Steroids

A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes which alter membrane fluidity; and as signaling molecules. Hundreds of steroids are found in plants, animals and fungi.

The steroid core structure is typically 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. Steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. Cutting Ring B produces secosteroids one of which is vitamin D3. 类固醇的核心结构通常由十七个碳原子组成，以四个‘融合’的环连接：三个六元环（第一幅图中的环A、B和C）和一个五元环（环D）。类固醇的差异主要体现在连接到这个四环核心的官能团以及环的氧化状态上。类固醇还可以发生更剧烈的修饰，例如改变环的结构，例如切断其中一个环。切断环B会产生次类固醇，其中一种是维生素D3。

Examples include the lipid cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.

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