Rhodopsin is a light absorbing pigment; it competently absorbs light in the middle of the visual spectrum with a maximum at 500nm. Rhodopsin is the major protein of the rod outer segment (ROS), a cellular domain that captures light and initiates the visual response. Rhodopsin is made in the rod inner segment (RIS) and is transform ported from there to the ROS on small membrane spheres called vesicles.
The proteins Rhodopsin hold the protonated retinal-Schiff's base complex, which naturally lies in the inter-membrane pocket formed by the 7 transform-membrane a-helical receptors. There are a lot of flat discs of rhodopsin within the outer segment of a rod cell, which upon light detection experience a photo-isomeric change from Rhodopsin (11-acisform) to all-transform retinal.
Rhodopsin consists of the protein opsin associated to 11-acisform retinal a prosthetic group. Retinal is the light absorbing pigment molecule and is a derivative of vitamin A. Opsin is a member of the 7TM receptor family. It catalyses the simply light responsive step in vision. The 11-acisform-retinal chromophore lies in a pocket of the protein and is isomerizes to all-transform retinal when light is absorbed. The isomerisation of retinal leads to a change of the shape of rhodopsin, which triggers a sequence of reactions, which lead to a nerve impulse. This is transformatted to the brain via the optical nerve.
Rhodopsin triggers an enzymatic cascade procedure consequential in the hydrolysis of GMP. This in turn closes cation-specific channels within the rod cell membrane, which are naturally open to influx of Na + in the dark, and due to the effect of hyperpolarisation, the inner synaptic body sends a nerve signal to other neurons in the Retina. In conclusion the light induce lowering of calcium levels aids recovery of keyed up neurons to a passive, dark state and the cycle starts once more upon detection of light. The photoreceptors of cone cells are also 7 a helical receptors with 11 aciform retinal as their chromophore.
Humans cannot create Rhodopsin, in its place they use and outside source, b -carotene that is found in food in order to synthesis it. Brightness falling on the retina brings about definite chemical change in the rhodopsin and extra substances present in the rods and cones. These changes happen very rapidly, but large quantities of vitamin A are desirable to bring this about. If there is any noticeable deficiency of vitamin A, night blindness might happen.
When light energy is absorbed by rhodopsin, the rhodopsin begins within trillionths of a second to decompose. The cause of this is photo activation of electrons in the retinal portions of the rhodopsin which leads to an immediate change of the aciform of retinal into an all transform form, which still has the same chemical structure as the aciform form but has a different physical structure- a straight molecule rather than an angulated molecule. It is the metarrhodopsin II also called activated rhodopsin that excites electrical changes in the rods that then transform it the visual image into the central nervous system.
The first stage in the reformation of rhodopsin is to reconvert the all Transform retinal into 11-acisform retinal. This process requires metabolic energy and is catalyzed by the enzyme retinal isomers. Once the 11-acisform retinal is formed it automatically recombines with the scotopsin to reform rhodopsin, which then remains stable until its decomposition is again triggered by absorption of light energy.
There is second chemical route by which all Transform retinal can be converted into 11-acisform retinal. This is by conversion of the all Transform retinal first into all Transform retinal, which is one form of vitamin A. then, the all Transform retinal, is converted into 11-acisform retinol under the influence of the enzyme isomers. And finally the 11-acisform retinol is converted into 11-acisform retinal that combines with scotopsin to form rhodopsin.
When the rhodopsin in the outer segment of the rod is exposed to light and begins to decompose, this decreases the outer segment conductance of sodium to the interior of the rod, even though the sodium ions continue to be pumped out of the inner segment.
When exposed to light rhodopsin is split up into the yellow compound retinene and colorless protein opsin. The retinene is in fact retinene 1. Under the influence of light, rhodopsin is converted to orange red compound lumirhodopsin, which becomes changed to metarrhodopsin, ultimately forming a yellow mixture of transform-retinene and opsin.
