A biophoton (from the Greek βίος meaning "life" and φῶς meaning "light") is a photon of non-thermal origin in the visible and ultraviolet spectrum emitted from a biological system. Emission of biophotons is technically a type of bioluminescence, but the latter term is generally reserved for higher luminance luciferin/luciferase systems. The term biophoton used in this narrow sense should not be confused with the broader field of biophotonics, which studies the general interaction of light with biological systems.
The typical observed radiant emittance of biological tissues in the visible and ultraviolet frequencies ranges from 10−19 to 10−16 W/cm2 (approx 1-1000 photons/cm2/second). This light intensity is much weaker than that seen in the perceptually visible and well-researched phenomenon of normal bioluminescence but is detectable above the background of thermal radiation emitted by tissues at their normal temperature.
While detection of biophotons has been reported by several groups, hypotheses that such biophotons indicate the state of biological tissues and facilitate a form of cellular communication are controversial.
Detection and measurement
Proposed physical mechanisms
Chemi-excitation via oxidative stress by reactive oxygen species and/or catalysis by enzymes (i.e., peroxidase, lipoxygenase) is a common event in the biomolecular milieu. Such reactions can lead to the formation of triplet excited species, which release photons upon returning to a lower energy level in a process analogous to phosphorescence. That this process is a contributing factor to spontaneous biophoton emission has been indicated by studies demonstrating that biophoton emission can be increased by depleting assayed tissue of antioxidants or by addition of carbonyl derivatizing agents. Further support is provided by studies indicating that emission can be increased by addition of reactive oxygen species.
Imaging of biophotons from leaves has been used as a method for Assaying R Gene Responses. These genes and their associated proteins are responsible for pathogen recognition and activation of defense signaling networks leading to the hypersensitive response, which is one of the mechanisms of the resistance of plants to pathogen infection. It involves the generation of reactive oxygen species (ROS), which have crucial roles in signal transduction or as toxic agents leading to cell death.
Biophoton have been observed in stressed plant's roots, too. In healthy cells, the concentration of ROS is minimized by a system of biological antioxidants. However, heat shock and other stresses changes the equilibrium between oxidative stress and antioxidant activity, for example, the rapid rise in temperature induces biophoton emission by ROS.
Hypothesized involvement in cellular communication
In the 1920s, the Russian embryologist Alexander Gurwitsch reported "ultraweak" photon emissions from living tissues in the UV-range of the spectrum. He named them "mitogenetic rays" because his experiments convinced him that they had a stimulating effect on cell division.
Biophotons were claimed to have been employed by the Stalin regime to diagnose cancer. The method has not been tested in the West. However, failure to replicate his findings and the fact that, though cell growth can be stimulated and directed by radiation this is possible only at much higher amplitudes, evoked a general skepticism about Gurwitsch's work. In 1953 Irving Langmuir dubbed Gurwitsch's ideas pathological science. Commercial products, therapeutic claims and services supposedly based on his work appear at present to be best regarded as such.
But in the later 20th century Gurwitsch's daughter Anna, Colli, Quickenden and Inaba separately returned to the subject, referring to the phenomenon more neutrally as "dark luminescence", "low level luminescence", "ultraweak bioluminescence", or "ultraweak chemiluminescence". Their common basic hypothesis was that the phenomenon was induced from rare oxidation processes and radical reactions. In the 1970s Fritz-Albert Popp and his research group at the University of Marburg (Germany) showed that the spectral distribution of the emission fell over a wide range of wavelengths, from 200 to 750 nm.  Popp proposed that the radiation might be both semi-periodic and coherent.
One biophoton mechanism focuses on injured cells that are under higher levels of oxidative stress, which is one source of light, and can be deemed to constitute a "distress signal" or background chemical process is yet to be demonstrated. The difficulty of teasing out the effects of any supposed biophotons amid the other numerous chemical interactions between cells makes it difficult to devise a testable hypothesis. A 2010 review article discusses various published theories on this kind of signaling.
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