monosaccharide (glucose)

A monosaccharide is a single sugar — the smallest, most basic unit of carbohydrate, the brick from which all longer sugars are built. The most famous one, glucose, is the molecule your body runs on day and night; it is the sugar your blood carries and your cells burn for energy.

Mono means one and saccharide means sugar, so a monosaccharide cannot be broken into smaller sugars. It is typically a short carbon chain dressed in hydroxyl groups, and in water it usually curls into a ring. Glucose has six carbons and the formula written as a kind of recipe (six carbons, twelve hydrogens, six oxygens). Other common single sugars include fructose (the sweetness of fruit) and galactose (part of milk sugar); some have the very same atoms as glucose arranged differently, which is enough to change their taste and behavior.

Glucose is special because it sits at the center of energy metabolism. Cells strip it apart step by step to capture its stored energy, ultimately powering nearly everything they do. Plants build glucose from sunlight, water, and carbon dioxide during photosynthesis; animals get it by digesting carbohydrates. From this one small ring flow the larger carbohydrate stories — link glucose units together and you get starch, glycogen, or cellulose.