Hi, today we're going to talk about the small intestine, along with two accessory organs, the liver and pancreas, and how they relate to chemical digestion and fat absorption in the small intestine. And this is a 3B topic. Now, the small intestine is the site of the most chemical digestion in the body, so we need a recap on chemical digestion, the breakdown of larger biomolecules into smaller ones. Read full transcript
We remember that peptides are cleaved by peptidases or proteases into amino acids. Lipids are cleaved by lipases into free fatty acids. Oligonucleotides and cleaved by nucleases into free nucleosides. And polysaccharides are cleaved by amylases into monosaccharides. Some enzymes are present on the surface of the enterocytes themselves. We call these the brush border enzymes after the brush border, or villi of duodenum, where these enzymes are the most prevalent.
There are a variety of lipases and amylases there, but memorizing them is probably overkill for the MCAT. If you take away just one thing about these enzymes, know that this is the site of lactase. Which is the enzyme in the digest lactose, the sugar found in milk. All people have active lactase enzymes as children, but adults who are lactose intolerant usually do not.
Adult lactose tolerance is actually due to a mutation that spread in the population after humans began drinking animal milk. Places that historically drank less milk, like East Asia, have higher rates of lactose intolerance for this reason. As this enzyme is deactivated in adulthood unless you have a mutation that keeps that from happening.
There are also several peptidases in the brush border. Some of them are what's called exopeptidases. Peptodidases like carboxypeptitase which cleave amino acids on the ends or the outside of an amino acid chain. Others are endopeptidases which cleave amino acids in the middle. Or inside of an amino acid chain.
Peptidases in the small intestine are usually highly specific, for specific residues, and will cut peptide bonds between some amino acids, but not others. Knowing the names of particular enzymes is probably not super high yield, just know the difference between endo and exopeptidases. There is one specific peptidase to know. Which is enteropeptidase, also known as enterokinaese.
We'll talk about why in just a second. Many of the enzymes in duodenum do not come from the small intestine itself, however, but from the pancreas, one of the accessory organs of digestion. The pancreas is located around here, and in addition to its endocrine functions, which we'll talk about in another lesson. The pancreas also releases several exocrine secretions into the small intestine.
In response to a meal, gastrin is secreted from the stomach. And cholecystokinin, or CCK, is secreted from the small intestine. This triggers the Acinar cells of the pancreas to secrete several enzymes into the duettino. These include various amylases, peptidases, lipases, and nucleases, whose names aren't super important to know, except perhaps for a couple of the peptidases.
Many of the enzymes secreted by the pancreas are secreted as zymogens. Inactive enzymes that need to be cleaved by another enzyme to become active. This prevents these strong enzymes from degrading the pancreas itself. Since they're only active once they reach the area of secretion. Now entrokinase, which we mentioned earlier as one of the brush border enzymes, cleaves the zymogen trypsinogen to make trypsin.
A peptidase. Trypsin, in turn, cleaves chymotrypsinogen into chymotrypsin, which is an additional peptidase. This is a good little enzyme cascade to file away in your memory. The pancreas also secretes bicarbonate anion, HCO3-. This happens after the small intestine secretes a hormone called secretin in response to acidity.
Bicarbonate acts as a base and neutralizes any stomach acid coming in with the chyme into the small intestine, preventing the acid from damaging any tissues. CCK and secretion also cause another accessory organ, the liver, to release bile salts through the bile duct. Now these are not enzymes. They don't break down chemical bonds and nutrients exactly.
But they do function to emulsify or break up large globules of fats and lipids in the small intestine, to make them easier to absorb. They do this because they are amphipathic or have a mix of hydrophilic and hydrophobic areas. Bile salts are actually a number of different molecules derived from cholesterol, such as this one, cholic acid.
And they typically have a charge group such as a deprotonated carboxylic acid on one end, which is then conjugated to a charged amino acid to make a salt. On the other end they have a large hydrophobic region. This dual nature gives them a strong affinity for water on the charged end while retaining affinity for the non polar tails of lipids on their hydrophobic end. Which helps them pull apart large globules of fat into smaller nacelles, creating a larger surface area for lipases to act on and break triglycerides down into free fatty acids and monoglycerides.
Bile also contains bilirubin. A waste product of hemoglobin breakdown. Excess bile is stored in the gallbladder, right here, until it is ready for secretion. Bile salts themselves are typically recycled by being reabsorbed in the ileum of the small intestine.
Now once the lipid droplets are broken down into micelles via bile salts. And then further broken down by lipases, the free fatty acids and monoglycerides diffuse directly through the membrane of enterocytes, where they eventually wind up in the endoplasmic reticulum and golgi apparatus of the cells. Here they are reformed into triglycerides and packaged into chylomicrons.
Chylomicrons are made up of a micelle of lipids with many embedded apoproteins. This micelle surrounds a core containing hydrophobic substances like triglycerides, cholesterol and vitamins A, D, E and K. We call this overall structure lipoproteins. Now once these lipoproteins known as chylomicrons are packaged, they are secreted into capillary sized lymph vessels called lacteals.
Through which they travel through the lymphatic system, eventually draining into the vena cava and ultimately going to the liver, where they are further processed, which we'll talk about in another lesson. So, to round everything out, here's some high-yield concepts to take away about chemical digestion and fat absorption in the small intestine. First know the various types of enzymes and what they digest.
Remember amylases digest sugars. Peptidases digest peptides and proteins. Lipases digest liquids and nucleases break down long stretches of DNA and RNA. You also wanna known where you can find them. So, in addition to knowing that a variety of these types of enzymes are present on the brush-border of the small intestine, and in pancreatic secretions, you should remember specifically, lactase for it's role in lactose digestion, and enterokinase or enteropeptidase, for it's role in the trypsinogen cascade.
You should also remember that the pancreas secretes these zymogens involved in that cascade. You should know the role that bicarbonate secretion fills in acid neutralization. You should also know that CCK and secretin are involved in causing the pancreas to secrete these. Additionally, you should also know the difference between endo and exopeptidases, specifically, where they cleave a peptide.
It's also good to know the role that bile salts play in fat absorbtion. Be able to recognize the terms of amphipathic and emulsifier, which describe a biosalt's structure and role, respectively. Amphipathic, remember, refers to the mixed polar and nonpolar nature of these molecules. While emulsifier refers to the ability to break up fat globules.
Finally be familiar with the pathway of fat absorption itself. In particular, how lipids do not go directly to the liver through the blood capillaries, but rather pass via chylomicrons to lymphatic lacteals, and enter the lymph circulation first.