In the process of

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, the

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

of ADP to form ATP using the energy of sunlight is called photophosphorylation. 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, ATP, 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

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. This stores energy in a proton gradient. As the protons flow back through an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

and NADPH.

ATP and reactions

Both the structure of ATP synthase and its underlying

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

are remarkably similar in all known forms of life. ATP synthase is powered by a transmembrane electrochemical potential 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 Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

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

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...

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 an activation 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 reaction ''if 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 be ''spatially'' separated into a series of intermediate redox reactions. This is an

(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

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

Cyclic photophosphorylation

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 photosystem I, flows in a cyclic pathway. The electron starts in photosystem I, passes from the primary electron acceptor to

ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...

and then to plastoquinone, next to cytochrome bf (a similar complex to that found in

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

), and finally to plastocyanin 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 ATP synthase during

chemiosmosis Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate, adenosine triphosphate (ATP) by the movement of hydrogen ion ...

. This pathway is known as cyclic photophosphorylation, and it produces neither O nor NADPH. Unlike non-cyclic photophosphorylation, NADP does not accept the electrons; they are instead sent back to the cytochrome bf complex. 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 compensation points.

Non-cyclic photophosphorylation

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 pigment P680 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 O + 2e ( photolysis or ''light-splitting''). An electron from the water molecule reduces P680 back to P680, while the H and oxygen are released. The electron transfers from pheophytin to plastoquinone (PQ), which takes 2e (in two steps) from pheophytin, and two H Ions from the stroma to form PQH. This plastoquinol is later oxidized back to PQ, releasing the 2e to the cytochrome bf complex and the two H ions into the thylakoid lumen. The electrons then pass through Cyt b and Cyt f to plastocyanin, 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. The photosystem II complex replaced its lost electrons from HO, 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 called ferredoxin-NADP reductase, which uses them to catalyze the reaction :NADP + 2H + 2e → NADPH + H This consumes the H ions produced by the splitting of water, leading to a net production of 1/2O, 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 the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.

Early history of research

In 1950, first experimental evidence for the existence of photophosphorylation ''in vivo'' was presented by Otto Kandler using intact '' Chlorella'' cells and interpreting his findings as light-dependent ATP formation. In 1954, Daniel I. Arnon et.al. discovered photophosphorylation ''in vitro'' in isolated

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s with the help of P³². His first review on the early research of photophosphorylation was published in 1956.

References

*Professor Luis Gordillo *Fenchel T, King GM, Blackburn TH. Bacterial Biogeochemistry: The Ecophysiology of Mineral Cycling. 2nd ed. Elsevier; 1998. *Lengeler JW, Drews G, Schlegel HG, editors. Biology of the Prokaryotes. Blackwell Sci; 1999. *Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 4th ed. Freeman; 2005. *{{Cite book, title=Bioenergetics, last1=Nicholls, first1=David G., author-link1=David G. Nicholls, last2=Ferguson, first2=Stuart J., isbn=9780123884312, edition=Fourth, location=Amsterdam, oclc=846495013, year=2013 *Stumm W, Morgan JJ. Aquatic Chemistry. 3rd ed. Wiley; 1996. *Thauer RK, Jungermann K, Decker K. Energy Conservation in Chemotrophic Anaerobic Bacteria. Bacteriol. Rev. 41:100–180; 1977. *White D. The Physiology and Biochemistry of Prokaryotes. 2nd ed. Oxford University Press; 2000. *Voet D, Voet JG. Biochemistry. 3rd ed. Wiley; 2004. Photosynthesis Light reactions