Movement of Ions and Electrogenesis in Higher Plant Cells

Abstract

During the past 10 years considerable information has accumulated on the electrochemical relationships of higher plant cells during transport of mineral ions. Using the Nernst equation as a criterion, none of eight ions (K+, Na+, Ca++, Mg++, NO3−, Cl−, H2PO4−, and SO4−) is in a passive equilibrium. Na+, Ca++, and Mg++ are subject to an exclusion mechanism, and all of the anions appear to be pumped inwardly. K+ apparently approaches an electrochemical balance under certain conditions but probably is actively accumulated. Compartmental analyses giving estimates of amounts in the cytoplasm and vacuole and of unidirectional fluxes permit application of the Ussing flux-ratio equation. The criterion in oat coleoptile cells suggests that at the plasmalemma Na+ is pumped out while K+ and Cl− are pumped in. K+ and Cl− appear to be coupled in active transport across the tonoplast into the vacuole. Good evidence has been found that the cell's electropotential arises from an electrogenic pump: CN− (cyanide) and DNP (dinitrophenol) reversibly block the potential and ionic transport; cell potentials are higher than can be accounted for by diffusion; the responses of respiration and potential to the concentration of CN− are nearly parallel; and CN− inhibited tissue approaches a fit to the Goldman constant field equation. Future objectives should be identification of the ion, or ions, subject to the electrogenic pump and discovery of the immediate energy source.