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 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 complex and has not yet been fully described - and the PS II atomic structure is not known.
Further reading : Florida State University, James Johnson.
Also see :