UN NOUVEAU MODELE DES CENTRES PHOTOCHIMIQUES DU SYSTEME II*

Abstract— The amount of oxygen evolved by Chlorella cells or by isolated chloroplasts has been measured after illumination by short saturating flashes. In all conditions, the amount of oxygen evolved by one flash is proportional to the fraction of the photochemical centers susceptible to produce oxygen. If dark adapted algae or chloroplasts are illuminated by a sene of flashes, no oxygen is produced by the first flash. This phenomenon is related to the activation process. If y n is the amount of oxygen evolved by the n th flash of the sequence, it appears that the sene y n shows large oscillations with a period 4. These oscillations are completely damped after 4–6 periods and the amount of oxygen evolved by a flash reaches a stationary value. For any value of n . the quantities y n and y n+2 are linked by a recurrent relation which is the same for Chlorella cells and spinach chloroplasts. No relation can be found between the terms y n and y (n+1). The mathematical properties of the series y n can be understood if one admits that a two memory process is involved in the photochemical reaction. The results have been interpreted in terms of a new model of the System II photochemical centers. The main characteristics of this model are: (1) Each photochemical center includes two electron donors ( Z ) and one electron acceptor ( Q ). (2) The formation of one atom of oxygen requires a two quantum process corresponding to the transfer of two electrons from the same electron donor (first memory). (3) The photochemical center acts as a switch which connects alternately each donor to the acceptor (second memory). The switch process occurs after each photoact with an efficiency of about 85 per cent. Other arguments in favor of this model are obtained from studies of the rate of oxygen production at the onset of a weak illumination.