A re-evaluation of the ATP :NADPH budget during C3 photosynthesis: a contribution from nitrate assimilation and its associated respiratory activity?

Abstract

The purpose of this review in reanalysing the ATP :reluctant balance in illuminated leaf cells is to stress that photosynthesis in vivo does not involve CO2 fixation alone, but embraces other processes, chief among which is N assimilation. Prior to the demonstration of CO2 fixation and photophosphorylation by isolated chloroplasts, the mitochondria were thought likely to provide all the ATP required for CO2 fixation (discussed in Arnon et al., 1954). During the 1960s, the development of techniques for the isolation of chloroplasts able to fix CO2 at rates approaching those of the parent tissue induced a paradigm shift, leading to the establishment of a dominant (if not unanimous) view that chloroplasts in vivo must by themselves meet all their ATP requirements. More recent studies, however, indicate that the reality lies somewhere between these two extremes. The present work places emphasis on the integrated nature of photosynthesis and proposes that much of the respiratory ATP necessary for whole cell photosynthesis may be generated during the production of C skeletons for N assimilation. Rather than considering dissipative electron transport pathways as necessary to uncouple respiratory precursor synthesis from ATP production, the present analysis emphasizes the metabolic value of ATP produced during N-linked respiration, with cellular ATP supply being tailored to ATP demand.