Hubi 2003 (MUN) - Chapter 2: Digestion (Digestion of Lipids)

Lipids are digested in the form of _____?

Triacylglycerols.

Step 1 of Lipid Digestion

Lipids are prepared for digestion in the stomach. The grinding and mixing that takes place in the stomach converts lipids into an emulsion, which is a mixture of lipid droplets and water.

Step 2 of Lipid Digestion

After lipids leave the stomach, emulsification is enhanced with the aid of bile salts.

Bile Salts

amphipathic molecules synthesized from cholesterol in the liver, stored in the gallbladder, and released into the small intestine, where they act as a detergent to solubilize dietary lipids, which makes it easier to digest triacylglycerols.

Step 3 of Lipid Digestion

The pancreas secretes enzymes that attach to the surface of a lipid droplet and degrade triacylglycerols into free fatty acids and monoacylglycerols.

Lipases

An enzyme that degrades a triacylglycerol into free fatty acids and monoacylglycerols.

Step 4 of Lipid Digestion

The pancreatic lipases are also released into the intestine as zymogens that are subsequently activated in a cascade involving trypsin. The final digestion products (free fatty acids and monoacylglycerol) are carried in micelles to the plasma membrane of the intestinal epithelial cells. In these micelles, the end products are absorbed.

Micelles

Globular structures formed by amphipathic molecules in which the hydrophilic part is exposed to water, and the hydrophobic part is sequestered inside, away from water.

Step 5 of Lipid Digestion

The free fatty acids and monoacylglycerols are transported into the intestinal cells by membrane proteins called fatty-acid-transport proteins (FATPs).

Step 6 of Lipid Digestion

Once inside the intestinal cell, fatty-acid-binding proteins (FABPs) ferry them to the cytoplasmic face of the Smooth Endoplasmic Reticulum (SER). Here, triacylglycerols are resynthesized from fatty acids and monoacylglycerols.

Step 7 of Lipid Digestion

The triacylglycerols associate with specific proteins and a small amount of phospholipid and cholesterol to form lipoprotein transport particles, known as chylomicrons (CMs).

Chylomicrons (CM)

A lipoprotein that transports dietary triacylglycerol from the intestine to other tissues.

Step 8 of Lipid Digestion

Chylomicron particles are then released into the lymph system, and then into the blood.