The light-sensitive cells in our eye - the photoreceptors - are known as rods and cones. Cone cells give us our colour-vision when the light is bright enough, and rods take over to give us a monochrome view of the world when the light levels are very dim. These two cell types have long been thought to mediate all of the light responses of the vertebrate eye. But according to a report in the 2 July 1998 issue of Nature, there are light-sensitive pigments in other nerve cells in the fish retina, suggesting that photoreception is not just restricted to rods and cones.
We can identify a photoreceptor cell by the photopigment it contains. Lots of cells in the eye contain pigments, such as melanin, to absorb light at the back of the eye, and shielding pigments to separate facets of the insect eye. But photopigments are a bit different. They are made up of a protein molecule, opsin, which usually sits in the cell membrane, and a so-called ?chromophore?, the active region, made from vitamin A. When light hits the chromophore, it changes shape and is released from the opsin. Opsin molecules activated in this way in turn set off nerve signals.
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