[Efficiency of photophosphorylation in chloroplasts with steady and pulsed illumination]

The quantum yield of noncyclic photophosphorylation in chloroplasts excited by a series of 8 mus flashes of the saturating intensity displays a two-fold decrease when the flash-frequency is reduced from about 1.1 to about 0.8 s-1, whereas further decrease of flash frequency does not affect the average ATP yield per flash. Under excitation by two-flashes series the ATP yield is also about half-maximal. These observations are inconsistent with the concept postulating accumulation of energy contributions from several parallel or consecutive one-electron transfers as a prerequisite for ATP formation. The two-state model of a thylakoid membrane and of a coupling site is put forward according to which only one of these states ensures ATP formation in response to one electron transfer through one coupling site, whereas the other state is nonphosphorylating.