Evidence from Chloroplast Fragments for Three Photosynthetic Light Reactions

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 67; no. 3; pp. 1404 - 1409
• Main Authors: Arnon, Daniel I., Chain, Richard K., McSwain, Berah D., Tsujimoto, Harry Y., Knaff, David B.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-11-1970; National Acad Sciences
• Subjects: Biological Sciences: Biochemistry; Chemical suspensions; Chlorophylls; Chloroplasts; Cytochromes; Electrons; Ferredoxins; Photosynthesis; Photosystem I; Photosystem II; Plants
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.67.3.1404

Previous reports from this laboratory described a new concept of three light reactions in plant photosynthesis comprising two short-wavelength (λ < 700 nm) photoreactions belonging to Photosystem II and one long-wavelength (λ > 700 nm) photoreaction belonging to Photosystem I. Among the electron carriers assigned to Photosystem II were cytochrome b559and plastocyanin and to Photosystem I, cytochrome f. According to a widely held view, the light-induced reduction of NADP by water requires the collaboration of Photosystems I and II and involves specifically cytochrome f and P700 (a portion of chlorophyll a peculiar to Photosystem I). By contrast, the new concept ascribes the light-induced reduction of NADP by water solely to the two photoreactions of Photosystem II, without the participation of Photosystem I and its components, cytochrome f and P700. Further evidence in support of the new concept has now been obtained from chloroplast fragments. Two kinds of chloroplast fragments have been prepared: (a) one with Photosystem II activity, capable--in the presence of plastocyanin--of photoreducing NADP with water but lacking P700 and functional cytochrome f and (b) another having only Photosystem I activity, lacking plastocyanin, and enriched in P700.