Oxygen Metabolism and Electron Transport in Photosynthesis

Abstract

Light drives photosynthesis, but too much light damages the photosynthetic apparatus (Jones and Kok 1966: Powles 1984). All photosynthetic organisms have developed strategies for avoiding the deleterious effects of high irradiances. During evolution, the metabolism of oxygen has become inextricably linked with the photosynthetic processes and their regulation. Oxygen is evolved in photosynthesis, but it is also used as a sink for energy, particularly in situations where electron transport capacity exceeds that of CO₂ assimilation. Aerobic autotrophs gain a significant advantage from using water as the electron donor for photosynthesis. In plants and cyanobacteria two separate photosystems (PSII and PSI) are connected in series to oxidize water and reduce NADP. The relative ability of a compound to accept or donate an electron is reflected by its midpoint redox potential. A compound with a higher (more positive) redox potential is generally more oxidizing, whereas one with a lower (more negative) redox potential tends to be more reducing. In photosynthesis, light energy is required to remove electrons from water (the H₂O/O₂ couple being highly positive) to reduce NADP (the NADP⁺/NADPH couple being highly negative). The oxidation of two molecules of water to yield molecular oxygen involves removal of four protons and four electrons and requires an average oxidizing potential of about +0.81 volts for each electron removed at pH 7.0 (Fig. 1).

The photosystem II (PSII) complex of oxygenic photosynthesis is unique among photosynthetic reaction centers in that it functions with a potentially destructive combination of light, pigments, high redox potentials, quinones,...