Capture frequency of excitations and energy transfer between photosynthetic units in the photosystem II

The connections which exist, in the photosynthetic apparatus, between the spatial arrangement of chlorophyll and the movement of excitation energy are discussed.The capture frequency of excitations in the photosystem II is analysed. At the microscopic level of a photosynthetic unit two stages are studied: the propagation of the excitation to the reaction centre and the photochemical utilization of the excitation by the centre. It is shown that the transport process is not a limiting one. It implies that the capture frequency depends on the reaction centre state. Thus it is possible to distinguish eight states for a photosynthetic unit of system II.At the macroscopic level of a set of units, the analysis of the fluorescence yield-fluctuations shows that these units are not isolated. It also indicates that the fluorescence emitted by the photosynthetic apparatus originates almost entirely from the system II, and that the reaction centres are traps for excitations whatever their states.