Fig. 8.1.

All these substances have a nucleus containing the four-ringed carbon skeleton of cyclopentenophenanthrene and are known as steroids. The sterols are one of the steroids and they are widely distributed in humans, animals, and plants. It is often called cholesterol in humans and animals and phytosterols (also called plant sterols) in plants. Notably, the general structural formula for the sterols includes the designation of the four rings with a side chain at C-17 and two methyl groups at C-18 and C-19. Cholesterol is one of the most abundant steroids in bile. Its hydroxyl group on the third carbon can react with the COOH group of a fatty acid molecule to form a cholesteryl ester. Plant sterols (e.g., β-sitosterol and β-sitostanol) are naturally occurring. Their chemical structures are very similar to cholesterol but with structural modifications of the side chain. In addition, stanols are saturated sterols, having no double bonds in the sterol ring structure, e.g., β-sitostanol