thumb|300px|Photophosphorylation in the [[light-dependent reactions of photosynthesis, which occurs at the thylakoid membrane in chloroplasts and cyanobacteria.]] Photophosphorylation is the process of phosphorylation of ADP to form ATP using the energy from photons (e.g. from sunlight) in photosynthesis. There are two types: cyclic photophosphorylation and non-cyclic photophosphorylation.

In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain, storing potential energy in a proton gradient. The protons return across the membrane though an enzyme called ATP synthase which synthesises ATP from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to supply energy for the synthesis of carbohydrates from carbon dioxide and NADPH.

Bioenergetic integration and universality of the proton gradient

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.

In other phototrophic bacteria

thumb|350x350px|Inferred [[metabolic pathways of "Ca. Thiodictyon intracellulare", an endosymbiotic purple bacterium. The circular bulge on the top right is the outline of a chromatophore, a vesicle where cyclic photophosphorylation occurs.]]

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 chloroplasts with the help of P<sup>32</sup>.

His first review on the early research of photophosphorylation was published in 1956.

References

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Sources

  • 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.
  • 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.