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a light-detecting, light-sensitive structure capable of generating physiological, that is, nerve or receptor, signals in response to the absorption of photons by its pigment molecules.
In a broad sense, the term “photoreceptor” is applied to all light-sensitive structures, ranging from the stigmata of unicellular organisms and from individual photosensitive cells scattered over the bodies of such organisms as worms and lancelets to the specialized visual cells of the eye, the complex organ of photoreception in animals and man. Photoreceptors also include various structures—such as the chloroplasts of plants, the plastids of algae, and the chromatophores of bacteria—that contain pigments and are responsible for such photobiological processes as photosynthesis, phototropism, phototaxis, and photoperiodism.
In the retina of the eye in man and other vertebrates the photoreceptors are the highly differentiated visual cells known as rod cells and cone cells; in invertebrates the retinular cells are photoreceptors. The photosensitive element of photoreceptor cells— the photoreceptor membrane—contains phospholipids and rhodopsin, which is a visual pigment that absorbs light. In the photoreceptors of vertebrates the photoreceptor membranes form the outer segments of the rods and cones; in invertebrates the photoreceptor membranes form numerous finger-like protruberances called microvilli; the densely packed system of the microvilli is called the rhabdomere of the visual cell.
The outer segment in vertebrates consists of numerous (up to 15,000 in deepwater fishes) disks or very flat sacs measuring about 160 angstroms in thickness and from 1–2 to 6–8 micrometers in diameter, depending on the species of animal. The disks are oriented strictly perpendicularly to the long axis of the cell. In the rods they float in the cytoplasm, since they are severed from the outer cell membrane; in the majority of the cones they maintain their connection with the membrane. In the rods, but not in the cones, the outer segment is continuously renewed through the formation of new apical disks and the atrophy and phagocytosis of the old ones.
Owing to the strict orientation of visual-pigment molecules in the photoreceptor membrane and to the special tubular packing of the membrane in the cell, many invertebrates are able to discern the direction of polarization of light and to orient themselves according to the light. The rods of vertebrates are the receptors of twilight, or scotopic, vision; the cones are responsible for day, or photopic, vision and for color vision. The compound eyes of insects are also capable of distinguishing color.
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M. A. OSTROVSKII [27–1760–1 ]