Steroids

A Steroid is an organic compound with four rings  arranged in a specific configuration . Examples include the dietary lipid cholesterol.

The sex hormones estradiol and testosterone and the anti-inflammatory  drugdexamethasone . Steroids have two principal biological functions: certain Steroids such as cholesterol are important components of cell membranes which after membrane fluidity and many steroids are signaling molecules which activate steroid hormones receptors.

The steroid core structure  is composed of seventeen carbon atoms , bonded in four fused rings.

Three six member cyclohexane ring ( ring A, B and C in the first illustration ) and one five member cyclopentane rind(the D ring). Steroids vary by the functional group attached to this four ring core and  by the oxidation  state of the rings. Sterols are forms of steroids with a hydroxyl group  at position three and skeleton derived from cholestane.

They can also vary  more markedly by change to the ring structure.  Hundreds  of Steroids are found in plants . animals and fungi. All steroids are manufactured in  from the sterols  lanosterol(animals and fungi). or cycloartenol(plants).  Lanosterols and cycloartenol are derived from thecyclization of the tritepene squalene.