Primary Photochemical and Photophysical Processes in Photosynthesis.

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Abstract
The paper summarizes observations and measurements relevant to our present concepts of the primary photophysical process in photosynthesis: the submicroscopic structure of chloroplasts; the existence and density of chlorophyll monolayers; their non-crystalline nature; the existence of "photosynthetic units" as areas over which the migration of excitons (excited electronic states), or separate migration of negative electrons and positive "holes" can extend. The tentative conclusion is drawn that the most important type of migration is that of excitons (which can be described as simultaneous transfer of electrons and holes); the separation of charges may occur however when the migrating exciton meets an electron (or hole) "trap" ("reaction center"), after which the remaining hole (or electron) may continue its migration. The possibility of a chlorophyll molecule participating in the primary photochemical process in the "reaction center" is discussed then on the basis of experiments on reversible photoxidation and photoreduction of chlorophyll in vitro, and of the "difference spectra" of algae (difference between the absorption spectra of algal cells in darkness and in bright light). It is suggested that 1 chlorophyll molecule in 200-300 can be chemically changed (perhaps, reduced) as consequence of its participation in the primary photochemical process in the photostationary state when photosynthesis approaches saturation.