Oxygen Dependency of in vivoPhotophosphorylation

THE mechanism by which ATP is formed when the intact green is illuminated is not yet fully understood. ATP formation in isolated chloroplasts has been shown to be dependent on illumination 1 and has been shown to proceed by either a cyclic or a non-cyclic process depending on the oxido-reduction catalysts used 2 . Chloroplasts are capable of photophosphorylation at low rates in the absence of catalysts. This endogenous activity has been demonstrated to be oxygen dependent 3,4 . Ion-exchange chromatographic studies of the ATP content of intact barley seedlings show that the steady-state level of ATP in light is several fold that in dark 5 . The level with illumination drops if the leaves are exposed to nitrogen or to carbon monoxide–oxygen mixtures. This was interpreted as evidence that formation of ATP in barley seedlings in light was an oxidative process, using molecular oxygen as a terminal electron acceptor.