The Cholesterol molecule - rotatable in 3 dimensions

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Steroids are like other lipids in that they are insoluble in water, but they cannot be broken down into fatty acids and glycerol. They are said to be unsaponifiable, meaning that they cannot be reacted with alkali to form soap-like compounds.

Cholesterol is a compound from which many steroids are derived. It is said to stabilise the plasma membrane's fluid mosaic structure composed of a bilayer of phospholipid molecules and other components. It is therefore present in all cells. However cholesterol is implicated in atherosclerosis. Some individuals synthesise excess cholesterol.

Its basic structure (cyclopentanoperhydrophenanthrene!) involves 4 carbon-carbon rings (three 6-membered and one 5-membered) combined , 17 carbons in all.

There are various groups attached to this which give each steroid its own particular function.

The -OH group (red with white) attached to carbon 3 is an alcohol group) - which explains the last 2 letters of its name.

The other important position is carbon 17 (numbered by the conventional system, not the one used by the definition file): in cholesterol there is a branched 8-carbon chain here.

Carbons 13 and 10 often carry other groups; here both have methyl groups.

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