Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 53

A final experiment that pointed the way to the existence of two distinct photochemical systems working in series in photosynthetic organisms was carried out by Louis Duysens and coworkers from the Netherlands (Duysens et al., 1961). Duysens was a pioneer in developing sensitive spectrophotometric methods to monitor photosynthetic systems. The experiment was to measure the oxidation–reduction state of cytochrome f in the sample upon illumination using various wavelengths of light. When the cytochrome is reduced, the absorbance spectrum changes, permitting quantitative measurements of its redox state (Fig. 3.5). Duysens found that far‐red light caused the cytochrome to become oxidized, whereas shorter‐wavelength light caused it to become reduced. The two colors of light had opposite, or antagonistic, effects. A particularly clear effect was observed using the red alga Porphyridium cruentum, which has phycobilisome antenna complexes. The effects are easily observed with this organism, because, as we now know, the phycobilisome antenna complex preferentially directs excitations mostly to one of the two photosystems. As is often the case, the choice of the experimental system in which an effect is emphasized was important to the initial understanding of the effect. Subsequent measurements have shown that the effect is, of course, a general one, observed in all oxygen‐evolving photosynthetic organisms.

Figure 3.5 Antagonistic effects on cytochrome oxidation. Irradiation with the light of one color causes the cytochrome to become more oxidized, while irradiation with light of a different color causes it to become more reduced. This experiment was the clearest early evidence for two photochemical systems connected in series in oxygenic photosynthetic organisms.

Source: Duysens et al. (1961)/Springer Nature.