Abstract
A hypothetical model for the structure and function of photosystem II is proposed that attempts to incorporate different phenomena related to the variable chlorophyll fluorescence inherent in this photosystem. The involvement of pheophytin redox chemistry on both the acceptor and donor side of photosystem II is postulated to achieve redox potentials high enough to oxidize water. The presence of this symmetry would be the cause of inefficient photochemistry in photosystem II when, under unbalanced carbon metabolism, a surplus charge remains on the reaction centre. In addition, such a scheme would enable an efficient dissipation of surplus energy in the reaction centre itself, and would be the origin ofthe ‘energy-dependent’ quenching of chlorophyll fluorescence,
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