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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, but we know many details about this process."

Source : Wikipedia

After many decades of research, much of the fine-detail of the highly complex photosynthesis (PS) process is now understood. It's now known, for example, that four photons are required to initiate PS, and that it takes only a millisecond or so to split water to produce oxygen as a by-product. (For complex technical details, see the two Nature papers linked below.)

Several crucial steps, however, remain undescribed.

Current research suggests that the oxygen moves through stages which include a complex peroxide - but the exact details are still not known :

The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide."

Source : Structural evidence for intermediates during O2 formation in photosystem IINature, May 2023

For further extensive technical details see : The electron–proton bottleneck of photosynthetic oxygen evolutionNature, May 2023

PS efficiency

The efficiency of photosynthesis (PS) is thought to be very close to 100% (exceptionally high for any complex system) which has led some researchers to suggest that quantum 'superposition' effects and/or quantum entanglementplugin-autotooltip__plain plugin-autotooltip_bigQuantum entanglement

Quantun Entanglement was first predicted by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935.

It has now been (unequivocally) experimentally demonstrated with photons, neutrinos, electrons, and even molecules and crystals :

In 2019, it was announced that 'superposition' had been achieved in an organic compound with more than 2,000 atoms. (ref.
may be in operation with respect to the way that photons are somehow channeled towards the most efficient pathways within the light-harvesting complexes. These theories have not yet been generally accepted.

Example research paper :

Photosynthetic organisms capture visible light in their light-harvesting complex and transfer the excitation energy to the reaction center which stores the energy from the photon in chemical bonds. This process occurs with nearly perfect efficiency." […] "The high efficiency of this process is not yet understood today."

Source : Gain and loss in open quantum systems, Institute for Quantum Science and Engineering, Texas A&M University, and Max Planck Institute for the Physics of Complex Systems, 2017

Also see Oxygen Evolving Process (EVP)plugin-autotooltip__plain plugin-autotooltip_bigOxygen Evolving Process (EVP)

"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.
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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