Photolysis is a term used to describe the mechanism of photochemical reactions initiated by the absorption of visible, infrared, or ultraviolet radiation. When a molecule absorbs a photon of radiant energy, it is energized to a higher level than is possible by ordinary heating. As a consequence, the molecules may be split into smaller entities in a manner different from normal thermal reactions. This initial splitting is the first stage in photolysis (splitting or loosening by light). The subsequent reactions of these energized components convert radiant energy into chemical energy. The immediate response to an absorbed photon is called the primary photochemical process. Subsequent changes are also a part of the photochemical reaction.

The best-known biological example of photolysis occurs in photosynthesis, in which two distinct photosystems, photosystem I and photosystem II, function in a coordinated manner to produce oxygen and fix carbon dioxide.

In photosystem I, light...