MCAT Biochemistry
Dietary fat consists mainly of triglycerides, but also includes free fatty acids, phospholipids, cholesterol, and cholesteryl esters. The bulk of lipid metabolism occurs in the small intestine. LLipids are hydrophobic and require emulsification in the intestinal lumen with the help of bile, which is synthesized by hepatocytes in the liver and secreted into the duodenum by the gallbladder. Emulsification allows lipids and water to mix together on a macroscopic level, but not on a molecular level.
This process is aided by pancreatic enzymes that hydrolyze the ester bonds of triglycerides, and cholesteryl esters. The lipid metabolites are then packaged into micelles and move into the brush border of intestinal mucosal cells. Inside the cytosol, lipid metabolites are re-esterified, reforming triglycerides and cholesteryl esters. These compounds are then packaged into chylomicrons to enter the bloodstream. However, not all lipids require chylomicrons to enter the bloodstream, as short-chain fatty acids can enter directly by simple diffusion.
Lesson Outline
<ul> <li>Dietary fat composition: <ul> <li>Triglycerides</li> <li>Free fatty acids</li> <li>Phospholipids</li> <li>Cholesterol</li> <li>Cholesteryl esters</li> </ul> </li> <li>Lipid metabolism occurs mainly in the small intestine</li> <li>Emulsification of lipids with bile: <ul> <li>Bile produced in the liver and secreted by the gallbladder</li> <li>Bile salts help mix lipids and water</li> </ul> </li> <li>Role of pancreatic enzymes: <ul> <li>Pancreatic lipase, colipase, cholesterol esterase</li> <li>Hydrolyze ester bonds of triglycerides and cholesteryl esters</li> </ul> </li> <li>Micelle formation and function: <ul> <li>Formed by amphipathic molecules like phospholipids and bile salts</li> <li>Shield hydrophobic lipid metabolites</li> <li>Facilitate transport through the intestine</li> </ul> </li> <li>Absorption process: <ul> <li>Micelles move into brush border of intestinal mucosal cells</li> <li>Lipid metabolites released into cell's cytosol</li> <li>Re-esterification of lipid metabolites to form triglycerides and cholesteryl esters</li> <li>Packaging into chylomicrons</li> </ul> </li> <li>Chylomicrons: <ul> <li>Water-soluble lipid transporters made of amphipathic lipid molecules</li> <li>Contain lipoproteins called apolipoproteins (or apoproteins)</li> <li>Least dense of lipid transporters</li> </ul> </li> <li>Chylomicron transport: <ul> <li>Leave intestinal mucosal cells</li> <li>Enter the lymphatic system through lacteals</li> <li>Travel through lymphatic system to thoracic duct</li> <li>Empty into left subclavian vein</li> </ul> </li> <li>Short-chain fatty acids can enter the bloodstream directly by simple diffusion</li> <li>Unabsorbed lipids continue to the large intestine and become part of stool</li> </ul>
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FAQs
Lipid digestion begins in the small intestine, where bile salts produced by the liver and stored in the gallbladder are released. These bile salts emulsify dietary lipids, increasing their surface area and making them more accessible to pancreatic enzymes. Pancreatic lipase then breaks down the lipids into fatty acids and monoglycerides, which can be absorbed by intestinal cells.
Micelle formation is essential for lipid absorption because it allows the water-insoluble fatty acids and monoglycerides to be solubilized in the watery environment of the small intestine. Micelles are formed when bile salts and phospholipids, which are amphipathic molecules, surround the lipids, forming a hydrophilic outer layer and a hydrophobic core. The micelles carry the lipids through the watery environment to the surface of intestinal cells, where they are absorbed.
Bile salts facilitate the emulsification of dietary lipids by reducing the surface tension between lipids and water. As amphipathic molecules, bile salts have both hydrophobic and hydrophilic regions. The hydrophobic part of the bile salt interacts with lipids while the hydrophilic part interacts with water. This creates small lipid droplets with a larger surface area, making them more accessible to digestive enzymes.
Pancreatic enzymes, primarily pancreatic lipase, play a crucial role in lipid digestion and absorption. Pancreatic lipase breaks down the emulsified lipids, specifically triglycerides, into fatty acids and monoglycerides. These products are then incorporated into micelles, which allows them to be transported through the watery environment of the small intestine and absorbed by intestinal cells. Other pancreatic enzymes, such as colipase and phospholipase, also contribute to the digestion of lipids.
Once lipids are absorbed by the intestinal cells, they are reassembled into triglycerides and combined with other lipids, cholesterol, and proteins to form chylomicrons. Chylomicrons are large lipoprotein particles that have a hydrophilic outer layer, allowing them to be transported in the watery environment of the bloodstream. Chylomicrons are released from the intestinal cells into the lacteals, which are lymphatic vessels, and transported through the lymphatic system before entering the bloodstream, where they deliver lipids to various tissues.
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