated bacteriorhodopsin liposomes is that of electroneutral total flow, i.e., the condition in which the net movement across the membrane of all chemical species adds up to no charge movement. It can be derived and shown that this condition is attained within seconds, considering the membrane resistance and electrical capacity in the usual salt media [28], Electroneutral total flow is mathematically expressed as ... 

From a biophysical point of view, the use of whole mitochondrion (or chloroplast) to test the "molecular" theory of Mitchell does not seem satisfying. The translocation of ions, such as protons, across a complex system, such as a cristae, is in itself ill-defined. The interactions between various fluxes (ion and water movements, electron and hole transport, etc.) are far too complex to be amenable to a simple analysis. At the present time, direct tests with the mitochondrial membranes are difficult. Experimental testing of the chemiosmotic hypothesis, using simpler model systems such as planar BLM and spherical liposomes are, therefore, in order. 

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