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Oxygen evolution is the process of generating molecular oxygen (O2) by a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breakin ...
, usually from water. Oxygen evolution from
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
is effected by oxygenic photosynthesis,
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from na ...
of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distillation of liquified air.


Oxygen evolution in nature

Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in earth's
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
. The reaction is part of the
light-dependent reaction Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...
s of photosynthesis in cyanobacteria and the
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. Water oxidation is catalyzed by a manganese-containing cofactor contained in photosystem II known as the
oxygen-evolving complex The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the portion of photosystem II where photo-oxidation of water occurs during the light reactions of photosynthesis. The OEC is surrounded by four core protein subunit ...
(OEC) or water-splitting complex. Manganese is an important cofactor, and
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
and
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
are also required for the reaction to occur. The stoichiometry this reaction follows: : 2H2O ⟶ 4e + 4H+ + O2 The protons are released into the
thylakoid lumen Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast th ...
, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via
photophosphorylation In the process of photosynthesis, the phosphorylation 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 ...
and coupling the absorption of light energy and oxidation of water to the creation of chemical energy during photosynthesis.


History of discovery

It was not until the end of the 18th century that
Joseph Priestley Joseph Priestley (; 24 March 1733 – 6 February 1804) was an English chemist, natural philosopher, separatist theologian, grammarian, multi-subject educator, and liberal political theorist. He published over 150 works, and conducted exp ...
discovered by accident the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by
vegetation Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic character ...
was ''"not at all inconvenient to a mouse."'' He was later awarded a medal for his discoveries that: ''"...no vegetable grows in vain... but cleanses and purifies our atmosphere."'' Priestley's experiments were followed up by
Jan Ingenhousz Jan (or John) Ingenhousz or Ingen-Housz FRS (8 December 1730 – 7 September 1799) was a Dutch-born British physiologist, biologist and chemist. He is best known for discovering photosynthesis by showing that light is essential to the process ...
, a Dutch physician, who showed that "restoration" of air only worked in the presence of light and green plant parts. Ingenhousz suggested in 1796 that CO2 ( carbon dioxide) is split during photosynthesis to release oxygen, while the carbon combined with water to form carbohydrates. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student C.B. Van Niel at
Stanford University Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is considere ...
found that
purple sulfur bacteria The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including h ...
reduce carbon to carbohydrates, but accumulate
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
instead of releasing oxygen. He boldly proposed that, in analogy to the sulfur bacteria's forming elemental sulfur from H2S ( hydrogen sulfide), plants would form oxygen from H2O (water). In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO2. This discovery was made by Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO2 was called the ''Hill reaction''. Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing isotopes of oxygen from water to oxygen gas.


Water electrolysis

Together with hydrogen (H2), oxygen is evolved by
electrolysis of water Electrolysis of water, also known as electrochemical water splitting, is the process of using electricity to decompose water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, or rem ...
. Electrons (e) are transferred from the cathode to protons to form hydrogen gas. The half reaction, balanced with acid, is: :2 H+ + 2e → H2 At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and releasing electrons to the anode to complete the circuit: :2 H2O → O2 + 4 H+ + 4e Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen: :Overall reaction: :2 H2O → 2 H2 + O2


Chemical oxygen generation

Chemical oxygen generators consist of chemical compounds that release O2 upon some stimulation, usually heat. They are used in submarines and commercial aircraft, providing emergency oxygen. Oxygen is generated by high-temperature decomposition of sodium chlorate: :2 NaClO3 → 2 NaCl + 3 O2 Potassium permanganate also releases oxygen upon heating, but the yield is modest. :2 KMnO4 → MnO2 + K2MnO4 + O2


See also

*
Great Oxygenation Event The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...


References

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External links

*Plant Physiology Online, 4th edition
Topic 7.7 - Oxygen Evolution
- Lecture notes by Antony Crofts, UIUC
Evolution of the atmosphere
– Lecture notes, Regents of the University of Michigan
How to make oxygen and hydrogen from water using electrolysis
Photosynthesis Breathing gases Oxygen Biological evolution