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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 drug dexamethasone. Steroids have two principal biological functions: certain steroids are important components of cell membranes which alter membrane fluidity, and many steroids are signaling molecules which activate steroid hormone receptors.

Complex chemical diagram
Steroid ring system: The parent ABCD steroid ring system (hydrocarbon framework) is shown with IUPAC-approved ring lettering and atom numbering.

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 drug dexamethasone. Steroids have two principal biological functions: certain steroids (such as cholesterol) are important components of cell membranes which alter membrane fluidity, and many steroids are signaling molecules which activate steroid hormone receptors.

The steroid core structure is composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups 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 a skeleton derived from cholestane. They can also vary more markedly by changes to the ring structure (for example, ring scissions which produce secosteroids such as vitamin D3).

Hundreds of steroids are found in plants, animals and fungi. All steroids are manufactured in cells from the sterols lanosterol (animals and fungi) or cycloartenol (plants). Lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene.

Filled-in diagram of a steroid
Space-filling representation
Ball-and-stick diagram of the same steroid
Ball-and-stick representation
5α-dihydroprogesterone (5α-DHP), a steroid. The shape of the four rings of most steroids is illustrated (carbon atoms in black, oxygens in red and hydrogens in grey). The apolar "slab" of hydrocarbon in the middle (grey, black) and the polar groups at opposing ends (red) are common features of natural steroids. 5α-DHP is an endogenous steroid hormone and a biosynthetic intermediate.

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