26. What is the structure of cholesterol?

Cholesterol is a type of lipid molecule and is classified as a sterol. Its structure consists of four fused hydrocarbon rings, which form the steroid nucleus. Here are the key features of cholesterol's structure: 1. **Steroid Backbone**: Cholesterol has a core structure made up of four interconnected carbon rings (labeled A, B, C, and D in steroid nomenclature). 2. **Hydroxyl Group**: It has a hydroxyl (-OH) group attached to the first carbon of the A ring, which makes it an alcohol and contributes to its amphipathic nature (having both hydrophilic and hydrophobic parts). 3. **Hydrocarbon Tail**: There is a hydrocarbon tail (a branched alkyl chain) attached to the D ring, which contributes to its hydrophobic characteristics. 4. **Double Bonds**: Cholesterol contains a double bond between carbons 5 and 6 in the B ring, which is characteristic of many sterols. The overall chemical formula for cholesterol is C27H46O. Its structure allows it to play a crucial role in cell membrane fluidity and serves as a precursor for the synthesis of steroid hormones, bile acids, and vitamin D.