¹ From the Biophysics Department, University of Edinburgh, Scotland

The distribution and rates of exchange of the ions sodium, potassium, and chloride in single internodal cells of the ecorticate characean, Nitellopsis obtusa, have been studied.

In tracer experiments three kinetic compartments were found, the outermost "free space" of the cell, a compartment we have called "protoplasmic non-free space", and the cell sap.

The concentrations in the vacuole were 54 mM Na⁺, 113 mM K⁺, and 206 mM Cl^-. The steady state fluxes across the vacuolar membrane were 0.4 pmole Na⁺/cm.² sec., 0.25 pmole K⁺/cm.² sec., and 0.5 pmole Cl^-/cm.² sec.

The protoplasmic Na/K ratio is equal to that in the vacuole but protoplasmic chloride is relatively much lower. Osmotic considerations suggest a layer 4 to 6 µ thick with sodium and potassium concentrations close to those in the vacuole. The fluxes between protoplasm and external solution were of the order of 8 pmoles Na⁺/cm.² sec. and 4 pmoles K⁺/cm.² sec.

We suggest that the protoplasm is separated from the cell wall by an outer protoplasmic membrane at which an outward sodium transport maintains the high K/Na ratio of the cell interior, and from the vacuole by the tonoplast at which an inward chloride transport maintains the high vacuolar chloride. The tonoplast appears to be the site of the principal diffusion resistance of the cell, but the outer protoplasmic membrane probably of the main part of the potential.

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