A Dynamical Model to study the Response of Microalgae to Pulse Amplitude Modulated Fluorometry
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Plastoquinone
Plastoquinone
DCMU
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Plastoquinone
P700
Cytochrome b6f complex
Electron acceptor
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Plastoquinone
Cytochrome b6f complex
Oxygen evolution
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The role of copper (Cu) in photosynthetic electron transport was explored by using Cu deficiency in sugar beet as an experimental approach. Copper influenced electron transport at two sites in addition to plastocyanin. Under mild deficiency (0.84 nmol of Cu per cm 2 of leaf area), electron transport between the two photosystems (PS) is inhibited but not electron transport within PS I or PS II measured separately. The chlorophyll/plastoquinone ratio was normal in Cu-deficient plants. However, the breakpoint in the Arrhenius plot of electron transport was shifted towards a higher temperature. It is concluded that Cu is necessary to maintain the appropriate membrane fluidity to ensure the mobility of plastoquinone molecules to transfer electrons between the two photosystems. Under severe deficiency (0.22 nmol of Cu per cm 2 of leaf area) both PS II and PS I electron transports were inhibited and to the same extent. PS II electron transport activity could not be restored by adding artificial electron donors. Polypeptides with M r s of 28,000 and 13,500 were missing in Cu-deficient chloroplast membranes. In PS II particles prepared from normal chloroplasts of spinach, 2 atoms of Cu per reaction center are present. We conclude that Cu influences PS II electron transport either directly, by participation in electron transfer as a constituent of an electron carrier, or indirectly, via the polypeptide composition of the membrane in the PS II complex.
Plastoquinone
Plastocyanin
P700
Electron donor
Electron acceptor
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Regulation of electron transport composition was studied with Synechocystis (Aphanocapsa) PCC 6714 grown under various trophic conditions. Photosystem (PS) composition, PS I/IIratio, was used as the index for the composition of the electron transport system. Special attention was paid to the relationship between changes in the PS I/II ratio and features of electron transport. Results indicated that (1) the change of composition of the electron transport system is induced not only by the light conditions but also by changes in the electron influx and efflux. (2) The change occurs so as to relieve the imbalance between the electron influx and the efflux, caused by changes in the conditions for the electron transport, and (3) the redox state of plastoquinone pool is probably related to the primary stimulus of the regulation.
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Cytochrome b6f complex
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Abstract Photosystem II herbicides act by blocking electron transport at the secondary electron acceptor ‘Q B ’, thought to be a non‐covalently bound plastoquinone. Recent evidence suggests that these compounds work by displacing the plastoquinone from its site in the thylakoid. Since the herbicides cannot act as electron carriers, electron transport is then blocked. In this report a model is presented for the site of action of Photosystem II herbicides that encompasses biochemical, biophysical, and structure‐activity considerations. The essence of the model is that Photosystem II herbicides are non‐reducible analogues of plastoquinone or its semiquinone anion. As examples of the ways in which known herbicidal classes fit the model, the possible interactions of diuron, atrazine, the putative urea‐triazine hybrid MBAT (the a‐methylbenzyl analogue of atrazine), and dinoseb with the active site are discussed. This model provides a stereochemical basis for herbicidal activity and offers a qualitative approach for the design of novel Photosystem II herbicides.
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P700
Cytochrome b6f complex
Triazine
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Abstract Isolated spinach chloroplasts (type A complete) were used to study the changes in the photo-chemical activities upon storage in order to establish optimum conditions for prolonged storage. Chloroplasts stored at - 5 °C were found to retain over 70% of their photosynthetic electron transport from H 2 O even after 5 days. Increases in the level of state 2 electron transport (due to uncoupling) with concomitant loss of state 3 phosphorylation activity was observed during the initial period of ageing. Addition of 1% bovine serum albumin decreased the level of uncoupling and maintained the phosphorylation activity for a longer period. Chloroplasts stored at 77 °K maintained their phosphorylation capacity for a period of 10 days even after repeated freezing and thawing. Photosystem I activity was found to be more stable over the whole ageing period of 15 days. Loss of plastoquinone may be responsible for the decrease in electron transport between photosystems II and I.
Plastoquinone
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Plastoquinone
P700
Electron acceptor
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Abstract— The significance of the accumulation of positive charge in the reaction center of the oxygen‐evolving system of photosynthesis is discussed. Many experiments on delayed and prompt fluorescence are explained by the electric field caused by positive charges on the various components at the oxidizing side of the photosystem 2 reaction center. A molecular model for this reaction center is proposed.
Oxidizing agent
Center (category theory)
Oxygen-evolving complex
Active center
Oxygen evolution
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Plastoquinone
DCMU
Acceptor
Electron acceptor
P700
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