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The retina is the part of the eye that contains cells that detect light. There are rod cells there that respond to dim light, and cone cells that give color vision in bright light. These types of cells are known as photoreceptor cells. They respond to light by transmitting a signal that triggers a cascade of biochemical reactions. This culminates in the transmission of the signal to the brain, which perceives it as vision.
The rod cells contain the pigment rhodopsin, which is highly sensitive to light. To enable humans to see at night, it absorbs green-blue light most strongly and appears purplish in color. Therefore, another name for this compound is visual purple.
There are a number of different light-sensing pigments and a large number of different opsins. These two types of compounds work together. The molecule that is responsible for the ability of rhodopsin to absorb light is bound to the opsin. This is a derivative of vitamin A known as retinal.
When light strikes the retinal molecule, it causes a change in the layout of the molecules. Studies of the structure of crystals of rhodopsin enable the detection of intermediates after the compound is irradiated. The immediate change is to a compound known as photorhodopsin, since light energy is transmitted as photons. This is very quickly followed by a second intermediate called bathorhodopsin.
This change in the structure of retinal causes a change in the conformation of the opsin. The opsin then activates a protein associated with it, which acts as a receptor. A receptor is a compound that transmits signals from an external response to cause a cascade of events. This culminates in the transmission of the signal to the brain to be perceived as night vision.
Rhodopsin binds to a very common class of receptors known as G proteins. It is considered to be a member of the G-protein coupled receptor family. This is a very large class of receptors that respond to many external signals, including hormones and odors.
Since rhodopsin is essential for night vision, mutations in the gene can lead to visual problems. There are inherited genetic disorders of these mutations. Some can lead to a form of night blindness. Others can lead to retinitis pigmentosa. This disease can start out with the loss of night vision and proceed through the loss of peripheral vision to total vision loss.