“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.”
“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
The efficiency is thought to be very close to 100% (exceptionally high for any complex system) which has led some researchers to suggests that quantum 'superposition' effects and/or quantum entanglement may be in operation with respect to the way that photons are somehow channelled towards the most efficient pathways within the light-harvesting complexes. These theories have not yet been generally accepted.