Like all objects, cells emit a characteristic "black body" distribution of wavelengths of photons, in a manner directly related to temperature. However, after compensating for this distribution, a number of photons (on the order of as many as 100 photons/cm²/sec) are detected over a range of wavelengths in the radio and microwave to ultraviolet range. The amount of light emitted is quite small; comparable to that observed from a candle viewed at a distance of 10 kilometers. The detection of these photons waited upon development of sensitive photomultipliers in the 1960s and 1970s.
It is not particularly surprising for a cell's metabolism to produce light; for example, many bacteria and other cells produce light through the use of a particular protein (luciferin). Given the extremely small number of photons produced (the above number corresponds roughly to a single photon per cell per hour, assuming a rather large cell diameter of 100 micrometers), for many years the predominant theory was that these photons were a random by-product of cellular metabolism.
Normal cell metabolism occurs in a chain of steps, each step involving a small energy exchange, for greater efficiency. With some degree of randomness ensured by thermodynamics, it would then be expected that some (unknown) number of these chains would possibly "skip" one or more steps. The resulting loss of efficiency would then be detected as a photon being emitted.
According to the simplest model of this theory, the observed frequency at which photons would be detected would then be expected to obey a standard random distribution. However, some scientists have claimed to detect a significant variance from the expected distribution of photons, as well as an additional coherence or coordination of the time when photons are emitted by distinct cells. The photons emitted as part of this (unknown) luminescent process were dubbed "biophotons" (by F. A. Popp) to indicate their origin.
At present there is no adequately tested theory for the production of these extra photons; and the final answer may require a careful examination of the experimental method, and could involve a variety of modes of production. For example, in keeping with the "random production" theory, biophotons are more prevalent in damaged cells, presumably due to the extra presence of free radicals.
In the absence of a mechanism which produces these photons, some have speculated that biophotons are involved in various cell functions such as mitosis; or alternatively that they are produced and detected by proteins in the cell nucleus, possibly DNA.
It is further speculated by some that these emissions are part of a system of cell to cell communication of more complexity than the modes of cell communication already known, such as chemical signalling; and that they are important in the development of larger structures such as the organss.
Some within eastern religions or new-age religions have claimed that one might be able to associate biophotons with the concept of "Qi", a mystical energy source within living beings posited by some religion. Scientists have found no evidence for the existence of Qi, and regard it as a purely religious construct; attempts to find a scientific basis for Qi have thus far failed, and are generally viewed as pseudoscience. A few people have posulated that these emissions are related to consciousness; no data exists to support these hypotheses.
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