Bicarbonate effects on the Hill reaction and photophosphorylation.

Abstract

Stimulation of the Hill reaction and of photophosphorylation of isolated spinach chloroplasts by added bicarbonate was compared directly with the carbon dioxide requirement for the Hill reaction. It is concluded that these represent 2 separate effects. The carbon dioxide requirement for the Hill reaction as discovered by Warburg only appeared after a CO2 depletion period, and its locus was the oxygen evolution site. The stimulating effect of added bicarbonate as discovered by Punnett worked quite well with fresh chloroplasts, required a higher level of bicarbonate, and seemed to relate to a region of coupled electron transport which was inhibited by use of a pH below the optimum of 8.0-8.3. The effect of added bicarbonate was abolished by a sufficient level of added uncouplers, whereas the CO2 requirement showed up best during uncoupled electron transport. CO2 depletion did not inhibit phosphorylation supported by cyclic electron flow. Added bicarbonate which stimulated phosphorylation, inhibited formation of a non-phosphorylated intermediate, "XE".