|
| quote: | Originally posted by Orbax
When light first strikes the retina a photon interacts with a molecule called 11-cis-retinal, which rearranges within picoseconds to trans-retinal (a picosecond is about the time it takes light to travel the breadth of a human hair) The change in the shape of the retinal molevule forces a change in the shape of the protein, rhodopsin, to which the retinal is tightly bound. The proteins metamorphosis alters its behavior. Now called metarhodopsin II, the protein sticks to another protein called transducin. before bumping into metarhodopsin II, transducin had tightly bound a small molecule called GDP. But when transducin interacts with metarhodopsin II, the GDP falls off, and a molecule called GTP binds to transducin. GTP-transducin-metarhodopsin II now binds to a protein called phosphodiesterase, located in the inner membrane of the cell. When attached to metarhopodpsin II and its entourage, the phosphodiesterase acquires the chemical ability to cut a molecule called cGMP. Initially there are a lot of cGMP molecules in the cell, but the phosphdiesterase lowers its concentration, just as a pulled plug lowers the level of water in a bathtub. Another membrane protein that binds cGMP is called an ion channel. It acts as a gateway that regulates the number of sodium ions in the cell. Normally the ion channel allows sodium ions to flow into the cell, while a seperate protein actively pumps them out again. The dual action of the ion channel and pump keeps the level of sodium ion in the cell within a narrow range. When the amount of cGMP is reduced because of the cleavage by the phophodiesterase, the ion channel closes, causing the cellular concentration of positively charged sodium ions to be reduced. This causes an imbalance of charge across the cell that causes a current to be transmitted down the optic nerve to the brain.
If the reactions were the only ones that operated in the cell, the supply of 11-cis-retinal, cGMP, and sodium ions would be quickly depleted. Something has to turn off the proteins that were turned on and restore the cell to its orignal state. Several mechanisms do this. First, in the dark the ion channel also lets calcium ions into the cell. The calcium is pumped back out by a different protein o that a constant calcium concentration is maintained. When cGMP levels fall, shutting down the ion channel, calcium ion concentration falls too. The phosphodiesterase enzyme, which destroys cGMP, slows down at lower calcium concentration. Second, a protein called guanylate cyclase begins to resynthesize cGMP when calcium levels start to fall. Third, while all of this is going on, metarhodopsin II is chemically altered by an enzyme called rhodopsin kinase. The modified rhodopsin then binds to a protein known as arrestin, which prevents the rhodopsin from activating more transducin. So the cell contains mechanisms to limit the amplified signal started by a singel photon.
Trans-retinal eventually falls off of rhodopsin and must be reconverted to 11-cis-retinal and again bound by rhodopsin to get back to the starting point for another visual cycle. To accomplish this, trans-retinal is first chemcially modified by an enzyme to trans-retinol-a form containing two more hydrogen atoms. A second enzyme then converts the molecule to 11-cis-retinol. Finally, a third enzyme removes the previously added hydrogen atoms to form 11-cis-retinal, a cyle is now complete.
**
That is what happens when a single photon of light strikes the eye. That isnt getting into color differentiation. The anatomy of the eye doesnt really matter. Neither does the fossil record. It doesnt matter that there are huge gaps in the record. It doesnt matter if the gaps can be explained plausibly. It doesnt tell us about whether or not the interactions of 11-cis-retinal with rhodopsin, transducin, and phosphodiesterase could have developed step by step. Niether do the patterns of biogeography, or population biology, nor the tradition explanations of evolutionary theory for rudimentary organs or species abundance. This is not to say random mutations is a myth, or that darwinism fails to explain ANYTHING, or that large-scale phenomana like population genetics dont matter. They do. Until recently however evolutionary biologists could be unconcerned with the molecular details of life because so little was known about them. Now that we do know, the infintesimal world that is revealed must be explained. |
Essentiallthy, this is exactly what I said in my post. A lot is known about the molecular details of life, and more is being discovered each day. You say that evolutionary biologists coule be unconcerned with the molecular details fo life because so little was known about them, yet you provided an even more complex chemical explanation of what I explained one post earlier, showing that you either understand that a lot is known about the complex chemical reactions of rhodopsin, or that you were able to find a site and cut and paste the information here.
Could it be possible that it is an commentary on darwins black box???
http://www.firstthings.com/ftissues...ws/johnson.html
One should not limit all their saying from one book, or few books. That person would risk exposing themselves to the mistakes and prejudices of few people. Expand your horizons. Try reading scientific journals from people that agree with evolution, and chemical processes that contribute to it, and see if you can explain why that would be wrong. By simply cutting and pasting and excerpt from the book. You obviously didnt have it in your own words, so I doubt if you understand the system very well.
Id like to refer you to another website, which i found by pasting your post in a search engine (the exact phrase you posted, listed in blue above, yeilded many exact results, which tells me you cut and pasted the material). This site discusses Evolution, and Creation both being led by blind faith, and it states that no historical event can be proven (see Introduction, section C and D). We both have to admit, that either of us could be right in our arguments, or both of us could be wrong as well (if we are honest). Although I do not beleive in a judgemental God, I choose to live a good life, which includes honesty as part of its backbone. You will see by my previous posts, and by this one, that I do not claim to know all the answers, and that I beleive in evolution because I have faith in the theory. I have also not said that you are wrong to beleive Creation, because it is also possibly correct. Neither can be proven.
http://www.geocities.com/Athens/Oly...perstition.html
___________________

I am enough of an artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.
Albert Einstein
|