HRR: lipids and bile

Question 1

Where does digestion of lipids begin?

Answer 1

In the mouth with lingual lipase

Question 2

Describe the basic process of digesting lipids.

Answer 2

1. Very basic digestion begins in the mouth and stomach via lingual and gastric lipases.
2. In the small intestine, fats get emulsified by bile salts.
3. Emulsified fats are digested by pancreatic lipases and esterases, and are then absorbed into enterocytes.
4. After reabsorption, lipids are re-esterified and packaged into chylomicrons.
5. Chylomicrons are secreted into lymph and will eventually enter the blood.

Question 3

What are major dietary lipids?

Answer 3

Triacylglycerol, phospholipid, cholesterols

Question 4

What is the optimal pH for lingual lipase?

Answer 4

4.5-5.4

Question 5

Briefly describe lingual lipase.

Answer 5

It breaks down triacylglycerols into diacylglycerol and fatty acid, and remains active in the stomach and upper small intestine.

Question 6

What is the optimal pH for gastric lipase?

Answer 6

3-6

Question 7

Briefly describe gastric lipase.

Answer 7

Breaks down triacylglycerol into diacylglycerol and fatty acids and remains active in the stomach and upper small intestine.

Question 8

What secretes gastric lipase?

Answer 8

Fundic chief cells

Question 9

What does pancreatic lipase do?

Answer 9

Works with colipase to convert triacylglycerol and diacylglycerol to monoacylglycerol and fatty acids.

Question 10

What does colipase do?

Answer 10

Helps bind bile salts and lipase, stabilizing the lipase.

Question 11

What do cholesterol esterases do?

Answer 11

Deacylate cholesterol esters to form cholesterol and fatty acids.

Question 12

What does PLA2 do?

Answer 12

Deacylates phospholipids into lysophospholipid and fatty acids.

Question 13

What is a mixed micelle?

Answer 13

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.

Question 14

How can lipids enter cells?

Answer 14

1. Diffusion
2. Incorporation into enterocyte membrane
3. Carrier-mediated transport

Question 15

What is NPC1L1?

Answer 15

A carrier-mediated transporter to bring cholesterol into enterocytes.

Question 16

How do lipids form chylomicrons?

Answer 16

The re-esterified lipids form lipid droplets that move through the RER and Golgi, and bud off of the Golgi to form chylomicrons.

Question 17

How do chylomicrons get secreted and transported?

Answer 17

They fuse with the enterocyte membrane and can be released to enter lymphatic capillaries.

Question 18

How are SCFA and MCFA secreted?

Answer 18

They pass directly through the enterocyte and into the blood capillaries (lacteals).

Question 19

What produces bile?

Answer 19

The liver.

Question 20

What is used to make primary bile acids?

Answer 20

Cholesterol.

Question 21

What is the rate-limiting step of bile production?

Answer 21

7-a-hydroxylase.

Question 22

Where is bile stored?

Answer 22

The gallbladder.

Question 23

What is necessary about the structure of bile salts?

Answer 23

They need to be amphipathic; the hydrophobic face interacts with lipids, and the hydrophilic face interacts with enzymes.

Question 24

What are secondary bile acids?

Answer 24

They’re dehydroxylated via bacteria in the terminal ileum and colon.

Question 25

What is the main function of bile?

Answer 25

Emulsify lipids to aid in their digestion.

Question 26

How does the body make bile usable?

Answer 26

It is conjugated to things like taurine or glycine to make bile salts, which are easier to work with.

Question 27

What do we do with bile when we’re done with it in the small intestine?

Answer 27

It is reabsorbed into the blood to be recycled.

Question 28

Between meals, how is bile diverted to the gallbladder?

Answer 28

Tonic contractions in the sphincter of Oddi prevent bile from entering the intestine when it is not needed between meals.

Question 29

What stimulates the gall bladder to release bile?

Answer 29

CCK.

Question 30

Describe how CCK stimulates bile release.

Answer 30

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.

Question 31

What are the mechanisms of action of CCK?

Answer 31

1. It can act directly on smooth muscle around the gallbladder. 2. Can activate vagal reflex to promote ACh release to promote contraction of gallbladder and relaxation of the sphincter of Oddi.

Question 32

How does removing the gallbladder impact lipid digestion?

Answer 32

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.

Question 33

Describe active transport of bile salts.

Answer 33

Uses ASBT, a sodium-independent transporter, to be brought into enterocytes.

Question 34

What is OSTaOSTB?

Answer 34

Allows bile salts to exit enterocytes and enter the interstitial space.

Question 35

What happens if we have a high level of bile salts in the enterocyte?

Answer 35

FGF19 secretion is increased, which reduces bile acid production in the liver.

Question 36

How are bile salts brought into hepatocytes?

Answer 36

Passively if they’re uncharged, by NTCP, or via OATP.

Question 37

How are bile salts pushed out of hepatocytes and back into interstitial space?

Answer 37

MRP4.

Question 38

How are bile salts pushed from the hepatocyte into the bile canaliculus?

Answer 38

BSEP or MRP2.

Question 39

How is bilirubin excreted?

Answer 39

It is brought into the liver, to hepatocytes, and is excreted along with bile via MRP2.