Photobiology

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Photobiology

 

a division of biology that deals with processes occurring in organisms upon exposure to visible, ultraviolet, and near-infrared radiation.

It has been known since antiquity that light affects the growth and development of organisms and their various functions. The foundation of photobiology was laid in the 18th and 19th centuries by a number of scientific advances, including the description of phototaxis, the development of the basis of color vision theory by H. Helmholtz, and the discovery of photosynthesis by the English chemist J. Priestley, the Dutch scientist J. Ingenhousz, and the Swiss researcher J. Senebier.

It was not until the second half of the 20th century, however, that photobiology emerged as a separate scientific field. Its new importance resulted from the development of the quantum theory of radiation, which forms the physical basis of photobiology, and from progress in biochemistry, biophysics, and physiology. The growth of photobiology was also facilitated by the introduction of new experimental methods, including differential and flash spectrophotometry, techniques of luminescence measurement, and electron paramagnetic resonance methods. The fundamental studies of A. N. Terenin and his followers in the spectroscopy, photochemistry, and photon characteristics of complex molecules stimulated the development of photobiology in the USSR.

Photobiology is involved in every vital activity of plants and animals. Photobiology is divided into a number of branches according to the functions performed by the various phenomena studied. One area of study is plant photosynthesis, that is, the energy processes associated with the storage of solar energy in synthesized biological compounds. Another area of study deals with the sensory and regulatory reactions of organisms upon exposure to light, for example, the vision of animals, phototaxis, phototropism, photoperiodism, the effect of light on the synthesis of such substances as vitamins and pigments, the stimulation by light of cell division and of the growth and development of the organism. Other areas of study are the biological effect of ultraviolet radiation; the effect of various kinds of radiation on the evolutionary process, origin of life, and maintenance of the ecological balance; and destructive photoprocesses. Examples of destructive processes are the photodenaturation and photooxidation of proteins, photoinactivation of enzymes and nucleic acids, damage to cells and tissues from ultraviolet radiation, the photodynamic effect of visible light, and the effect of visible light on repair processes after cell damage by ultraviolet radiation. Bioluminescence, that is, the emission of light by organisms as a result of the transformation of chemical energy into light energy, is often an object of study in photobiology.

In spite of the diversity of the photobiological phenomena mentioned above, their initial physical and chemical stages are similar. This fact has led to the development of a branch of photobiology that studies the principles and molecular mechanisms of photobiological processes. The general problems of photobiology include the establishment of the principles of the transformation of the energy of photons into the energy of chemical bonds and into membrane potential, the coupling of the photochemical and dark enzymatic phases in photobiological processes, the study of the function and molecular organization of photoreceptors, and the investigation of the causes of the high efficiency of photobiological processes. Clearly, in order to solve these problems, it was necessary to shift to the study of subcellular and molecular levels. The new approach led to a rapid development of molecular photobiology.

For photobiological processes to occur, the presence of photo-receptors is required. These photoreceptors absorb light selectively and are localized in special cell structures—the chloroplasts in higher plants, chromatophores in algae and bacteria, and melanophores in animal cells—and in the rods and cones of the retina of the eye. Photoreceptors of plants include chlorophylls, their various analogues and derivatives, carotenoids, phycobilins (including phytochromes), and certain coenzymes (for example, flavins). Animal pigments are the visual pigments and melanins; the latter are the most important animal pigments. The aromatic amino acids of proteins, the nucleic acids, and many other biologically active compounds are photoreceptors in the ultraviolet spectral region.

According to present-day theory, the molecular mechanism of. photobiological processes may be represented as a sequence of three stages. The first stage is the absorption of a photon by a photoreceptor together with the formation of singlet and triplet excited states or, in some cases, together with the migration of the energy of electron excitation to an active site. In the second stage the initial photochemical or structural changes of the molecules occur. In the third stage coupling of the photochemical and enzymatic stages leads to the final physiological effect.

Photobiology serves as the theoretical basis for increasing the efficiency of the photosynthesis of agricultural plants, for raising plants by artificial means, for accelerating the growth of farm animals, and for using radiation in medical practice and in the control of environmental pollution. Studies in the field of photobiology are closely linked with such problems as the use of laser radiation in biology, the biological use of solar energy, and the development of artificial systems based on the principles of photobiological phenomena, for example, systems for obtaining hydrogen by the biophotolysis of water.

In the USSR, studies in photobiology are conducted in such scientific institutes of the Academy of Sciences of the USSR as the A. N. Bakh Institute of Biochemistry, the K. A. Timiriazev Institute of Plant Physiology, the Institute of Photosynthesis, and the Institute of Biophysics. Research is also carried on at such other institutions as the Institute of Photobiology of the Academy of Sciences of the Byelorussian SSR in Minsk, the department of biology of Moscow State University, and the Second Moscow Medical Institute. Papers on photobiology are published in such journals as Doklady Akademii Nauk SSSR (since 1922), Biofizika (since 1956), Biokhimiia (since 1936), and Molekuliarnaia biologiia (since 1967). The international journal Photochemistry and Photobiology has been published in the USA since 1962.

World research in photobiology is coordinated by the International Committee of Photobiology, which was founded in 1951 and since 1955 has been part of the International Union of Biological Sciences. The committee is concerned with the development of photobiological studies and arranges international congresses. A total of seven congresses has been held: in Amsterdam (1954); Turin, Italy (1957); Copenhagen (1960); Oxford, Great Britain (1964); Hanover, USA (1968); Bochum, Federal Republic of Germany (1972); and Rome (1976).

REFERENCES

Terenin, A. N. Fotonika molekul krasitelei i rodstvennykh organicheskikh soedinenii. Leningrad, 1967.
Smith, K., and P. Hanawalt. Molekuliamaia fotobiologiia. Moscow, 1972. (Translated from English.)
Konev, S. V., and I. D. Volotovskii. Fotobiologiia. Minsk, 1974.
Krasnovskii, A. A. Preobrazovanie energii sveta pri fotosinteze: Molekuliarnye mekhanizmy. Moscow, 1974.
Wolken, J. J. Photobiology. New York, 1968.
Photophysiology, vols. 1–7. New York-London, 1964–75.

A. A. KRASNOVSKII and F. F. LITVIN

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