HRR: lipids and bile Flashcards
Where does digestion of lipids begin?
In the mouth with lingual lipase
Describe the basic process of digesting lipids.
- Very basic digestion begins in the mouth and stomach via lingual and gastric lipases.
- In the small intestine, fats get emulsified by bile salts.
- Emulsified fats are digested by pancreatic lipases and esterases, and are then absorbed into enterocytes.
- After reabsorption, lipids are re-esterified and packaged into chylomicrons.
- Chylomicrons are secreted into lymph and will eventually enter the blood.
What are major dietary lipids?
Triacylglycerol, phospholipid, cholesterols
What is the optimal pH for lingual lipase?
4.5-5.4
Briefly describe lingual lipase.
It breaks down triacylglycerols into diacylglycerol and fatty acid, and remains active in the stomach and upper small intestine.
What is the optimal pH for gastric lipase?
3-6
Briefly describe gastric lipase.
Breaks down triacylglycerol into diacylglycerol and fatty acids and remains active in the stomach and upper small intestine.
What secretes gastric lipase?
Fundic chief cells
What does pancreatic lipase do?
Works with colipase to convert triacylglycerol and diacylglycerol to monoacylglycerol and fatty acids.
What does colipase do?
Helps bind bile salts and lipase, stabilizing the lipase.
What do cholesterol esterases do?
Deacylate cholesterol esters to form cholesterol and fatty acids.
What does PLA2 do?
Deacylates phospholipids into lysophospholipid and fatty acids.
What is a mixed micelle?
A way to carry lipids that features the fatty acid tails on the inside and the polar heads along the outside, allowing the fat to be carried in hydrophilic environments and move them through enterocytes.
How can lipids enter cells?
- Diffusion
- Incorporation into enterocyte membrane
- Carrier-mediated transport
What is NPC1L1?
A carrier-mediated transporter to bring cholesterol into enterocytes.
How do lipids form chylomicrons?
The re-esterified lipids form lipid droplets that move through the RER and Golgi, and bud off of the Golgi to form chylomicrons.
How do chylomicrons get secreted and transported?
They fuse with the enterocyte membrane and can be released to enter lymphatic capillaries.
How are SCFA and MCFA secreted?
They pass directly through the enterocyte and into the blood capillaries (lacteals).
What produces bile?
The liver.
What is used to make primary bile acids?
Cholesterol.
What is the rate-limiting step of bile production?
7-a-hydroxylase.
Where is bile stored?
The gallbladder.
What is necessary about the structure of bile salts?
They need to be amphipathic; the hydrophobic face interacts with lipids, and the hydrophilic face interacts with enzymes.
What are secondary bile acids?
They’re dehydroxylated via bacteria in the terminal ileum and colon.
What is the main function of bile?
Emulsify lipids to aid in their digestion.
How does the body make bile usable?
It is conjugated to things like taurine or glycine to make bile salts, which are easier to work with.
What do we do with bile when we’re done with it in the small intestine?
It is reabsorbed into the blood to be recycled.
Between meals, how is bile diverted to the gallbladder?
Tonic contractions in the sphincter of Oddi prevent bile from entering the intestine when it is not needed between meals.
What stimulates the gall bladder to release bile?
CCK.
Describe how CCK stimulates bile release.
Partially digested molecules in the intestine are sensed by I cells, which then release CCK into the blood. It will then stimulate relaxation of the sphincter of Oddi and contraction of the gallbladder to allow bile to flow into the gallbladder.
What are the mechanisms of action of CCK?
- It can act directly on smooth muscle around the gallbladder.
- Can activate vagal reflex to promote ACh release to promote contraction of gallbladder and relaxation of the sphincter of Oddi.
How does removing the gallbladder impact lipid digestion?
The gallbladder stores bile and helps concentrate it, but we are still capable of producing and using bile without the gallbladder. It doesn’t impact it too much.
Describe active transport of bile salts.
Uses ASBT, a sodium-independent transporter, to be brought into enterocytes.
What is OSTaOSTB?
Allows bile salts to exit enterocytes and enter the interstitial space.
What happens if we have a high level of bile salts in the enterocyte?
FGF19 secretion is increased, which reduces bile acid production in the liver.
How are bile salts brought into hepatocytes?
Passively if they’re uncharged, by NTCP, or via OATP.
How are bile salts pushed out of hepatocytes and back into interstitial space?
MRP4.
How are bile salts pushed from the hepatocyte into the bile canaliculus?
BSEP or MRP2.
How is bilirubin excreted?
It is brought into the liver, to hepatocytes, and is excreted along with bile via MRP2.
