A biotransformation (bioconversion) uses living cells or isolated enzymes to perform one or a few specific chemical transformations on a substrate — producing a structurally modified product. The cell is used as a catalyst, not primarily for its biosynthetic capacity. Examples: steroid hydroxylation, chiral resolution, acrylamide production from acrylonitrile by nitrile hydratase. In contrast, fermentation refers broadly to microbial production processes, usually involving complex metabolic pathways.