11.9: Digestion of Triglycerides
- Describe the process of digestion of triglycerides.
Triglycerides are esters and esters can undergo hydrolysis. When this hydrolysis happens in the digestive system the process is called digestion. Lipid digestion begins in the stomach. Lingual lipases present in the saliva are activated when they reach the acidic conditions of the stomach. Gastric lipases secreted in the stomach start the hydrolysis of the ester functional groups in triglycerides. Most of the digestion of triglycerides happen in small intestine. A hormone secreted in this region stimulates the gallbladder to discharge bile. The principal constituents of bile are the bile salts, which emulsify large, water-insoluble lipid droplets, disrupting some of the hydrophobic interactions holding the lipid molecules together and suspending the resulting smaller globules (micelles) in the aqueous digestive medium. These changes greatly increase the surface area of the lipid particles, allowing for contact with the enzyme lipases and thus rapid digestion of the fats.
The lipases catalyze the digestion of a triglyceride first to diglyceride and a fatty acid. The diglyceride is then hydrolyzed to a monoglyceride and a fatty acid as shown in figure 11.9.2.
The large and hydrophobic long-chain fatty acids and monoglycerides are not so easily suspended in the watery intestinal chyme. However, bile salts resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar (hydrophilic) ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids. The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the cells.
The free fatty acids and monoglycerides then enter the cells that line the small intestine and are converted back into triglycerides. The triglycerides are eventually carried through blood by lipoproteins.
Summary
Bile salts help to emulsify the hydrophobic triglycerides in the aqueous gastric solution. The enzyme lipase catalyze the hydrolysis (digestion) of the triglycerides to fatty acids and monoglycerides. These products of digestion then diffuse into the cells that line the small intestine and then converted back to triglycerides.