This book will bring you closer towards understanding how photosynthetic oxygen evolution works in high plants and micro algae, or cyanobacteria. To the time when this work was done, information on photosynthesis was largely based on experimental results obtained from studies on high plants, mainly spinach. Yet no crystal structure of photosystem 2 was available from high plants. In 2001, Dr. Zouni and co-workers from TU-Berlin had reported a crystal structure of photosystem 2 obtained at resolution 3.8 Å from a thermophilic cyanobacteria Thermosynechococcus elongatus. Thus an important question to answer was: how similar photosynthetic water oxidation mechanisms are in high plants and cyanobacteria? The author used Joliot-type electrode to produce and afterward to compare flash induced oxygen evolution patterns from cyanobacteria and high plants to understand functional similarities and differences in oxygen evolving complex (OEC) of photosystem 2. The author also analyzed effects of H/D isotope exchange on reactions of OEC under different pL and temperature. The author also went beyond the classic Kok model to characterize “super” -reduced states of OEC.