20:
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of the reactants relative to the products. If donor and acceptor (the reactants) are of higher free energy than the reaction products, the electron transfer may occur spontaneously. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free
142:(ETC). Electron transport chains often produce energy in the form of a transmembrane electrochemical potential gradient. The gradient can be used to transport molecules across membranes. Its energy can be used to produce ATP or to do useful work, for instance mechanical work of a rotating bacterial
126:
It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an osmotic gradient), in such a way that the overall free energy of the
213:
The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems in the thylakoid membrane. First, a photon is absorbed by chlorophyll pigments surrounding the reaction core center of photosystem II. The light excites an electron in the
273:
O, so electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, they are transferred to the photosystem I complex, which boosts their energy to a higher level using a second solar photon. The excited electrons are transferred to a series of acceptor
35:, which in chemistry means making a substance by combining simpler substances. So, in the presence of light, synthesis of food is called 'photosynthesis'. Noncyclic photophosphorylation through light-dependent reactions of photosynthesis at the
122:
or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex macromolecular structures to lower the activation energies of biochemical reactions.
266:, using energy from photosystem I to pump hydrogen ions (H) into the thylakoid space. This creates a H gradient, making H ions flow back into the stroma of the chloroplast, providing the energy for the (re)generation of ATP.
55:. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of energy available to living organisms, which is sunlight. All organisms produce a phosphate compound,
103:, usually in the form of a proton gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient, or a so-called proton motive force (pmf).
117:
The fact that a reaction is thermodynamically possible does not mean that it will actually occur. A mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an
127:
system decreases (making it thermodynamically possible), while useful work is done at the same time. The principle that biological macromolecules catalyze a thermodynamically unfavorable reaction
201:
In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation. It is favored in anaerobic conditions and conditions of high irradiance and CO
154:
This form of photophosphorylation occurs on the stroma lamella, or fret channels. In cyclic photophosphorylation, the high-energy electron released from P700, a pigment in a complex called
114:
energy of a system will proceed spontaneously (given that the system is isobaric and also at constant temperature), although the reaction may proceed slowly if it is kinetically inhibited.
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before returning to photosystem I. This transport chain produces a proton-motive force, pumping H ions across the membrane and producing a concentration gradient that can be used to power
59:, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an
359:[On the relationship between the phosphate metabolism and photosynthesis I. Variations in phosphate levels in Chlorella pyrenoidosa as a consequence of light-dark changes]
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The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the
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reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the
357:"Über die Beziehungen zwischen Phosphathaushalt und Photosynthese. I. Phosphatspiegelschwankungen bei Chlorella pyrenoidosa als Folge des Licht-Dunkel-Wechsels"
401:; Allen, M.B.; Whatley, F.R. (1954). "Photosynthesis by isolated chloroplasts. II. Photophosphorylation, the conversion of light into phosphate bond energy".
234:). An electron from the water molecule reduces P680 back to P680, while the H and oxygen are released. The electron transfers from pheophytin to
505:
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nor NADPH. Unlike non-cyclic photophosphorylation, NADP does not accept the electrons; they are instead sent back to the cytochrome b
515:
Thauer RK, Jungermann K, Decker K. Energy
Conservation in Chemotrophic Anaerobic Bacteria. Bacteriol. Rev. 41:100–180; 1977.
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Fenchel T, King GM, Blackburn TH. Bacterial
Biogeochemistry: The Ecophysiology of Mineral Cycling. 2nd ed. Elsevier; 1998.
481:
275:
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The scientist
Charles Barnes first used the word 'photosynthesis' in 1893. This word is taken from two Greek words,
158:, flows in a cyclic pathway. The electron starts in photosystem I, passes from the primary electron acceptor to
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are remarkably similar in all known forms of life. ATP synthase is powered by a transmembrane electrochemical
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with the help of P. His first review on the early research of photophosphorylation was published in 1956.
