Photolysis
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 energizes an electron associated with a chlorophyll molecule and is captured by an electron acceptor that is thereby reduced. This initiates a series of reactions that eventually lead to the reduction of NADP+ (nicotine adenine dinucleotide phosphate) to reduced NADPH. NADPH, required for the synthesis of fatty acids, is used in a series of reactions that fix carbon dioxide and reduce it to sugar. An electron passed along by photosystem II replaces the electron lost by the chlorophyll molecule.
Light absorbed by a chlorophyll molecule in photosystem II excites an electron in the pigment to a higher energy level. The high-energy electron is captured by a system of molecules that function as electron carriers. The electron carrier molecules can alternately accept an electron (and become reduced), and donate an electron to the next carrier (to become oxidized). Some of the energy that the electron gradually loses in this process is captured and stored in the form of adenosine triphosphate (ATP). The electron lost by the chlorophyll molecule is replaced from water in a series of reactions that lead eventually to the release of oxygen gas. This process is sometimes called the photolysis of water, although water molecules do not directly absorb the light energy.
The nature of the two photosystems has been extensively studied. It has been possible to separate the two systems by treating lamellar (thin-tissued) fragments of disrupted chloroplasts with detergents. Further treatment with charged detergents, and separation of components with electrophoretic techniques has led to the identification of components of the two systems. Each photosystem contains a light-harvesting complex of pigment molecules and a core complex containing a reaction center where the photolytic reactions actually take place.
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