micelle
Meaning of micelle: A ball of soap-like molecules hiding a greasy core inside a water-friendly shell, which is how soap lifts fat and how the gut absorbs it.
Definition of micelle
A micelle is a tiny sphere that forms when molecules with a water-loving end and a fat-loving end gather together in water. The fat-loving tails turn inward, away from the water, and the water-loving heads face outward. The result is a droplet with a greasy interior and a soluble surface, able to carry fatty material through watery fluid that would otherwise reject it.
Why do they form at all?
Water molecules cling tightly to one another, and a greasy chain cannot join in. Rather than sit as an obstacle, such chains are pushed together until they end up shoulder to shoulder with their water-loving heads outward, which costs the water far less. Below a certain concentration these molecules drift about singly. Above it they assemble into spheres all at once, which is why detergents and bile salts behave so differently at low and high concentration. Nothing holds a micelle together except that arrangement, so it forms and breaks apart continually.
Where does the body use them?
In the small intestine, and the arrangement is essential rather than incidental. The liver makes bile salts, which are shaped exactly this way, and releases them after a meal. Fat broken down by pancreatic enzymes is not soluble in the watery contents of the gut and cannot reach the lining on its own. Packed into micelles it is ferried across, released at the surface and absorbed. Vitamins A, D, E and K travel the same route, which is why they are grouped apart from every water-soluble vitamin. When bile cannot reach the gut, fat passes through undigested and those four vitamins fall short with it.
How does a micelle differ from similar structures?
| Structure | Build | What it carries |
|---|---|---|
| Micelle | Single layer, greasy center | Fat and fat-soluble drugs |
| Liposome | Double layer, watery center | Water-soluble drugs |
| Emulsion droplet | Much larger, film of surfactant | Bulk oil, as in milk |
| Cell membrane | Double layer, spread as a sheet | Nothing; it is the boundary |
Used in a sentence
The compound was formulated in micelles to keep a poorly soluble drug in solution.
Good to know
The molecules forming a micelle are not simply water-soluble; each carries a water-repelling section, and it is that section being driven out of contact with water that builds the structure in the first place.
Word origin
Where “micelle” comes from
From the Latin mica, meaning a crumb or grain, with a diminutive ending, so the word reads as a very small particle.