Cytosolic ion exchange dynamics: Insights into the mechanisms of component ion fluxes and their measurement

The quantification of cellular pool sizes of ions is essential for the understanding of the energetics of metabolic and membrane transport processes. No less important is the quantification of ion fluxes into, out of, and within cells. Of the variety of analytical methods available, only one, compartmental analysis by tracer efflux ( CATE), can be used to simultaneously determine subcellular ion pool sizes and resolve ion fluxes. Thus, this methodology can be used to provide steady-state isotherms for major flux processes not amenable to direct measurement, such as effluxes or xylem fluxes, and to develop hypotheses about mechanisms underlying them. The exchange half-time for an ion in a cellular compartment emerges as a key CATE parameter that relates pool sizes with fluxes, and is a term that can be used to estimate errors in a wide range of findings in plant ion relations, and verify their plausibility. Case studies involving the flux and compartmentation of Ca2+, K+, and inorganic N are presented.