Molecular oxygen of the biosphere is formed during photosynthesis of plants, which takes place in the complicated permolecular structure of the oxygen-evolving complex (OEC) of chloroplast membranes as a result of oxidizing water molecules by using solar light energy. The absorption of light energy, its transformation into electrochemical energy of separated charges, and accumulation of oxidative equivalents in the water-oxidizing center (WC) on Mn cations performing water oxidation with O2 formation are implemented in the complex. In order to decipher the mechanism of the functioning of the OEC, we have developed the method for chromatographic isolation of the complex from chloroplast membranes and pursued its comprehensive research. Based on the obtained data, we have ascertained the OEC to represent a dimer of pigment-lipoprotein complexes of photosystem 2 (PLPC PS-2) associated by the mirror symmetry rule on the basis of hydrophobic bonds. The dissociation of the OEC into monomeric complexes was observed under the action of heat (33-34о С), solubilizing detergents, and heavy metals and resulted in the inhibition of the oxygen formation function. As a result of studying the structural transitions and regularities of the functioning of the OEC, we have first suggested the model of the molecular organization of the dimeric OEC and topographic location of its main proteins that has already been proved. The formation of the WC has been shown to be determined by the symmetry in the location of two unidentant ligands (TyrZ) coordinationally bound with Mn2+ cations in the structure of a hydrophobic boiler formed during the association of the monomeric PLPC PS-2. The oxygen formation mechanism has been developed, according to which the photochemical reactions taking place in turn in the left and right PLPC PS-2 in the process of the four-quantum cycle result in the removal of four electrons from two oppositely located TyrZ–liganded Mn cations. This determines the formation of a molecular two-anode reactor, in which photooxidized manganese cations enter into the reaction of photohydrolysis with water molecules and form a dihydroxide association [Mn4+(OH)···(HO)Mn4+] that determines the spontaneous electron density disproportionation between Mn cations and oxygen atoms of the hydroxyl groups of water molecules incorporated into the association. The synchronous and equalrate but oppositely directed transfer of two pairs of electrons from the hydroxyl groups of water molecules to Mn4+ cations is the condition for the formation of molecular σ-π-orbitals of O2. As a result, two Mn2+ cations are formed, molecular oxygen evolves, protons are released, and the initial state of the WC is regenerated. The quantum-chemical modeling has proved the suggested mechanism of molecular oxygen formation.