The photoreceptors—rods and cones (fig. 10.36)—are activated when light produces a chemical change in molecules of pigment contained within the membranous discs of the outer segments of the receptor cells. Rods contain a purple pigment known as rhodopsin. The pigment appears purple (a combination of red and blue) because it transmits light in the red and blue regions of the spectrum, while absorbing light energy in the green region. The wavelength of light that is absorbed best—the absorption maximum—is about 500 nm (blue-green light).
Green cars (and other green objects) are seen more easily at night—when rods are used for vision—than are red objects. This is
■ Figure 10.36 Rods and cones. (a) A diagram showing the structure of a rod and a cone. (b) A scanning electron micrograph of rods and cones. Note that each photoreceptor contains an outer and inner segment.
because red light is not absorbed well by rhodopsin, and only absorbed light can produce the photochemical reaction that results in vision. In response to absorbed light, rhodopsin dissociates into its two components: the pigment retinaldehyde (also called retinene or retinal), which is derived from vitamin A, and a protein called opsin. This reaction is known as the bleaching reaction.
Retinene can exist in two possible configurations (shapes)—one known as the all-trans form and one called the 11-c/s form (fig. 10.37). The all-trans form is more stable, but only the 11-c/s form is found attached to opsin. In response to absorbed light energy, the 11-cis-retinene is converted to the all-fraws isomer, causing it to dissociate from the opsin. This dissociation reaction in response to light initiates changes in the ionic permeability of the rod plasma membrane and ultimately results in the production of nerve impulses in the ganglion cells. As a result of these effects, rods provide black-and-white vision under conditions of low light intensity.
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