Steroli

Steroli, in English typically referred to as sterols, are a class of steroid alcohol lipids that are essential components of cell membranes in all eukaryotes and also serve as precursors to a variety of bioactive molecules. They share a common tetracyclic ring structure and a hydroxyl group, usually at the third carbon, with a hydrophobic side chain. The term sterol is used for the general class, while steroli is the plural form in several languages such as Italian.

Common examples include cholesterol, the principal sterol in animal membranes; ergosterol, the main fungal sterol; and plant sterols (phytosterols) such as sitosterol, campesterol, and stigmasterol. In plants, sterols contribute to membrane properties and also act as precursors to other signaling molecules, including brassinosteroids.

Biosynthesis of steroli begins with the mevalonate pathway, producing isoprenoid units that are assembled into squalene. In animals and fungi, squalene is cyclized to lanosterol and subsequently converted to cholesterol or ergosterol, respectively. In plants, cycloartenol formation leads to a variety of plant sterols.

Functions of steroli include modulating membrane fluidity, permeability, and thickness, and organizing membrane microdomains such as lipid rafts. They are precursors to important steroids: in animals, bile acids and steroid hormones derive from cholesterol; in plants, sterols support growth and development and can be converted to brassinosteroids.

Dietary steroli, particularly plant sterols, can reduce intestinal absorption of cholesterol and help lower LDL cholesterol levels. High intakes may interfere with absorption of fat-soluble vitamins, and rare genetic disorders like sitosterolemia can cause abnormal sterol accumulation.