How do cones work at a molecular scale ?

                 Now let’s talk a bit about how do cones work at a molecular scale. Important to notice that rods work a bit like cones but it won’t be developped because this has nothing to do with colorblindness.

pd_5-02 detail rod photoreceptor

Constitution of cones and rods and molecule of rhodopsin.

                The image above shows how cones and rods are constituted. In particular there is a part of these which is called the « outer segment ». This part is cone-shaped and that is why cones are called that way. In the outer segment there are discs which are made of rhodopsin which is a protein made of an other protein (opsin) and a molecule called  « chromophore » (retinal). We already talked about opsin on this blog but what is the retinal ? It is one of the three forms of the vitamin A and it is able to catch the light.

pd_5-03 cones and rods light sensitivity

Associations of opsin and retinal.

                As you should remember (and if you don’t go check the article !) there are three types of opsins in the human body. According to the association of retinal/opsin the rhodopsin created will react differently to the light. There are three combinations possible which will allow the capture of the different wavelengths (small, medium and large).

pd_5-04 retinal photoisomerization

Change of shape of the retinal when it catches a photon.

                The previous diagram shows the molecule of retinal and how it reacts to the light. In fact when a photon is absorbed by the retinal its form changes. But as it is shown in the next picture the retinal is right in the middle of the rhodopsin, so when its form changes, the form of the rhodopsin changes. But this will allow a chemical transporter to come within the protein and that will send a chemical message to the synapse of a nerve which will eventually arrive to the brain and it will interpret this signal as a color. After the change of shape, the retinal will be sent out of the rhodopsin and it will find its original shape back thanks to a process that won’t be developed. Depending on the opsin that makes the rhodopsin considered the signal sent won’t be the same. This explains the different colors that we can see.

pd_5 quelque chose

Transformation of the rhodopsin in metarhodopsins.

                To summerize colorblindness can be caused by a problem in this process. But most of the time it comes from a lack of one of the opsins which avoids one or more of the colors to be recognized by the rhodopsin.

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