Photosynthetic water oxidation to yield the oxygen of the atmosphere is of paramount biological and also technical relevance, in the light of decreasing fossil fuel reserves. The splitting of water into hydrogen (on carriers) and oxygen takes place in a multimeric protein called Photosystem II (PSII). The rigorous understanding of nature´s solution for this thermodynamically and mechanistically highly demanding reaction is one approach towards the construction of an artificial hydrogen technology under exploitation of almost unlimited energy sources, sunlight and an ubiquitous substrate, water. This thesis aims at two aspects: (i) Electron and proton transferring amino acids and (ii) so far undetected chemical intermediates between water and O2(i) D1-Glu189 has been claimed to be involved (a) in the proton conducting network around the Mn4Ca-cluster and (b) as a direct ligand to Mn. We exchanged the negative Glu against the positive Arg or Lys or the neutral Glu without any effect on the relaxation times (ns-ms) of the various electron transfers in PSII. Our data exclude these postulated roles of D1-Glu189 and qualify a recently published structural model.(ii) Dioxygen is produced in what seems to be a single reaction step, although it involves the transfer of four electrons from bound water to the fourfold oxidised catalytic centre. No chemical intermediate (e.g. peroxide) has been detected by high resolving optical and magnetical spectroscopy. To overcome the detection problem of short lived intermediates we pushed the process backward by elevated oxygen pressure and found the first evidence for such an intermediate. The astonishing half suppression of oxygen evolution at only 2.3 bar O2 emphasised the small driving force of this important reaction. PSII operates at the energetic limits; this is why the atmospheric oxygen level cannot be pushed much above the present level.