小肠（三）： 吸收

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 gonna get them inside of our bloodstream 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 is 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 unit of life adenosine tri as in three phosphate and so if we look at a single and taro site or an intestinal cell there'd 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 to die phosphate and in doing so would allow our amino acid to enter into our entero site or our intestinal cell from there the amino acid undergo a couple of different steps but eventually we'll leave the intera site and go to a blood capillary where it enters the bloodstream and then can be shuttled anywhere else in the body for use monosaccharides or 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 setting 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 a 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 and Oh site and just to make sure we're complete I'm gonna 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 intera site 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 entero site while the sugar leaves and goes to the blood capillary so this also ends up in our bloodstream and can go anywhere in the body to be used the nucleoside in the base sort of used the same mechanism that amino acids do so I'm just gonna 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're going to end up now 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 int arrow site like this guy all it has to do is just diffuse across the membrane and then it ends up on the inside in the entire site all of our fatty acids are gonna 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 inteiro site 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'll end up in veins that will send the digested fat the heart and eventually two arteries that can then distribute them wherever they need to go in the body and so you can appreciate a lot that'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're 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