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Emulsification of Fats
Fats are hydrophobic and as a result will not mix very well with the solution in the lumen of the small intestine nor with the chyme. Instead the fat molecules such as triglycerides and cholesterol will aggregate together to form large spherical bundles called fat globules. Due to the large size of the fat globule, pancreatic lipase (a water-soluble molecule) will have no way of actually reaching the inside portion of the fat globule. This means that the lipase can only cleave ester bonds of the triglycerides on the surface and it cannot access the inside portion, which makes the lipase very inefficient. To increase the efficiency and the rate at which lipase cleaves ester bonds, the liver produces and releases a fluid called bile. Bile is composed of amphipathic molecules such as phospholipids and bile salts. When bile enters the small intestine, it will mix with the fat globules and will cause them to break down into smaller units called emulsion droplets. This process is called emulsification. Emulsification greatly increases the surface area of the fat on which the lipase can actually act on. As a result, lipase is now in a position to begin digesting the ester bonds of the lipids efficiently. With the help of colipase, lipase binds onto the surface of these emulsion droplets and begins breaking them down. This is where digestion takes place. Eventually, the emulsion droplets are broken into fatty acids. Since fatty acids are hydrophobic, the bile phospholipids or bile salts can surround the fatty acids and form a tiny spherical structures called a micelles. The micelles are about two hundred times smaller than the emulsion droplets and can therefore easily cross the membrane of enterocytes and enter the cytoplasm of the cell.
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