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小肠(二): 消化

Raja Narayan 创建




so the best way to talk about the digestion process in the duodenum is to talk about the four major macromolecules that make up everything in our body starting off with protein proteins are just chains of amino acids so there's one amino acid and here's another amino acid and they're connected by something called a peptide bond after proteins we also have things that are called carbohydrates carbohydrates or our carbs and these are just repeating units of simple sugars attached to each other you name one simple sugar yeah I think you said glucose and so something like glucose attached to say galactose would be a disaccharide and this particular disaccharide would be called lactose so a single sugar would be considered a monosaccharide two of them a disaccharide and then a whole bunch would be a polysaccharide in addition to our carbs we also have nucleotides nucleotides and these guys make up our nucleic acid our DNA and they're composed of three major parts there's a phosphate group that sits on a ribose sugar that I'm drawing here this little Pentagon and then right here you're gonna have a base that's present and this base is the coding part of your DNA like adenine or guanine if you've heard those names before and then finally we also have FAT FAT has two parts to it it's got this tri glycerol head that has these three oxygen groups on it and then each of these oxygens have a fatty acid tail so there's one there's two there's three so three fatty acid tails that are found here so now that we know what the main macromolecules are we can talk about what are the main processes that are involved in breaking these guys up so they can be absorbed into our bloodstream starting with our proteins there's going to be brush border enzymes that are present to help break our peptide bonds or the amino acid amino acid bond that's present and so I'll just write peptidases here peptidases because as you might recall whenever you name an enzyme that ends in ace that means that whatever comes before it is what's broken so a peptide bond is broken by a peptidase in addition the pancreas brings along a couple of important enzymes that will help break up our proteins first of all it's got trypsinogen that's present as well as some chymotrypsinogen but whenever you see Jen at the end of an enzymes name that means that it's not ready to work yet that means it still needs one more cut or cleavage before it can start being completely functional well this is where our brush border comes in handy again because there's an enzyme that's present on the brush border it's called entero peptidase and Turo peptidase that is specifically present to break down trypsinogen into its active form called trypsin and the same thing with chymotrypsinogen into kymo trypsin these guys are going to be able to break down your peptide bond as well just like peptidases are able to do but need a little kick or a little cut in order to start working and once we gobbled up our proteins we'll have our single amino acid subunits then and be absorbed when we get to carbs we'd notice that the pancreas is also pretty helpful here as well because the pancreas releases an enzyme that's called amylase that can help break up the sugar bond or this glycosidic bond is another name for this link right here in certain carbohydrates the brush border is also helpful here because they have a whole bunch of very specific enzymes that break the glycosidic bond between very specific simple sugars for instance the brush border enzymes known as lactase lactase can only be used to break apart lactose like I've drawn here now the interesting thing about lactose that I should mention is that when you get the stomach flu you've got a bug that's just sitting in your tummy it actually can start inflaming the duodenal wall and by doing so it can knock off some of the lactase brush border enzymes and so you won't have lactase you won't be able to break down lactose you won't be able to separate our glucose from our galactose and so temporarily you're going to be lactose intolerant most people gain disability back but you can understand now how precious these brush border enzymes are to us and so they'll help break up certain disaccharides the pancreas will do its job as well and eventually you'll just have your simple sugars that are gonna be left over here and remember the name for each single unit is called a mono saccharide a monosaccharide when it comes to nucleotides the brush border helps us as well the specific enzyme set that we have on our brush border that helps break apart nucleotides are called nucleo wait for it side aces nucleoside aces and the reason why we have nucleoside aces and not nucleotide aces is because it breaks this bond right there resulting in this phosphate group sticking on the ribose sugar but now we've got our base as a completely separate unit from the nucleoside finally when it comes to fat we're gonna get some help from bile as I mentioned earlier and bile if you recall comes from our liver and our gallbladder so these guys are gonna release some bile into our duodenum and they're going to help emulsify or organize our fat so all right here in parentheses to organize because they're not really involved in breaking them down they're there to help organize our fats one of the main things that help us actually break it down is from the pancreas the pancreas released is something that's called lipase and the lipase comes in and Cleaves right about there so that way each of these single fatty acid chains break off and so you'll have these three fatty acid chains that are hanging out right here including your triglyceride head that's going to be a separate unit altogether so now you these four things that came off of this one fat molecule