People with lactose intolerance lack functional lactase in the small intestine. Which reaction goes unfinished, and what does that missing reaction normally involve?

Core Concept

Lactose is a disaccharide whose two monosaccharides are joined by a glycosidic bond formed through condensation when milk is made. Lactase performs the reverse chemistry: it positions a water molecule so its OH and H fragments insert across the bond, freeing one glucose and one galactose. Without functional lactase the intact sugar travels to the colon, where microbes ferment it and release gas and acids that cause discomfort. Absorbed monosaccharides, by contrast, pass easily into the bloodstream.