Lipid Absorption And Digestion

Question 1

Explain the 5 phases of lipid digestion and what happens in every step.

Answer 1

I) Hydrolysis of triacylglycerols to free fatty acids and monoacylglycerols within the lumen of GI tract.

II) Solubilization of lipids by detergents (bile acids) and transport from intesrinal lumen to surface of the lining epithelial cells.

III) Uptake of free fatty acids and monoacylglycerols into epithelial cells and again converted to triacylglycerols.

VI) Packaging of newly synthesized triaglycerols into chylomicrons.

V) Exocytosis of chylomicrons from intestinal epithelial cell to lymph, from lymph to thoracic duct and into systemic venous system.

Question 2

Emulsification provides…

Answer 2

More sites for absorption of lipase molecules.

Question 3

What is the major enzyme for triacylglycerol hydrolysis?

What does it do to the lipid?

Answer 3

Pancreatic lipase us the major enzyme.

It hydrolyzes the esters in the alpha position of the glycerol.
Products are Beta-monoacylglycerols.

Question 4

What inhibits pancreatic lipase?

Answer 4

Bile acids!

Question 5

What does colipase protein do?

Answer 5

It exists in pancreatic juice and helps to avoid the inhibition of lipase by bile acids.

(It is secreted as procolipase, depends on tryptic removal of NH2 terminal for activation.)

Question 6

Pancreatic juice also contains unspecified lipid esterase which gets activated by bile acids. What does it act upon?

Answer 6

• Cholesteryl Esters
• Monoacylglycerols
• Lipid Esters
• Esters of vitamine A

Question 7

Pancreatic juice is rich with phospholipases. What is their function? What activates them?

Answer 7

It hydrolyzes phospholipids. Phospholipases are activated by trypsin and require bile acids for activity.

Question 8

What is a mixed micelle?

Answer 8

A monoglyceride micelle together with phospholipids.

Question 9

What is the micelles greatest function in absorption of lipids?

Answer 9

Micelles are the major vehicle for transferring lipids from lumen to mucosal surface where absorption occurs!

Question 10

What happens to unabsorbed lipids?

Answer 10

They reach the lower part of the intestine and can be metabolized by bacteria.

(The remaining fraction of unabsorbed lipids is however excreted with the stool / Diarrhea)

Question 11

What property of chokesterol makes cholesterol form stone formation in gallbladder?

Answer 11

Its limited solubility

Question 12

What is secreted by the liver inte bile?

Answer 12

Phospholipids, cholesterol and bile acids.

Question 13

What are the primary bile acids synthesized by the liver?

Answer 13

Cholic and chenodeoxycholic acid.

Question 14

How are secondary bile acids made?

Answer 14

They are derived from primary bile acids by bacterial reduction resulting in:

Cholic acid-> litocholate
Chenodeoxycholic acid-> deoxycholate

Question 15

Why is a small pool advantageous to have bile acids in?

Answer 15

Because bile acids become toxic at high conc due to their detergent properties.

Question 16

How are chylomicrons made?

Answer 16

They are synthesized in ER from which they migrate through the golgiand then in vessels in contraluminal membrane. Then out in lymph, thoracic duct and then venous system.

Question 17

Describe the enteroheoatic circulation?

Answer 17

I) Secretion of bile acids by liver cells across canalicular lasma membrane into the bile is by primary active transport.

II) Ileal absorption of bile acids by Na+ secondary active transporters

III) Bile acids are removed from enterocytes to the blood by anion exchanger

IV) Uptake into the liver from blood is by secondary active Na+ taurocholate cotransporting polypeptide NTCP