Permeation of redox species through a cell membrane of a single, living algal protoplast studied by microamperometry.

Permeation of several redox species through a cell membrane of a single algal protoplast (radius 100 microns) was investigated by amperometry with a Pt microdisk electrode (disk radius, 6.5 microns) located near the membrane. The redox current observed at the microelectrode decreased as the microelectrode approached the cell membrane since the membrane acted as a barrier for diffusion of redox species from bulk to the microelectrode. Permeability coefficient (Pm) of the protoplast membrane was determined by the quantitative analysis of the variation of the redox current with microelectrode-membrane distance using digital simulation. The Pm values for Fe(CN)6(4-), Fe(CN)6(3-), Co(phen)3(2+), ferrocenyl methanol(FMA) and p-hydroquinone(QH2) were < or = 1.0 x 10(-4), < or = 1.0 x 10(-4), 1.0 x 10(-3), 5.0 x 10(-3) and 2.0 x 10(-2) cm/s, respectively. Using these Pm values, the concentration changes inside a model cell and chloroplast were theoretically calculated.