Photopigments
Rod and cone cells contain photopigments. These are the compounds that that react when exposed to light and start the chemical cascade that results in a neural stimulus .
Photopigments in Rods
All rod cells are identical in terms of the photopigments they contain. The photopigments are located in a specialized region of the cell, often called membrane disks.
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The photopigment in rod cells is called rhodopsin. Rhodopsin is composed of two components. The first is a membrane bound protein called opsin. Opsin is covalently bound to a derivative of Vitamin A, retinal. The structure of retinal can be seen below.
11-cis-retinal
Retinal has multiple double bonds that usually exist in a mostly trans conformation with the exception the C-11 double bond which is in the cis conformation. This conformation can be seen to the left. When retinal is exposed to light, the electrons in the highest energy pi orbital are excited from pi to pi* orbitals. Once these electrons are excited to antibonding orbitals, the carbons between C-11 and C-12 are able to rotate freely around this bond. This allows retinal to change its conformation. Retinal can now take on an all trans conformation that can be seen below.
trans-retinal
In most compounds visible light does not have enough energy to promote electrons from one energy level to another. Typically ultraviolet (UV) light is required for such transitions. Retinal is unique in that it has an extensive conjugated diene system. Conjugated diene systems typically have low energy differences between their highest occupied molecular orbital (HOMO) and their lowest unoccupied molecular orbital (LUMO). Because the HOMO/LUMO gap is so small in retinal due to its conjugated system, visible light is able to cause this change in conformation. When retinal is bound to opsin in rod cells, it optimally absorbs light at 498 nm, but absorbs light ranging in wavelength from 400 - 600 nm. This means that rod cells are unable to absorb wavelengths greater than 600 nm.
After the Change
Retinal cells fire tonically without activation. This means that unless a stimulus (light) is presented, a rod cell continually releases a chemical signal (neurotransmitter) to the bipolar cells with help to process visual information. When light causes the conformational change in retinal. This conformational change alters how opsin interacts with its surroundings. This alteration causes a chemical cascade that results in less neurotransmitter release to the bipolar cell. This can indicate to the larger nervous system that that cell has been actvated.
Photopigments in Cones
There are three different kinds of cone cell. Each type of cone cell is structurally similar to rod cells. As in rods retinal is bound to an opsin protein. However each cone can express one of three very similar opsin proteins. These protiens differ from one another by only a few alterations in their amino acid sequence. These alterations in the opsin protiens alter how retinal absorbs light. In one kind of cone cell called an L cone, retinal optimally absorbs light at 564 nm. These cone cells are also called red cones because they are the cells that most strongly absorb red light. In another kind of cone cell called an M-cone cell, retinal most strongly absorbs light at 534 nm. This kind of cone cell is also called a green cone. In the third cone cell, the S-cone cell, light of 420 nm is most strongly absorbed. This is called the blue cone cell.