Photoreception(redirected from photoreceptor cell)
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The process of absorption of light energy by plants and animals and its utilization for biologically important purposes. In plants photoreception plays an essential role in photosynthesis and an important role in orientation. Photoreception in animals is the initial process in vision. See Photosynthesis, Taxis, Vision
The photoreceptors of animals are highly specialized cells or cell groups which are light-sensitive because they contain pigments which are unstable in the presence of light of appropriate wavelengths. These light-sensitive receptor pigments absorb radiant energy and then undergo physicochemical changes, which lead to the initiation of nerve impulses that are conducted to the central nervous system. See Eye (invertebrate), Eye (vertebrate)
the detection of light by unicellular organisms or by photoreceptors, that is, specialized structures containing light-sensitive pigments.
In photoreception, which is one of the basic photobiological phenomena, light serves as a source of information. In photosynthesis, the energy of light is used to power chemical reactions; in photoreception, it performs a triggering informational function that sets off a complex chain of molecular, membranous, and cellular processes. These processes are responsible for comparatively simple types of photoreception, such as phototropism, or the orientation of sessile animals and plants toward a source of light; phototaxis, or the directed movement of freely moving organisms toward or away from light; and photokinesis, or the undirected increase or decrease in the mobility of an organism in response to changes in the level of light. The most complex and highest form of photoreception is vision; it is effected by special organs, which vary in their degree of perception fidelity.
The study of the evolution and comparative physiology of photoreception is of great interest. In protozoans the primitive photoreceptor system consists of an eyespot and flagellum, that is, a receptor and an effector. In Chlamydomonas the photosensitive eyespot is connected to a chloroplast; in Euglena the eyespot is connected directly to a flagellum. Most invertebrates and some vertebrates, such as certain species of fish and amphibians, have a diffuse sensitivity to light, and photoreceptors are not used. In some species light is perceived through chromatophores, that is, special cellular organelles. Nonspecialized photosensitive elements may be scattered over the body or may be concentrated on the surface or below it.
Visual photoreception is accomplished in photoreceptors. The stigmata and ocelli of protozoans and the ocelli of coelenterates, of flat and segmented worms, and of arthropods can be regarded as the simplest forms of the organ of vision. The structure and function of the photoreceptor system is more complex in mollusks. In the octopus and cuttlefish, for example, the system is comparable to the eye of vertebrates. The highly specialized photoreceptors in the compound eye of arthropods and in the chambered eye of vertebrates form the most developed organs of vision. The primary processes of vision are common to all animals, and they take place in the light-sensitive photoreceptor membrane of the visual cell. The composition and molecular structure of the membrane is essentially identical in vertebrates and invertebrates. The differences are, as a rule, in the methods by which the membranes are packed in the light-detecting parts of the various photoreceptors. The principal light-sensitive element of the photoreceptor membrane is visual pigment, a typical and well-studied example of which is rhodopsin.
From the point of view of comparative biochemistry, it is of particular interest that retinal, a β-carotene derivative, serves as a chromophore for absolutely all the visual pigments. Moreover, of all the possible isomers, only 11-cis-retinal can be the chromophore part of a visual pigment molecule. Thus, the molecular biochemical principle underlying the photoreceptor mechanism proved to be phylogenetically fixed. The protein part of the visual pigment molecules is species-specific. The specificity of the protein is apparently responsible for the differences in spectral sensitivity of the retinal cones in color vision. The physicochemical mechanism of photoreception is based on complete photoisomerization of retinal from 11-cis-retinal into trans-retiqal. This photoreaction alters the structure of the protein part of the visual pigment molecule and the functional properties of the photoreceptor membrane. As a result, the ions shift in the visual cell, and the rates of some enzymatic reactions may change. The light-induced changes in the visual pigment molecule and photoreceptor membrane ultimately produce in the receptor cell a visual signal, that is, a propagated photoreceptor electric potential.
REFERENCESProsser, L., and F. Braun. Sravnitei’naia fiziologiia zhivotnykh. Moscow, 1967. Chapter 12. (Translated from English.)
Fiziologiia sensornykh sistem, part 1. (Rukovodstvo po fiziologii.) Leningrad, 1971. Pages 88–119.
Handbook of Sensory Physiology, vol. 7/1–7/2. Berlin, 1972.
M. A. OSTROVSKII