222:, leaving behind P680. The energy of P680 is used in two steps to split a water molecule into 2H + 1/2 O
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139:
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White D. The
Physiology and Biochemistry of Prokaryotes. 2nd ed. Oxford University Press; 2000.
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a thermodynamically favorable reaction occurs simultaneously, underlies all known forms of life.
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This consumes the H ions produced by the splitting of water, leading to a net production of 1/2O
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Lengeler JW, Drews G, Schlegel HG, editors. Biology of the
Prokaryotes. Blackwell Sci; 1999.
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297:, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.
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at the core of photosystem II, which is transferred to the primary electron acceptor,
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Nelson DL, Cox MM. Lehninger
Principles of Biochemistry. 4th ed. Freeman; 2005.
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190:. This pathway is known as cyclic photophosphorylation, and it produces neither O
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In 1950, first experimental evidence for the existence of photophosphorylation
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The transfer of electrons from a donor molecule to an acceptor molecule can be
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238:(PQ), which takes 2e (in two steps) from pheophytin, and two H Ions from the
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75:, ATP is generated from ADP and inorganic phosphate. ATP is essential in the
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246:. This plastoquinol is later oxidized back to PQ, releasing the 2e to the
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separated into a series of intermediate redox reactions. This is an
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Stumm W, Morgan JJ. Aquatic
Chemistry. 3rd ed. Wiley; 1996.
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The photosystem II complex replaced its lost electrons from H
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molecules, but this time are passed on to an enzyme called
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of ADP to form ATP using the energy of sunlight is called
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cells and interpreting his findings as light-dependent
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Voet D, Voet JG. Biochemistry. 3rd ed. Wiley; 2004.
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Both the structure of ATP synthase and its underlying
431:(1956). "Phosphorus metabolism and photosynthesis".
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79:to assist in the synthesis of carbohydrates from
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278:, which uses them to catalyze the reaction
504:: CS1 maint: location missing publisher (
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327:et.al. discovered photophosphorylation
258:. The electrons then pass through Cyt b
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67:. As the protons flow back through an
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16:Biochemical process in photosynthesis
174:(a similar complex to that found in
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445:10.1146/annurev.pp.07.060156.001545
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433:Annual Review of Plant Physiology
209:Non-cyclic photophosphorylation
476:(Fourth ed.). Amsterdam.
472:; Ferguson, Stuart J. (2013).
365:Zeitschrift für Naturforschung
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254:and the two H ions into the
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150:Cyclic photophosphorylation
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282:NADP + 2H + 2e → NADPH + H
63:. This stores energy in a
301:Early history of research
276:ferredoxin-NADP reductase
29:, which means light, and
140:electron transport chain
61:electron transport chain
456:Professor Luis Gordillo
40:
377:10.1515/znb-1950-0806
323:formation. In 1954,
205:compensation points.
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53:photophosphorylation
415:10.1021/ja01653a025
470:Nicholls, David G.
178:), and finally to
101:potential gradient
43:In the process of
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37:thylakoid membrane
409:(24): 6324–6329.
309:was presented by
120:activation energy
111:Gibbs free energy
91:ATP and reactions
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333:chloroplasts
331:in isolated
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311:Otto Kandler
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295:Calvin cycle
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264:plastocyanin
248:cytochrome b
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188:chemiosmosis
184:ATP synthase
180:plastocyanin
176:mitochondria
168:cytochrome b
162:and then to
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77:Calvin cycle
73:ATP synthase
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439:: 325–354.
242:to form PQH
198:f complex.
532:Categories
339:References
228:photolysis
220:pheophytin
166:, next to
160:ferredoxin
500:cite book
492:846495013
316:Chlorella
252:f complex
136:spatially
32:synthesis
385:97588826
355:(1950).
329:in vitro
214:pigment
144:flagella
307:in vivo
186:during
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240:stroma
226:+ 2e (
69:enzyme
47:, the
26:photos
381:S2CID
361:(PDF)
107:Redox
85:NADPH
506:link
488:OCLC
478:ISBN
216:P680
97:gene
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321:ATP
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