KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY KENNETH EWARD / BIOGRAFX / SCIENCE PHOTO LIBRARY
Visual pigment structure. Computer model of the structure of the pigment visual purple, present in rod cells in the retina of the human eye. The blue represents the protein rhodopsin, which is bound to a molecule of the light-sensitive compound (known as a chromophore) retinal (yellow). When light strikes a rod cell, the retinal molecule undergoes a structural change caused by the light energy. This in turn alters the conformation of the rhodopsin protein, which sets off a reaction leading to a signal being sent to the brain. The pigment is then converted back to its original purple is used for vision at low light levels.
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