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 'The most common enzyme on Earth'.

Under well-lit conditions, the enzyme bonds with Carbon Dioxide from the atmosphere - but, around 25% of the time it links to Oxygen instead. The plant then needs to go through a complex set of conversions to eliminate the (apparently unwanted) oxygenated products.

Some researchers assert that the O₂ absorption - known as Photorespiration - is, in effect, a 'faulty' process somehow locked into the plants' evolution in the distant past.

Others point to possible benefits - e.g. that it may play a role in facilitating Nitrogen assimilation from the soil.

A third viewpoint is that the O₂ absorption process serves some as-yet-undiscovered purpose.

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 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.

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