>>Dr. Ketchum: In this video lecture we are going to discuss digestion and absorption of lipids.
So in a typical diet, we consume 25 to 160 grams of lipids; that’s 90% triglycerides.
The lipids that we ingest, we have to digest. So lipids face very special problems in digestion
and absorption, and the reason for that is because they’re not water soluble. Remember
that carbohydrates and proteins are water soluble. So the problem with lipids then,
because they’re not water soluble, they don’t mix with the stomach or the intestinal contents
and they tend to form water droplets. Remember that lipids are lipophilic; that means they’re
hydrophobic. So that presents a little bit of a problem to us, and we’re going to figure
out—or we will discuss how lipids get around this, or how our body has adapted for—so
that we can digest and absorb these lipids. So the enzymes that are necessary for lipid
digestion are called lipase. We have three different types of lipase. There is lingual
lipase, so that’s secreted from the mouth.
We also have another form of lipase called gastric
lipase that’s secreted from the stomach, and then we also have our third enzyme which
is perhaps our most important one, and that’s pancreatic lipase. And as you could guess
from the name, that is secreted from the pancreas. When it’s secreted from the pancreas, it
will then enter into the duodenum. So pancreatic lipase actually functions in the duodenum
to break down fat, but it is synthesized and secreted by the pancreas. The most important
of these, or where does most lipid digestion occur, and most lipid digestion occurs in
the duodenum.
So lipases only act on molecules near the edge of the fat droplet. And so if you have
a large fat droplet and lipases are only acting near the edge of this very large fat
droplet, it’s going to take a very long time to break down this fat. So what we have
to aid or to assist with lipid digestion are bile salts. So bile salts are going to take
this very large fat droplet and break it down into smaller droplets. Bile salts have increased
the surface area, so now these enzymes that remember only function near the edge of the
fat droplet, they have a lot more surface area, right, to try to digest this lipid.
So this process where bile salts take a large droplet and break it down into smaller droplets,
this process is called emulsification. So let’s focus then on these bile salts. So
bile salts are synthesized in the liver from cholesterol. So one of the reasons that cholesterol
is necessary, right, you can’t cut out all the cholesterol in your diet. The bile salts
then get secreted in the bile to the duodenum, right? Where the liver, which produces the
bile and the gallbladder which stores the bile, when bile leaves the gallbladder or
leaves the liver, how does it make it to the duodenum? Be sure you understand what structures
that passes through and what sphincters have to be open in order for this to happen. Okay,
so bile salts are what we call an amphipathic molecule. And so you learn about amphipathic
molecules when you learned about the phospholipid bilayer. So remember, an amphipathic molecule
is a molecule that has both a polar side and—a non-polar side rather, and a polar side for
you. And the function for the bile salt then is to emulsify that fat. And it’s able to
do that because the polar ends get attracted to water, and then the non-polar end is afraid
of the water. So you’ll see shortly how this bile salt will bind to a fat droplet.
So before, here we have a large fat droplet okay? Very little surface area, right, is
exposed to lipases. And now we have the bile salts. Notice the direction that they’re
binding. The non-polar part of the bile salt is here, so that’s the part that’s lipid
soluble. Notice where it’s binding to the fat droplet; it is the non-polar side of your
bile salt. So it binds to the lipid—lipophilic things bind to lipids, right? The other side
of your bile salt on this side here, this is the side that’s polar, right? And it’s
important that it orients that way, because outside here is a whole bunch of water molecules,
right, with charges on it. And that explains why these bile salts orient the way that they
do. So these bile salts then, because they’re amphipathic, can take this large fat droplet
and break it down into smaller fat droplets. So remember that this bile is dropped or dumped
into the duodenum. So this is why most lipid digestion occurs in the duodenum, because
that’s where you have bile salts emulsifying your fat, which makes it easier for your lipase
to act. There is no bile in your mouth, normally, and there is no bile in your stomach normally.
