In the process ofphotosynthesis, thephosphorylation ofADP to formATP using the energy of sunlight is calledphotophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main source of energy available to living organisms, which is sunlight. All organisms produceATP, 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 anelectron transport chain. This stores energy in aproton gradient. As the protons flow back through anenzyme calledATP synthase, ATP is generated from ADP andinorganic phosphate. ATP is essential in theCalvin cycle to assist in the synthesis of carbohydrates fromcarbon dioxide andNADPH.
The scientist Charles Barnes first used the word 'photosynthesis' in 1893. This word is taken from two Greek words,photos, which means light, andsynthesis, which in chemistry means making a substance by combining simpler substances. So, in the presence of light, synthesis of food is called 'photosynthesis'.
Photophosphorylation represents a specific instance of a more general bioenergetic principle: the conservation of energy through transmembrane electrochemical gradients. The synthesis of ATP by ATP synthase, driven by a proton motive force, is a highly conserved mechanism across all domains of life, occurring in chloroplasts, cyanobacteria, mitochondria, and the plasma membranes of many prokaryotes.[1][2]
This structural and functional conservation indicates that photophosphorylation and oxidative phosphorylation share a common evolutionary foundation, differing primarily in the source of energy used to generate the proton gradient—light energy in photosynthetic systems and redox energy derived from chemical substrates in respiratory systems.[3][4]
From a physiological perspective, photophosphorylation supplies ATP not only for the Calvin–Benson cycle but also for maintaining redox balance and ionic homeostasis within the chloroplast.[5] In photosynthetic prokaryotes such as cyanobacteria, photophosphorylation is functionally integrated with other energy-conserving pathways, highlighting that cellular bioenergetics operates as a coordinated network of energy fluxes rather than as isolated reaction sequences.[6][7]
The formulation of the chemiosmotic theory unified these observations by demonstrating that the transmembrane proton gradient itself constitutes the central intermediate of biological energy conversion, replacing earlier models based on discrete high-energy chemical intermediates.[3] Within this framework, photophosphorylation is understood as part of a broader class of chemiosmotic processes in which membrane structure, electron transport, and ATP synthesis form an inseparable functional unit.
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Both the structure of ATP synthase and its underlyinggene are remarkably similar in all known forms of life. ATP synthase is powered by a transmembrane electrochemicalpotential gradient, 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).
Redox reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is theGibbs free energy 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 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.
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 anactivation energy 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.
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 system decreases (making it thermodynamically possible), while useful work is done at the same time. The principle that biological macromolecules catalyze a thermodynamically unfavorable reactionif and only if a thermodynamically favorable reaction occurs simultaneously, underlies all known forms of life.
The transfer of electrons from a donor molecule to an acceptor molecule can bespatially separated into a series of intermediate redox reactions. This is anelectron transport chain (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 bacterialflagella.
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In plants, 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 calledphotosystem I, flows in a cyclic pathway. The electron starts in photosystem I, passes from the primary electron acceptor toferredoxin and then toplastoquinone, next tocytochrome b6f (a similar complex to that found inmitochondria), and finally toplastocyanin 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 powerATP synthase duringchemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH[citation needed]. Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons; they are instead sent back to the cytochrome b6f complex.[8]
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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 CO2 compensation points.[citation needed]
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 pigmentP680 at the core of photosystem II, which is transferred to the primary electron acceptor,pheophytin, leaving behind P680+. The energy of P680+ is used in two steps to split a water molecule into 2H+ + 1/2 O2 + 2e- (photolysis orlight-splitting). An electron from the water molecule reduces P680+ back to P680, while the H+ and oxygen are released. The electron transfers from pheophytin toplastoquinone (PQ), which takes 2e- (in two steps) from pheophytin, and two H+ Ions from thestroma to form PQH2. This plastoquinol is later oxidized back to PQ, releasing the 2e- to thecytochrome b6f complex and the two H+ ions into thethylakoid lumen. The electrons then pass through Cyt b6 and Cyt f toplastocyanin, 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.[citation needed]
The photosystem II complex replaced its lost electrons from H2O, 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 molecules, but this time are passed on to an enzyme calledferredoxin-NADP+ reductase, which uses them to catalyze the reaction
This consumes the H+ ions produced by the splitting of water, leading to a net production of 1/2O2, ATP, and NADPH + H+ with the consumption of solar photons and water.
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 theCalvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.
In 1950, first experimental evidence for the existence of photophosphorylationin vivo was presented byOtto Kandler using intactChlorella cells and interpreting his findings as light-dependentATP formation.[10] In 1954,Daniel I. Arnon et.al. discovered photophosphorylationin vitro in isolatedchloroplasts with the help of P32.[11] His first review on the early research of photophosphorylation was published in 1956.[12]
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