Oxygen reduction and optimum production of ATP in photosynthesis

Abstract

THE accepted pathway of CO2 fixation in plant photosynthesis requires that the photosynthetic light reactions produce ATP and reduced pyridine nucleotide (NADPH) in the molar ratio 3:2 (ref. 1). Early studies of photosynthetic phosphorylation suggested that non-cyclic electron transport could produce only equimolar amounts of ATP and NADPH, and the source of the extra ATP was presumed to be cyclic electron flow2. The view that the non-cyclic system is by itself able to produce twice as much ATP as NADPH has been expressed3,4, and removes the need for the in vivo operation of a cyclic electron flow which can be demonstrated in vitro only in artificial conditions5. An inflexible ATP–NADPH ratio of 2:1 for the products of the light reactions would, however, result in a feedback inhibition of electron transport, with ADP concentration as the limiting factor. One way of achieving flexibility in the relative production of ATP and NADPH would be for a low absolute stoichiometry of non-cyclic photophosphorylation (corresponding perhaps to a 1 : 1 ratio) to be accompanied by an ancillary reaction producing ATP but not NADPH. Such an ancillary reaction would be especially important in situations where ATP might be required for phosphorylations additional to those of the reductive pentose phosphate pathway.