plastoquinone pool (PQ/PQH 2 ) and plastocyanin (PC), respectively. The proton gradient that is built up
during a light phase “drives” the formation of ATP via another membrane-spanning complex, the ATP
synthase. The ultimate electron donor of the redox chain is molecular water, whose oxidation takes place
at the luminal side of the thylakoid membrane. Upon illumination, the reaction supplies electrons, which
are fed into the photosynthetic electron transport chain; protons (which in some organisms are subse-
quently reduced again to give molecular hydrogen); and molecular oxygen. A detailed model of photo-
system II is shown in Figure 4. The photosynthetic electron transport through photosystem II can, in parts,
be effectively followed by absorption spectroscopy. By means of this technique the dependence of redox
reactions in the region of photosystem II on the temperature has been analyzed by Renger and his cowork-
ers by recording absorption changes at 830 nm (Figure 5). The results clearly showed that the direct light-
dependent electron flow from Yzto P680is virtually independent of temperature (at least in the range
between 0 and 33°C). Interestingly, however, the electron abstraction from the OEC up to the formation
of S 3 (see later) is not invariant or steady but showed a significant change of EAat a discrete temperature
(inset in Figure 5). From these and other experiments it could be concluded that the reaction coordinates
of the OEC remained essentially constant and unmodified during evolution (from cyanobacteria to higher
plants); this interpretation implies that the basic functions of photosynthesis were optimized in the early
stages after invention [9,10].
It can easily be imagined that the oxidation of molecular water requires more than one oxidation step
(absorption of one photon), but in the early phases of photosynthesis research illumination was always
done with continuous light lasting for at least seconds. Thus, single oxidation steps could not be followed.
As early as in 1955, technically remarkable experiments by Allen and Franck [25] showed that photo-
synthetic preparations lost their capacity to photoevolve molecular oxygen as the consequence of one
short light pulse if a sufficiently long dark adaptation period preceded the light phase. Following the im-
provement of highly sensitive electrode systems and the development of illumination regimes with short
light flashes triggered at 1 Hz or more by suitable pulse generators, the phenomenological studies by Jo-
liot and coworkers [26] became possible (Figure 6). The observations have been described and unequiv-
ocally explained by the so-called Kok model [9,10,27] (Figure 7). The model says that four photons have
to be absorbed and five (four tangible) redox states (S states Siwith i 0–4) have to be consecutively at-
tained before molecular oxygen is evolved from the dark reaction out of S 4 and the reaction center “falls”
back to the ground state S 0. The oscillation that can be observed in the course of a so-called oxygen evo-
304 BADER AND ABDEL-BASSETFigure 4 Detailed cartoon of the photosystem II complex integrated in the thylakoid membrane: P680, reac-
tion center; Pheo, pheophytin; PQ, plastoquinone; D1/D2, intrinsic membrane-spanning polypeptides; E1, E2,
E3, extrinsic polypeptides with molecular masses of 16, 23, and 33 kDa, respectively; YZ/YD, redox active ty-
rosines of polypeptides D1 and D2, respectively, with YZdirectly participating in the electron transport; CP,
core protein. (From Ref. 10.)