Photosynthetic Phosphorylation and Electron Transport

Arnon, Daniel I.; Tsujimoto, Harry Y.; McSwain, Berah D.

doi:10.1038/2071367a0

Cited by 67 publications

(24 citation statements)

References 47 publications

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“…Cytochromes b6 and f have previously been assigned to cyclic photophosphorylation (92,93) and are now also considered components of System I. The present concept places cytochrome f and P700 in System I and, in contrast to the older hypothesis, predicts that neither would be required for the photoreduction of ferredoxin-NADP by water via System II.…”

Section: Three Light Reactionscontrasting

confidence: 40%

“…4 also shows that the wavelength dependence of the phosphorylation associated with electron flow from an artificial electron donor (reduced 2,6-dichlorophenol indophenol) to NADP (91) resembles the cyclic system. This similarity suggests that electron transport involved in the photoreduction of NADP by DPIPH2 proceeds via (a portion of) System I and is not involved in the photoreduction of NADP by water via System II (92,93). In this view (further elaborated below), ferredoxin-NADP could be reduced either by water via System II or by an artificial electron donor via System I.…”

Section: Two Photosystems In Plant Photosynthesismentioning

confidence: 93%

“…In this view (further elaborated below), ferredoxin-NADP could be reduced either by water via System II or by an artificial electron donor via System I. System I identified with cyclic, and System II identified with noncyclic, electron transport could thus be regarded as parallel processes in chloroplasts, each basically capable of proceeding independently of the other (92,93 . 92)-that the two photosystems must operate in series and, through such collaboration, bring about the light-induced reduction of ferredoxin-NADP by water.…”

Section: Two Photosystems In Plant Photosynthesismentioning

confidence: 99%

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The Light Reactions of Photosynthesis

Arnon

1971

Proc. Natl. Acad. Sci. U.S.A.

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Historically, the role of light in photosvnthesis has been ascribed either to a photolysis of carbon dioxide or to a photolysis of water and a resultant rearrangement of constituent atoms into molecules of oxygen and glucose (or formaldehyde). The discovery of photophosphorylation demonstrated that photosynthesis includes a light-induced phosphorus metabolism that precedes, and is independent from, a photolysis of water or CO2. ATP formation could best be accounted for not by a photolytic disruption of the covalent bonds in C02 or water but by the operation of a light-induced electron flow that results in a release of free energy which is trapped in the pyrophosphate bonds of ATP.Photophosphorylation is now divided into (a) a noncyclic type, in which the formation of ATP is coupled with a light-induced electron transport from water to ferredoxin and a concomitant evolution of oxygen and (b) a cyclic type which yields only ATP and produces no net change in the oxidation-reduction state of any electron donor or acceptor. Reduced ferredoxin formed in (a) serves as an electron donor for the reduction of NADP by an enzymic reaction that is independent of light. ATP, from both cyclic and noncyclic photophosphorylation, and reduced NADP jointly constitute the assimilatory power for the conversion of C02 to carbohydrates (3 moles of ATP and 2 moles of reduced NADP are required per mole of C02).Investigations, mainly with whole cells, have shown that photosynthesis in green plants involves two photosystems, one (System II) that best uses light of "short" wavelength (X < 685 nm) and another (System I) that best uses light of "long" wavelength (X > 685 nm). Cyclic photophosphorylation in chloroplasts involves a System I photoreaction. Noncyclic photophosphorylation is widely held to involve a collaboration of two photoreactions: a short-wavelength photoreaction belonging to System II and a long-wavelength photoreaction belonging to System I. Recent findings, however, indicate that noncyclic photophosphorylation may include two short-wavelength, System II, photoreactions that operate in series and are joined by a "dark" electron-transport chain to which is coupled a phosphorylation site. Early conceptsThe first hypothesis about the role of light in photosynthesis came very appropriately from Jan Ingenhousz, who some years earlier had made the epochal discovery that it is "the influence of the light of the sun upon the plant" (1) that is responsible for the "restorative" effect of vegetation on "bad" air-an observation first made in 1771 by Joseph Priestley without reference to light (2). In 1796, Ingenhousz wrote that the green plant absorbs from "carbonic acid in the sunshine, the carbon, throwing out at that time the oxygen alone, and keeping the carbon to itself as nourishment" (3).The idea that light liberates oxygen by photodecomposing C02 had, with some modifications, persisted for well over a century. It seemed to have had a special attraction for some of the most illustrious chemists in their day, e.g., von ...

