gin of the evolved oxygen in the case of oxygenic photosynthesis. In the early phases of photosynthesis
research, it was not obvious that the light-evolved oxygen originated from water and not from carbon
dioxide. By means of mass spectrometry and the application of stable oxygen isotopes containing water
(H 218 O), it became clear that the transformation of carbon dioxide to carbohydrates did not entail any lib-
eration of molecular oxygen because the isotopic oxygen showed up exclusively in the gas phase so that
apparently the water from the aqueous part of the reaction assays had been oxidized. (Consequently, in
the case of C^18 O 2 no liberation of isotopic oxygen was detectable.) Again surprisingly, the simplest car-
bohydrate, formaldehyde—the molecule that is structurally identical to the chemical formula known from
all photosynthetic schemes, CH 2 O—was never observed in the course of CO 2 assimilation. Instead, the
rather complex molecule ribulose-1,5-bisphosphate was found to serve as carbon dioxide acceptor cat-
alyzed by the enzyme known as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This en-
zyme is relatively unique with respect to its bifunctionality: here it catalyzes the assimilation of carbon
dioxide (carboxylase function), but it can, depending on the reaction conditions, also react with molecu-
lar oxygen (oxygenase function)—at first glance a useless and even lavish and wasteful reaction.
Relevant details of this process and the significance of the phenomenon, e.g., for crop yield, are dis-
cussed in following sections of this chapter. The photosynthetic electron transport includes specific car-
riers (redox components) that operate sequentially via one or two photosystems. These components have
been relatively well investigated by now and are described in textbooks and reviews. Therefore, we con-
centrate in this chapter on relevant mechanistic details and aspects of photosynthetic water cleavage as an
extraordinary (without disregarding others) evolutionary achievement in plant physiology.
A. Mechanism of Water Oxidation
In the introduction we listed quite a few bioenergetic estimates to describe the importance of photosyn-
thesis for life in general. Figure 1 illustrates the significance of oxygenic photosynthesis for the evolution
of higher life forms. The picture is based on the classical experiment by Joseph Priestley, who designed
this setup in 1780 to demonstrate that the mouse survived better in a closed system under an artificial gas
atmosphere when a green plant was added to the system provided that the plant was illuminated. The
plant, in this case mint, had “restored” the air (as cited in Ref. 23). Under this condition, water was oxi-
dized, oxygen was evolved, and this oxygen served for the respiration of the mouse. (The plant also prof-
its from this system as the mouse breathes out carbon dioxide, which increases the amount of substrate
CO 2 for the plant.)
In recent years, evolutionary aspects of photosynthesis and many details of the water oxidation mecha-
302 BADER AND ABDEL-BASSETFigure 1 Cartoon based on the experiments by Joseph Priestley demonstrating the mutual interrelationship
between an animal and an illuminated plant with respect to the oxygen and carbon dioxide gas exchange.