nism have been worked out and published. Figure 2 depicts a “heterologous” fusion model of ancestral forms
of anoxygenic types of both photosystems (PSI and PS II) in early photosynthetic organisms leading to the
evolution of the “modern” oxygen-evolving photosystem II. This model derives the core polypeptides D1 and
D2 from homodimeric forms similar to the L and M polypeptides in purple bacteria. Gene fragmentation from
common ancestors with subsequent duplication is thought to account for the development of the CP43 and
CP47 core polypeptides within photosystem II (see Ref. 8 and references therein).
The present idea about the arrangement of the involved electron transport complexes and redox com-
ponents within the thylakoid membrane of plants and other eukaryotes is summarized in Ref. 24 and Fig-
ure 3. The major complexes PSII(OEC), cytochrome b 6 /ƒ (Cyt b 6 /ƒ), and PSI are linked by the mobile
PHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 303Figure 2 A heterologous fusion model for the evolution of oxygenic photosynthesis based on phylogenetic
analysis. (Modified from Ref. 8.)
Figure 3 Schematic diagram of the electron transport system of oxygenic photosynthesis in eukaryotic or-
ganisms. (Courtesy of Donald R. Ort.)