Bioluminescence is a type of reaction that involves the emission of electromagnetic radiation in the visible and near infrared, by certain living organisms. Bioluminescence is to be considered a particular case of another phenomenon, luminescence. The latter is characterized by the emission of radiation without the emission of heat. For luminescence to occur, there must be an electronic transition (quantum jump). This means that electrons, carriers of charge, pass from a higher energy state – more excited, in jargon – to a lower one, releasing a photon (simplifying … light).

Normally, electrons are excited if they are exposed to heat sources, but it may happen that the trigger is instead a set of chemical or biochemical reactions. In the second case, we speak of chemiluminescence, in the third of bioluminescence, rather than luminescence.

Bioluminescence is found in both terrestrial (e.g., fireflies) and marine animals (Crustacea, Selaci, Hydrozoa, Teleostei, etc.). Among plants are luminescent some varieties of Fungi, saprotrophic bacteria that develop on decaying animal tissues, numerous symbiotic microorganisms of fish and marine molluscs. Bioluminescence is a phenomenon of biochemical origin related to the synthesis of particular luminous substances that accumulate in the luminescent organism (intracellular bioluminescence) or that are secreted into the external environment, as occurs in some marine organisms (extracellular bioluminescence).

The emission of light occurs as a result of the oxidation of a photogenic substance generically called luciferin, whose molecular structure varies from species to species. This oxidative reaction involves the use of atmospheric oxygen or dissolved in water, and is catalyzed by the enzyme luciferase. In some species, including fireflies, the oxidation of luciferin requires not only the presence of oxygen, but also an adequate supply of ATP.

Little is known about the biological significance of luminescence, but it is known that fireflies have the function of sexual attraction and defense, and that various species of fish have the function of defense, attraction or illumination of the environment (which gives rise to marine luminescence).

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