It is clear that photosynthetic water oxidation also includes the liberation of protons. These pro-
tons, however, do not necessarily originate directly from the substrate water, at least not before a wa-
ter molecule is definitively oxidized as the consequence of a turn of the Kok cycle. In any case, the the-
oretical function of water as the exclusive source of protons would have been difficult to bring into line
with the isotope experiment illustrated in Figure 8. Rather, most of the protons are initially liberated as
a consequence of protolytic reactions of specific redox cofactors (Mn ligands) or via the deprotonation
306 BADER AND ABDEL-BASSETFigure 7 Functional scheme of the Kok model of photosynthesis. By successive absorption of light quanta,
the reaction centers involved cycle through about five redox states (S 0 –S 4 ) before molecular oxygen is liber-
ated. S 1 is (together with S 0 ) stable in the dark, and this explains why the maximum amplitude is observed with
the third flash of a sequence. Under specific conditions and depending on the organism investigated, “overre-
duced” states S 1 /S 2 /S 3 have to be inserted before the “ground state” S 0. (Modified from G Renger, sub-
mitted, 2000.)
Figure 8 Photosynthetic water oxidation from H 218 O in the blue-green alga Oscillatoria chalybea. This
mass spectrometric experiment showed that flash-induced water oxidation can be observed as a consequence
of a single analyzing flash to to a prefabricated S 3 redox state (two preflashes). Oxygenic photosynthesis does
not necessarily require a water/oxygen intermediate or partially oxidized water molecules. (From Ref. 30.)