Biological solar energy conversion: Approaches to overcome yield, stability and product limitations

The chloroplast aging and rate limitations in the photosynthetic electron transport system were studied. It was confirmed that the photosystem II acceptor, phenylenediamine, accepts electrons from two sites in the electron transport chain, one of which is uncoupler insensitive. These two sites have approximately equal rate constants and contribute about equally when photochemistry operates with maximum rates. Therefore, it was concluded that when uncoupler is present, the primary rate limitation precedes the uncoupler-insensitive transport to phenylenediamine. Fluorescence measurements have confirmed the results showing that chloroplast storage at high pH inhibits only the oxygen evolution system and not the functioning of the trap.