Ergosterol is a sterol compound primarily found in fungal cell membranes, serving a structural role analogous to cholesterol in animal cells. It maintains membrane fluidity and integrity. Upon exposure to ultraviolet light, ergosterol is converted to ergocalciferol (vitamin D2), making it a key precursor in vitamin D production. It is insoluble in water but dissolves readily in lipids and organic solvents. Ergosterol is used in pharmaceuticals, dietary supplements, and as a marker for fungal contamination in food. Its structure features a tetracyclic steroid nucleus with specific double bonds at positions 5-6, 7-8, and 22-23, and a hydroxyl group at C3.