Biology Question #1038

harish, a 14 year old male from scarborough asks on November 11, 2002,

What is the difference between fats and cholestrol?

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The answer

David Winsemius answered on November 27, 2003

Cholesterol is a fat-soluble steroid whereas fats are long chain fatty acids hooked up in twos and threes to form di- and tri-glycerides. Cholesterol has multiple carbon rings and has hydroxyl groups hanging off in a few places. Most cholesterol is made by the liver. The body needs cholesterol so that membranes work properly. More on cholesterol metabolism at Indiana State University.

The fats, phospho-lipids, and fatty acids are not attracted to water as much as water is attracted to water. The fats and water tend to separate because it lowers the overall energy in a system. So fats get labelled hydo-phobic. They are not really hydro-phobic but they are not as hydrophilic as water and many proteins are. The glycerol or phosphate end are more attracted to water, so those ends stay close to the water and the fat ends line up together in sheets. Two sheets can face each other on their fat sides and this is the basis for membranes in animals and plants.

Cells use this behaviour to build up membrane barriers and do specialized stuff inside these walled off areas. Typical membranes are the cell's outer membrane, the membrane that surrounds the nucleus, Golgi apparatus and the different types of endoplasmic membranes. They are all made of fat layers, cholesterol, and special proteins that add structure and special capabilities. It is almost too beautiful to comprehend. I was working in a genetics lab when the fluid mosiac model of membranes was described. I thought it was absolute genius at the time and the years have fulfilled my expectations. Here is a picture of a model membrane, and here is an explanation of the fluid mosaic model.

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