Rods and Cones

photochemistry of vision

There are four parts that we photoreceptors can be distinguished, namely:

1.The outer segments consisting of membranous or discs. On these membranes are the light sensitive photo chemical (photopigment) of what needs to change before an action potential will occur.

2.The inner segment that is stuck to the outside through a narrow neck. In the inner segment, in addition to other organelles, there are many mitochondria so energy is saved.

3. The core of the receptor cell.

4. The synaptic body. Rods and cones are basically the same except that their outer segments and the cones more conical and shorter.

Regeneration of the receptor

The lifetime of a membrane disk is about 10 days . The discs degenerate constantly at the distal tip and then gets "phagocyted" by the pigment layer. New disks and membrane photopigments are always on the basal side of the outer segment added by the activity of the inner segment , which requires energy .

The Rods

The photopigment for vision in rods is rhodopsin. This substance consists of a protein known as opsin combined with an aldehyde of Vitamin A, namely the cis form of retinal (11-cis-retinal). When light falls on the rods in well-lit conditions, the light is absorbed by rhodopsin . The result is that retinal changes from the cis form to the trans isomer, all-trans retinal. In short, it means that the retinal changes physically (straight) and breaks its bond with the protein opsin . The remaining disk, comprised of only opsin, is said to have been bleached. This occurs gradually and goes through several stages (stages for interest: prelumirhodopsin → lumirhodopsin → metarhodopsin → 11-pararhodopsin , which is finally divided by hydrolysis into opsin and all-trans retinal) . The excess all-trans retinal in the eye changes to all-trans retinol and is then stored in the pigment as vitamin A. Some of these vitamins are lost in the circulation . In poorly lit conditions (dark adaptation)...