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Micelles are spheres of lipids that form in aqueous solutions. In humans, they form from bile salts. These micellar aggregates help transport the digestive products of lipids to the intestine to be absorbed. Also, they are used as detergents.
A portion of the food we eat is made of oils and fats. These are degraded during digestion in the stomach, by a lipase enzyme, into lipid-like compounds, including fatty acids. Additional degradation takes place in the pancreas using a different lipase. The lipases are water soluble, unlike the compounds they degrade. Degradation requires the help of biological detergents known as bile salts, secreted by the gall bladder and liver.
Micelles are produced from the bile salts that help make the fatty acids, and other lipid breakdown products, available for degradation by a lipase. The principle of micelle formation is like that of oil not mixing with water. Many classes of lipids have a head group that is polar and interacts well with water. They also have two tail groups that are hydrophobic. These tail groups do not interact well with water and prefer to be in clusters with other hydrophobic molecules, like a form of oil.
At low concentrations, lipids and fatty acids are soluble in water. The driving factor for micelle formation is when these compounds reach a higher concentration, known as the critical micelle concentration (CMC). At concentrations greater than the CMC, the hydrophobic tail groups spontaneously come together to avoid the water. The polar head groups stick out, facing the water, while the center is full of hydrophobic tails that exclude water. This generally gives the miceller aggregates the structure of a sphere, although they can be shaped like a disk.
Mixed micelles can be composed of various compounds that are not very soluble in water. These compounds are dissolved in the center of the sphere where they can blend in with the hydrophobic tails. In this manner, they are transported by micellar aggregates of bile salts to the intestine to be further broken down. These structures are the major way in which lipids get to the cell surface of the intestine to be absorbed. The rate of transport can be increased 1,000 times over that of individual fatty acids.
Cholesterol secreted by the liver can also travel in bile, in micelles. It is virtually insoluble in aqueous solutions, but is soluble in mixed micelles. Sometimes the body produces super-saturated bile with a high ratio of cholesterol. This can lead to gallstone formation. Other types of compounds that travel in these micellar aggregates are lipid-soluble vitamins, such as A, D, E, and K.
The structure of micelles is very similar to the lipid bilayer of biological membranes. The bilayers have the same type of interactions. The lipids, however, face each other giving a double layer of lipids instead of a sphere. The principle is the same as the structure of micellar aggregates with a hydrophobic interior and a polar exterior. Some types of lipids can readily exchange into the cellular membrane, from the micellar aggregrate.
Lipid bilayers can join ends to form a circle known as a liposome. These structures have an internal compartment. Liposomes are being used medically as carriers for drugs and enzymes. This allows doctors to target particular organs. It helps to avoid side effects like tissue damage and drug breakdown that can happen as drugs are introduced into the body by normal methods.
The compounds that make up micelles are also known as surfactants. These are compounds that are soluble in both oil and water. They allow compounds that are barely soluble in water to accumulate to higher concentrations within the micellar aggregates. The surfactant properties of these aggregates make them useful as detergents. They can dissolve oily deposits on clothes that will not wash off in water.
There is another type of micelle that is the reverse of the oil-in-water type. It has water-soluble substances dissolved in an organic solution. In this case, however, the polar head groups are in the center of the micelles, while the hydrophobic groups are on the outside, interacting with the organic solvent.