The Systems I, II and III in the Photosynthetic Cycle of Electron Transfer

• 1 Two separate light reactions may be distinguished: (I) the reduction of ferredoxin and NADP probably by oxidation of carotene to xanthophyll; (II) the oxida tion of cytochrome f by chlorophyll (probably a). Reaction II implies a return of electrons to the pigments, system III, thus maintaining its normal steady state of oxidation-reduction. The xanthophyll is hereby again reduced to carotene.
• 2 System I is sensitive to violet-blue-green and probably also infrared light. Carotene absorbs in these regions, ferredoxin in blue-violet. System II is primarily sensitive to red light but also to blue-violet. Both chlorophyll and cytochromes absorb in the latter region, the cytochromes also in green.
• 3 The response of systems I and II to different spectral regions was studied by means of a special spectrophotometric flash technique, enabling precise measure ments of the band-height of the enzymes involved. The initial photic reactions of systems I and II, viz., the reduction of ferredoxin-NADP and the oxidation of cyto chrome f show a full turnover in less than 0.1 ms but the transfer between systems I and II by means of which the cytochromes are reduced is slowed down to about 10^?1-10^?2 s. The initial effect may thus be observed during ca. 0.1 s. At continuous illumination the displacement of the steady states of the enzymes may last up to several seconds and then return to a state of only partial reduction. Erroneous inter pretations of these phenomena are corrected.
• 4 In the blue-sensitive system I ferredoxin alone mediates the reduction of NADP but the possibility of the presence of other factors capable of dark chemical elec tron transfer is discussed. In the red-sensitive system II three cytochromes operate, viz., f, b₃ and b₆. Spectrophotometric evidence for the existence of two cytochromes b is presented. Cytochromes b₆ and f are approximately synchronously oxidized and reduced, whereas b₃ reacts somewhat independently. Cytochrome b₃ probably acts as a decharger of OH^? and compensates for the capture of H⁺+ e^? at the reduction of triphosphopyridine nucleotide (NAD) or of electrons by other oxidants.
• 5 The transfer of electrons between systems I and II maintains a reversible steady state of oxidation-reduction that may be moved to one side or the other not only by monochromatic light but also in the dark under influence of N₂, O₂, the ratios NADP/NADPH and ADP/ATP, and various added substances. Spectrophotometric measurements in UV show that a flavoprotein participates in the multiple steady state.
• 6 The investigations illustrate many intricate technical problems that are too frequently overlooked. Photostructural reactions must be eliminated by referring band-heights to an isosbestic level. The photosynthetic activity is strongly dependent on light-scattering. Reliable measurements of cytochromes must be made in the α-region owing to a strong interference of rapid changes of the ratio carotene/xanthophyll in the region of the γ-bands.