Photosystems, green plant

Green plants split water under irradiation by sunlight The reaction pathway of the green plant photosystem is shown in Fig. 18.11. With the aid of light energy the plants first extract electrons by oxidizing water The electrons are then transported into the Calvin cycle through ferredoxin. The arrows in the... 

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

A similar conclusion was reached by Feiler, Albouy, Pourcet, Mattioli, Lutz and Roberts from reso-nance-Raman studies of active reaction-center complexes of Chlorobium. They further showed a similarity in the binding of this Chl-a isomer in green-bacterial reaction centers to that of the Chl-type acceptor molecules in other bacteria and green-plant reaction centers. Finally, one may note that the identification of BChl 663 as an isomer of Chi a has placed more emphasis on the similarities between the reaction centers of green sulfur bacteria and the green-plant photosystem I, where the primary electron acceptor is Chi a, and the recently discovered heliobacteria, where the primary electron acceptor is an 8 -hydroxy-Chl a. 

Both green-plant photosystem I and green sulfur bacteria have for sometime known to have the FeS-type reaction centers. Now a new group of photosynthetic bacteria, the heliobacteria, discovered about twenty years ago by Gest and Favinger, have also been found to belong to this type. The first dis-... 

Fig. 11. Electron-transfer schemes for the reaction centers of the green sulfur bacteria (A) and green filamentous bacteria (B). The reaction-center components of the green sulfur bacteria are compared to green-plant photosystem I and those of the green filamentous bacteria are compared to green-plant photosystem II or purple bacteria. The decay times and redox potentials are for Prosihecochloris aestuariiand Chloroflexus aurantiacus. See text lor discussion. Figure adapted from Amesz (1987)...

R1. B Ke (1973) The primary electron acceptor of photosystem I. Biochim Biophys Acta 301 1-33 R2. B Ke (1978) The primary electron acceptors in green-plant photosystem I and photosynthetic bacteria. In DR Sanadi and LP Vernon (eds) Current Topics in Bioenergetics. Vol 7 pp 75-138. Acad Press. 

Fig. 15. Plot ofthe extent of absorbance change due to P7007P430" recombination measured at 695 nm vs. the redox potential of 14 quinones and 7 non-quinone carbonyl compounds, the fluorenones (individual compounds are identified below the plot). The solid curve is the theoretical, one-electron Nernst curve centered near the redox potential of FeS-X in vivo. Data adapted from Itoh and Iwaki (1992) Exchange ofthe acceptor phylloquinone by artificial quinones and fluorenones in green plant photosystem I photosynthetic reaction center. In N Malaga, T Okada and H Masuhara (eds) Dynamics and Mechanism of Photoinduced Transfer and Related Phenomena. p.533. Elsevier,...

S Itoh and M Iwaki (1991) Full replacement of the function of the secondary electron acceptor phylloquinone (=vitamin KO by non-quinone carbonyl compounds in green plant photosystem I photosynthetic reaction centers. Biochemistry 30 5340-5346... 

The model in Fig. 1 is depicted again in Fig. 11 (A) to show the intermediary position of the b(f complex in the thylakoid membrane of the green-plant photosystem. In Fig. 11 (B), the R-[2Fe 2S] and Cyt / are shown located near the membrane-lumen interface and Cyt b has the unique transmembrane arrangement of its two hemes located on the opposite sides of the membrane. This assumed heme arrangement is significant for the important role played by Cyt b(, in electron transport and proton translo-... 

To survey in detail the current state of knowledge of photosynthetic light harvesting would require an encyclopedic article. In this article we focus on the general principles that have been learned from studies of the purple bacterial and cyanobacterial systems. We discuss briefly the implications for green plant photosystems. We conclude with a discussion of questions that highlight areas that we feel are currently in need of investigation or resolution. 

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

Iwaki, M. and Itoh, S., Function of quinones and quinonoids in green-plant photosystem I reaction center, Adv. Chem. Ser., 228,163, 1991. 

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