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Section: Three Light Reactionscontrasting

confidence: 40%

Section: Two Photosystems In Plant Photosynthesismentioning

confidence: 93%

Section: Two Photosystems In Plant Photosynthesismentioning

confidence: 99%

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The Light Reactions of Photosynthesis

Arnon

1971

Proc. Natl. Acad. Sci. U.S.A.

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“…A currently popular hypothesis (which this laboratory adopted in 196129 and abandoned in 19653°) holds that the photoreduction of ferredoxin by water requires the collaboration in series of System II and System I. An alternative hypothesis put forward by this laboratory" 13 30 envisages that the photoreduction of ferredoxin by water involves only System II and limits the function of System I to cyclic electron transport and its experimental variant: the reduction of the ferredoxin-NADP couple by an artificial electron donor such as reduced dichlorophenol indophenol. Store recently, Arnold and Azzi3l have also concluded, on the basis of quite different considerations, that the photoreduction of ferredoxin by water is accomplished solely by System II and that System I is limited to cyclic electron flow (and phosphorylation).…”

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confidence: 99%

A Concept of Three Light Reactions in Photosynthesis by Green Plants

Knaff

Arnon

1969

Proc. Natl. Acad. Sci. U.S.A.

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Abstract.-It has generally been accepted that plant photosynthesis involves two light reactions, one that proceeds best in short-wavelength light and is identified with oxygen evolution (System II) and another that proceeds best in long-wavelength light and is identified with a cyclic electron flow (System I). This paper presents a concept of three light reactions in photosynthesis, based on new evidence that System II comprises two rather than one short-wavelength light reaction. These appear to operate in series and to be connected by an electron transport chain peculiar to System II. Parallel to System II is the longwavelength light reaction of System I.There is now wide agreement that photosynthesis in green plants involves two light reactions, one that proceeds best in short wavelength (X < 685 my) light (known as Photosystem II or simply System II) and another-known as System I-which proceeds best in long wavelength (X > 685 my) light. System I is identified with a light-induced cyclic electron transport among chloroplast constituents (cytochromes --chlorophyll ferredoxin --cytochromes) that produces ATP without any net change in the redox state of any electron donor or acceptor (cyclic photophosphorylation). System II is identified with a lightinduced electron transport from water to a terminal electron acceptor with a concomitant evolution of oxygen and production of ATP (noncyclic photophosphorylation) (references cited in reviews1' 2).We have recently reported3' 4 two new light reactions characteristic of System II which, because of their insensitivity to temperature (they occur even at -1890C), appear to lie close to the primary photochemical events in that system. The first of these light reactions is the photooxidation of cytochrome b559 (ref. 3) and the second is a spectral change, which now appears to be a reduction, of a new photoreactive chloroplast component which we have provisionally named C550.4 This paper presents evidence in support of a hypothesis which holds that System II includes one light reaction (JIb) in which C550 is reduced and water is oxidized (liberating oxygen) and a second light reaction (Iha) in which cytochrome b559 is oxidized and ferredoxin is reduced. Our results suggest that light reactions IIb and Ila operate in series and are linked by an electron transport chain which includes (but is not limited to) C550, cytochrome b559, and plastocyanin. This formulation leads to a concept of three light reactions in photosynthesis, comprising the two short-wavelength light reactions of System II and, parallel to it, the one long-wavelength light reaction of System I.Methods.-"Broken" spinach chloroplasts were prepared according to the method of Whatley and Arnon' and Tris-treated chloroplasts by a modification of the procedure of Yamashita and Butler. Sonicated chloroplasts were prepared by sonicating a chloroplast suspension (0.5 mg chlorophyll/ml) with a Branson sonifier for 3 min at power setting 3.

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“…Particles prepared by treating chloroplasts with high concentrations of detergents which destroy photosystem II activity often require added plastocyanin for photosystem I activity [S,61. Arnon et al [7] have recently used detergents to extract from spinach chloroplasts a particle which shows plastocyanin-dependent photoreduction of NADP from water.…”

Section: Discussionmentioning

confidence: 99%