AJourneytoCollagenedited 2010 - 1 A Journey to Collagen:...

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1 A Journey to Collagen: This morning I drank orange juice so I have a lot of ascorbic acid (Vitamin C) in my system . Later I ate a steak and the proteins in the steak were broken down in my intestines to amino acids. So, there are lots of glycine, lysine, and proline (amino acids ) in my intestinal lumen. Now there are multipass transmembrane proteins (receptors) in the apical surface of the epithelial cells lining my intestine. It is thought that there is a specific receptor protein for each amino acid. So, the glycine, lysine, and proline have each found their transmembrane receptor protein and have attached to the receptor site. But nothing happens! Those three amino acids (and all amino acids) need help to get into the cell. I forgot to mention that I put salt on the steak and the salt or rather the Na + ion provides the help needed to get the amino acids into the cell. Na + attaches to the same receptor protein and the combination of the amino acid and Na + attaching to the receptor protein brings about a configurational change in the protein that now allows both the amino acid and the Na + ion to enter the cell –“the gate is open”. Some amino acids require more than one sodium ion. This type of transport across the cell membrane is called active transport driven by ion gradient . Note that no ATP is required for this type of transport. The energy is in the form of an electrochemical gradient: the sodium concentration is high in the lumen and low within the cell, so Na + is moving down its gradient while the amino acids may be going against its concentration. By the way, Vitamin C was taken up by simple diffusion. So now the three amino acids and Vitamin C are happily inside the enterocyte and the enterocyte already had sufficient amino acids and Vitamin C for its own needs. But, somewhere in my connective tissue, a fibroblast in the dermis of my skin needs to make collagen. So, my three friends (glycine, lysine, and proline) as well as Vitamin C are transported out the basolateral surface of the enterocyte into the connective tissue and run into a capillary. They cross the membrane of the endothelial cell and cross through the cytoplasm of the endothelial cell and exit the other membrane of the endothelial cell to get into the blood plasma in which they are carried to the dermis of my skin. Having arrived in the dermis, my friends again cross two membranes of an endothelial cell to enter the ground substance of the connective tissue where they are taken up by a fibroblast. Thus far, my friends have crossed the apical and basolateral membrane of the enterocyte, both membranes of the endothelial capillary cell in the villus of the intestinal tract, and both membranes of the endothelial capillary cell in the connective tissue, and finally the cell membrane of the fibroblast. I think that adds up to seven membranes!! It has been a tough
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This note was uploaded on 11/08/2011 for the course SCIENCE 3254 taught by Professor Dr.shoupe during the Spring '11 term at Fort Valley State University .

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AJourneytoCollagenedited 2010 - 1 A Journey to Collagen:...

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