Human Physiology – Digestion and Absorption of Lipids

Human Physiology – Digestion and Absorption of Lipids

January 9, 2020 38 By Ewald Bahringer


>>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.