Small intestine 2: Digestion

Voiceover: So the best way to talk about the digestion process and the duodenum is to talk about the four major macro molecules 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 carbs and these are just repeating units of simple sugars attached to each other. Can you name one simple sugar? Yeah, I think you said glucose. Something like glucose attached to say galactose would be a disaccharide. 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. 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 going to have the base that's present. This base is the coating 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 triglycerol head that has these three oxygen groups on it and then each of these oxygens has 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 macro molecules 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 blood stream. 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 bond that's present. So I'll just write peptidases here, because as you might recall, whenever you name an enzyme that ends in 'ase', that mean 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 'gen' at the end of an enzymes name, that means 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 present on the brush border that's called enteropeptidase. That is specifically present to break down trypsinogen into it's active form called trypsin. And the same thing with chymotrypsinogen into chymotrypsin. 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 gobble up our proteins we'll have our single amino acid sub-units then and be absorbed. When we get to carbs we 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 enzyme know as 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 duodenum 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 this ability back, but you can understand now how precious these brush border enzymes are to us. They'll help break up certain disaccharides, the pancreas will do it's job as well, and eventually you'll just have your simple sugars that are going to be left over here. And remember the name for each single unit is called a monosaccharide. When it comes the nucleotides the brush border helps this as well. The specific enzyme set that we have on our brush border that helps break apart nucleotides are called nucleo-, wait for it, -sidases, nucleosidases. And the reason why we have nucleosidases and not nucleotidases, 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 going to get some help from bile, as I mentioned earlier. And bile, if you recall, comes from our liver and our gall bladder. So these guys are going to release some bile into our duodenum. They're going to help emulsify or organize our fat. So I'll write right here in parenthesis 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 releases something that's called lipase. The lipase comes in and cleaves right about there. 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 all together. Now you've got these four things that came off this one fat molecule.