Slow Passive Diffusion of Orthophosphate between Intact Isolated Chloroplasts and Suspending Medium

Abstract

Isolated spinach [Spinacia oleracea] chloroplasts purified by isopycnic centrifugation in density gradients of Percoll were highly intact, were devoid of extrachloroplastic contaminations and retained a high rate of CO2-dependent O2 evolution. When suspended in a medium which avoided rupture of the envelope, intact purified chloroplasts progressively lost their PO4 content by passive diffusion. This led to a slow decrease in the uptake of labeled 3-phosphoglyceric acid or Pi and in the rate of CO2-dependent O2 evolution by isolated chloroplasts. Under these conditions, there was a good correlation between the rate of CO2-dependent O2 evolution and the concentration of Pi in the stroma space. Addition of Pi to the suspending medium at a final concentration of 10 mM, which counterpoised the slow efflux of Pi from the chloroplasts, slowed considerably the decrease in the rate of CO2-dependent O2 evolution.