Ion Permeability of the Plasmalemma of the Plant Cell

Abstract

IT has recently been suggested by several workers¹ that the plasmalemma of the plant cell does not constitute a barrier to the diffusion of ions, and that there is a Donnan distribution of ions between the cytoplasm of the plant cell and the extracellular medium. As a corollary, it has been suggested^1c that the electric resistance and capacity of plant tissue are to be attributed wholly to the properties of the tonoplast and bulk cytoplasm. However, it has been suggested that recent measurements of resistance and capacity on Nitella give the properties of the plasmalemma²; and a.c. measurements on Chlorella³, which has no significant vacuole, appear to require a similar explanation. Results which I have obtained with cells of Nitella indicate that the plasmalemma of the Nitella cell acts as a barrier to the inward diffusion of calcium ions, and is responsible for the d.c. resistance of the cell bathed in a solution of potassium, sodium or calcium chloride. Thus if the cytoplasm contributes to the apparent free space for ionic diffusion, it cannot be in the simple way proposed for higher plant tissue¹. Although one cannot argue from single cells of an alga to pieces of angiosperm tissue, it may be noted that there is evidence from Nitella that labelled ions do cross the plasmalemma much more rapidly than the tonoplast⁴; this situation is comparable with that in angiosperm tissue^1c.