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Journal of Experimental Biology, Vol 196, Issue 1 439-442, Copyright © 1994 by Company of Biologists

JOURNAL ARTICLES

BACTERIAL AND PLANT ANTIPORTERS

P. C. Maloney

Articles in this chapter emphasize the phenomenon of ion exchange across biomembranes. This topic has been of interest at least since 1947, when Hans Ussing articulated his model of 'exchange diffusion' to account for an unexpected elevation in the rate of 22Na+ efflux from skeletal muscle (Ussing, 1947). Ussing had tried to measure net Na+ movement by following tracer flux, but found that this approach greatly overestimated the true value. To explain this overestimate, Ussing postulated that isotopic flux reflected only isotopic exchange rather than net movement of mass, and he proposed the following model. Perhaps a negatively charged carboxyl group on some membrane element accepts the positively charged 22Na+ so that the neutral complex can move through the membrane (mechanism unspecified) without interference from the electric field? If so, at the opposite surface, tracer could be discharged, replaced by (non-radioactive) 23Na+, and the neutral complex would return to the original membrane surface, and at that point 23Na+ would be released to let the cycle begin again. Clearly, by following the movement of 22Na+, one might considerably overestimate the true net movement of Na+ itself.
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© The Company of Biologists Ltd 1994