Abstract
This review deals with the production of oxygen by photo-oxidation of water, which is a topic fitting a journal devoted to oxygen. Most of the present biosphere, including mankind, depends on oxygen. Elucidating the mechanism is of importance for solving the present energy crisis. Photosynthesis evolved in bacteria, first in a form that did not produce oxygen. The oxygen-producing version arose with the advent of cyanobacteria about three billion years ago. The production of oxygen by photo-oxidation of water requires the co-operative action of four photons. These are harvested from daylight by chlorophyll and other pigments (e.g., phycobiliproteins) and are channeled to photosystem II and photosystem I. The oxygen-evolving complex resides in photosystem II, surrounded by protein subunits, and contains one ion of calcium, four ions of manganese, and a number of oxygen atoms. For each quantum of energy it receives from absorbed light, it proceeds one step through a cycle of states known as the Kok–Joliot cycle. For each turn of the cycle, one molecule of oxygen (O2) is produced.