Small intestine 3: Absorption
Voiceover: All right, great. So, now we have all of our monomers ready to be absorbed. How does the absorption process work? Let's take a look. So, now we're so close. We've got all of our monomers, but we need to figure out how the heck are we going to get them inside our blood stream. Well, starting with our amino acids here. These guys are going to be shuttled into cells using what's called primary active transport, primary active transport. Now, if I say primary here, what does that specifically indicate as used? Now, you might recall when we use active transport, that means we need a little bit of energy to get something to happen. And the form of energy that we use in primary active transport comes from ATP, the energy, the unit of life, adenosine tri as an three, phosphate. And so if we look at a single enterocyte or an intestinal cell, there would be a protein that's here on the cell membrane. This protein would break apart our ATP, cleaving off one of the phosphate groups to release adenosine diphosphate, so that's two, diphosphate. And in doing so, would allow our amino acid to enter into our enterocyte or our intestinal cell. From there, the amino acid could undergo a couple of different steps, but eventually will leave the enterocyte and go to a blood capillary, where it enters the blood stream and then can be shuttled anywhere else in the body for use. Monosaccharides are sugars, sort of have a similar thing going on, but instead of primary active transport, we have what's called secondary active transport going on. So, if we use the ATP for primary active transport, what do we use for secondary? Well, the fact that we're saying this is still active transport means that there was some energy that was used at some point, and the energy actually was invested in sending up an ion gradient. And so the ion gradient then could be used by allowing something like sodium to flow down its gradient, to go from the place of high concentration to low concentration where it can relax. And by allowing that to occur, energy is then harnessed allowing a monosaccharide or a sugar to enter into our enterocyte. And just to make sure we're complete, I'm going to draw the protein transporter we have here as well as one on the other side, and show that there is a sodium ion that's flowing into our enterocyte down its concentration gradient to end up in the enterocyte with the sugar. And sort of the same thing happens on the other side, except as the sugar leaves, sodium on this side is entering. So, the sodium is still flowing down its concentration gradient, but it ends up inside the enterocyte while the sugar leaves and goes to the blood capillary. So this also ends up in our blood stream and can go anywhere in the body to be used. The nucleoside in the base sort of use the same mechanism that amino acids do. So, I'm just going to write primary active transport right here. And you can take a look above to see how that happens. And by doing that, you can imagine where they are going to end up. That's right, the blood capillary as well. And that takes us to our last macro molecule, fat. Now, the thing about fat that's rather redonkulous is that because it's got this really nonpolar tail. If it ever shows up next to an enterocyte like this guy, all it has to do is just diffuse across the membrane, and then it ends up on the inside. In the enterocyte, all of our fatty acids are going to be reorganized into what are called chylomicrons, chylomicrons. And like the name, chylomicrons themselves are too big to fit directly into a blood capillary. I couldn't even fit it here in this enterocyte. So, it doesn't actually directly go into the blood capillary. It is too big to do that. Too big to go to the blood capillary. Instead, chylomicrons will be absorbed into what are called lymphatic, lymphatic capillary, also known as a lacteal, a lymphatic capillary. And these are big enough to accommodate our chylomicrons. Here, they're going to be further digested and broken down into smaller bits, and by the time that happens, they will end up in veins. That will send the digested fat through the heart and eventually to arteries. That can then distribute them wherever they need to go in the body. And so you can appreciate a lot it's going on here. We've talked about how all four of our major macromolecules are digested in the duodenum, the place where the most digestion occurs in the GI tract. And now, we just talked about how they are absorbed, most likely in the jejunum, right. Because the jejunum is where the most absorption occurs anywhere in the GI tract. And that's how our small intestine works.