Oxygen evolution is the process of generating molecular

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

(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 break ...

, usually from water. Oxygen evolution from

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

is effected by oxygenic

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

, electrolysis 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 reactions 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 ...

s of

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( Embryophytes) have emerged deep in the Charophyte alg ...

and

plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...

s. It utilizes the energy of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

to split a water molecule into its protons and

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

s 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. ...

. Water oxidation is catalyzed by a

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

-containing cofactor contained in

photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photos ...

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 subunits ...

(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 t ...

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 s ...

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, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for

ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals, men's professional tennis governing body * American Technical Publishers, employee-owned publishing company * ', a Danish pension * Armenia Tree Project, non ...

synthesis via photophosphorylation 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 e ...

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 charact ...

was ''"not at all inconvenient to a

mouse A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

."'' 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, 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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

) is split during photosynthesis to release oxygen, while the

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

combined with water to form

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ...

s. 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 Cornelis Bernardus van Niel (also known as Kees van Niel) (November 4, 1897 – March 10, 1985) was a Dutch-American microbiologist. He introduced the study of general microbiology to the United States and made key discoveries explaining the ...

at Stanford University found that purple sulfur bacteria 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 formul ...

instead of releasing oxygen. He boldly proposed that, in analogy to the sulfur bacteria's forming elemental sulfur from H₂S (

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The und ...

), 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

(H₂), oxygen is evolved by electrolysis of water.

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 Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

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

References

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External links