Rhodopsin

rhodopsin, also called visual purple, pigment-containing sensory protein that converts light into an electrical signal. Rhodopsin is found in a wide range of organisms, from vertebrates to bacteria. In many seeing animals, including humans, it is required for vision in dim light and is located in the retina of the eye—specifically, within the tightly packed disks that make up the outer segment of the retina’s photoreceptive rod cells, which are specially adapted for vision under low-light conditions.

Rhodopsin was discovered in 1876 by German physiologist Franz Christian Boll, who observed that the normally reddish purple frog retina turned pale in bright light. The fading of colour was later attributed to the destruction of rhodopsin, via a process known as bleaching. Bleaching and the subsequent regeneration of rhodopsin are major steps in the visual cycle—the series of biochemical reactions that is critical for vision in low light.