Digestion Lipid digestion begins in the mouth with lingual lipases and continues in the stomach where gastric lipase is added to the mixture. Most digestion of lipids occurs in the small intestines where chyme is mixed with lipases in pancreatic juice. Lipids being hydrophobic form globules. Lipases are hydrophilic and can only act upon the lipid on the globule's surface.

Action of Bile Salts Bile salts enhance the digestive action of lipases by breaking down fat globules into smaller droplets in a process called emulsification. This increases the surface area where lipases can act.

Bile salts are synthesized by hepatocytes and are secreted in bile which mixes with chyme. Bile salts are derived from cholesterol. Cholesterol is hydrophobic but when converted to bile salts possess a number of oxygen containing polar groups that are hydrophilic. Because all the polar groups are located on one side of the molecule, the molecule is amphipathic. The bile salts break up the fat globules into smaller hydrophilic coated droplets that more readily mix with water.

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Action of Pancreatic Lipase Pancreatic lipases work on the lipids on the surface of the lipid droplets by breaking the bonds of fatty acids on either end of the glycerol backbone. The fatty acids and monoglycerides are released into solution. Of these end-products of digestion some are absorbed by the epithelial cells while others remain in aggregations called micelles.

The fat droplets gradually disappear as they move down the small intestines toward the colon. In the ileum the bulk of the bile salts are absorbed and recycled by the liver by a pathway referred to as the enterohepatic circulation.

Absorption of Lipids Fatty acids and monoglycerides enter the enterocytes by simple diffusion. Inside the enterocytes the molecules are reassembled into triglycerides and are packaged into large particles called chylomicrons乳糜微滴 . The chylomicrons are secreted across the basolateral membrane by exocytosis. The chylomicrons enter the lymphatic capillaries. The flow of lymphatic fluid carries the chylomicrons to the bloodstream.