Editing Lipid (section)

===Sterols===
[[File:Cholesterol.svg|thumb|280px|alt=Chemical diagram|Chemical structure of [[cholesterol]]]]
{{Main|Sterol}}
Sterols, such as [[cholesterol]] and its derivatives, are an important component of membrane lipids,<ref name="Bach_2003" /> along with the glycerophospholipids and sphingomyelins. Other examples of sterols are the [[bile acid]]s and their conjugates,<ref name="Russell_2003" /> which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the [[phytosterols]], such as [[β-sitosterol]], [[stigmasterol]], and [[brassicasterol]]; the latter compound is also used as a [[biomarker]] for [[algae|algal]] growth.<ref name="Villinski_2008"/> The predominant sterol in [[fungal]] cell membranes is [[ergosterol]].<ref name="Deacon 2005"/>

Sterols are [[steroid]]s in which one of the hydrogen atoms is substituted with a [[hydroxyl group]], at position 3 in the carbon chain. They have in common with steroids the same fused four-ring core structure. Steroids have different biological roles as [[hormone]]s and [[signaling molecules]]. The eighteen-carbon (C18) steroids include the [[estrogen]] family whereas the C19 steroids comprise the [[androgen]]s such as [[testosterone]] and [[androsterone]]. The C21 subclass includes the [[progestogens]] as well as the [[glucocorticoid]]s and [[mineralocorticoids]].<ref name = "Stryer_2007" />{{rp|749}} The [[secosteroid]]s, comprising various forms of [[vitamin D]], are characterized by cleavage of the B ring of the core structure.<ref name="Bouillon_2006"/>