So let’s take a look at how lipase acts then. How does it break down this fat
droplet? We’re going to assume, here we already have your bile salts that are bound
to this fat droplet. So you have several bile salts already bound. Now your lipase needs
to come in and bind to the fat. So here they’re illustrating to you lipase and how it’s
interacting with one triglyceride. And so lipase is going to take that triglyceride,
so when we break down a triglyceride using lipase, we’re going to be getting monoglycerides
and fatty acids. Okay, so they’re no longer coupled together, right? So now what happens?
So here we see how you have your fatty acid and that fatty acid, remember, those are lipophilic.
So fatty acids can freely diffuse across the epithelial cell. Once they get inside of the
cell, their ultimate goal is to diffuse all the way over into the blood, right? You want
to absorb these into the blood. We’re going to come back to that. Now not all of your
fatty acids are going to be absorbed right away. Some of the fatty acids that do not
freely diffuse right away—they’re not absorbed. They’re going, what’s going
to happen with them is they’re going to aggregate with bile salts and cholesterol
and some other lipid soluble substances. And when they all aggregate, what we’re going
to call this aggregation is a micelle. As your fatty acids get absorbed and the body
needs to start absorbing more fatty acids, the micelle can actually release more products
to be absorbed. So the micelle may release this fatty acid and then it may get absorbed,
okay? But that says the body needs it. Okay, so let’s go ahead and return to the situation
where we freely diffuse our fatty acids or our lipid, right, our lipid into the epithelial
cell. Remember the goal is to get this lipid across and into the blood. And let’s look
at how this happens.
Here comes your monoglyceride and your free, your free fatty acid, right? And we,
we know how they got across the apical membrane—by freely diffusing. Now what happens to them?
So they’re going to enter into this organelle here called the smooth ER, which you, you’re
familiar with. Okay, remember that the smooth ER does a lot with fat, right? It synthesizes
triglycerides; that’s its function. And step two, the smooth ER is going to reassemble
the triglyceride. We broke it down before, now we’re going to reassemble it. So that
reassembled triglyceride then, and I’m going to label this number three, enters the Golgi apparatus.
The Golgi apparatus then we know modifies and packages, and sends it out. When it gets
exported, it’s exported as a chylomicron. So this chylomicron contains the triglyceride,
the modified triglyceride. So it’s in a vesicle, right? That’s what this chylomicron looks
like; it looks like a secretory vesicle. So the way for any secretory vesicle to get out
of a cell is via exocytosis. So number four here is exocytosis. So this cell has to exocytose
the chylomicron that contains your modified triglyceride. This chylomicron is too large
to go directly into the blood, so the chylomicron is going to bypass the blood completely and
it’s going to enter a structure called the lacteal. Lacteals are blind-ended lymphatic
vessels that are in the villus of the small intestines. So now we have this chylomicron
that’s inside of this lacteal, and it enters into the lymphatic system. So once that chylomicron
gets into the lymphatic system, the lymphatic system is directly connected to circulation—to
the blood stream. So this triglyceride then enters into the blood via the lymphatic system.
Then, it’s going to go where? Where do we store fat? Well we store fat in adipose tissue,
and when necessary, we can break it down to use it as a source of energy as well. These
triglycerides can also be stored in other cells as well. So we talked about bile salts,
and we talked about how the liver produces these bile salts and they get stored in the
gallbladder and then when needed, they pass through the common bile duct through the ampulla
of Vater and through an open sphincter of Oddi. So when they leave the common bile duct
and they pass through the open sphincter of Oddi, they enter into the duodenum. So the
bile salts do their job, okay? They emulsify the fat. And the thing about these bile salts
is we can recycle them. When the bile salts reach the illeum, they can be absorbed back
into circulation. So they go back into the circulation and they go back to the liver,
and that way you can recycle them, right? Then you can secrete the bile salts again
without having to make new ones. This video once again described the digestion and absorption
of lipids, and this concludes the video. I hope this was helpful.