Energy sources for photosynthetic carbon dioxide fixation

Abstract

The high-energy intermediates, or high-energy states, involved in photophosphorylation (PSP), may be used directly to drive CO2 fixation, without the intercession of ATP. This paper reports an investigation on the amounts of CO2 fixed in isolated spinach chloroplasts, when ATP synthesis was inhibited, but when electron flow and high-energy intermediate synthesis was unaffected. As a corollary to this work, the role of pyrophosphate (PPi) as a source of energy was studied. Izawa, Winget and Good (1966) reported that phloridzin inhibited ATP formation in PSP, in a manner similar to oligomycin inhibition of mitochondrial ATP synthesis. Nobel (1967) noted that phloridzin, while strongly inhibiting PSP had only a marginal effect on the uptake of Ca++ by chloroplasts. It was hoped that the use of this inhibitor would point to a role for high energy states other than for ATP formation. The second inhibitor used in this investigation was quinacrine. This compound was shown by Izawa (1965) and Dilley and Vernon (1966) to inhibit the synthesis of ATP, while allowing high energy intermediates to perform mechanical work in the chloroplasts.