Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in earth's biosphere. The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts 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.

Source : Wikipedia

The reaction 2H₂O ⟶ 4e^− + 4H⁺ + O₂ is responsible for the continuation of all aerobic (i.e. oxygen breathing) life on Earth.

The production of oxygen from plants was first noted in the late 18th century, and has been very extensively studied since then. It's known that the oxygen (which originates from the plant's internal water) is generated in the plant's Oxygen Evolving Complex (OEC), using the so-called 'PS II' system. It involves the transfer of electrons (possibly one at a time), facilitated by a set of proteins.

A widely accepted theory from the 1970's (the Joliot and Kok cycle) proposes that the OECs have five 'states' and that manganese and calcium ions are essential to the process.

The process is extremely complicated and has not yet been fully described. There appear to be some steps involving complex peroxides which are not yet understood.

For 2023 research technical details see Structural evidence for intermediates during O2 formation in photosystem II

In addition to the process puzzles, the PS II atomic structure is also unknown.

Further reading : Florida State University, James Johnson.

Also see : Photosynthesisplugin-autotooltip__plain plugin-autotooltip_bigPhotosynthesis

"Photosynthetic water splitting (or oxygen evolution) is one of the most important reactions on the planet, since it is the source of nearly all the atmosphere's oxygen [...] The mechanism of water oxidation is still not fully elucidated, and RuBisCO anomalyplugin-autotooltip__plain plugin-autotooltip_bigRuBisCO anomaly

RuBisCO (full name : Ribulose-1,5-bisphosphate carboxylase-oxygenase) is a crucial enzyme involved in the conversion of CO2 into sugars during photosynthesis. Without it, photosynthesising plants (and all other organisms which depend on plants, or require oxygen) could not exist. It's so widespread, some have called it

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