monosaccharide

A monosaccharide is a single sugar unit — the smallest, simplest carbohydrate that cannot be broken into a smaller sugar by hydrolysis. Glucose, fructose and galactose are the famous ones; ribose, the sugar in RNA, is another. If a polysaccharide like starch is a long necklace, a monosaccharide is one bead. They are the building blocks from which every larger carbohydrate is assembled.

Each monosaccharide is a polyhydroxy aldehyde (an aldose) or polyhydroxy ketone (a ketose), with a backbone of three to seven carbons. Glucose is an aldose with six carbons (an aldohexose); fructose is a ketose with six (a ketohexose); ribose is an aldopentose with five. Here is the twist that the open-chain formula hides: in water a five- or six-carbon sugar does not stay open. One of its own hydroxyl groups reaches over and attacks the carbonyl carbon — the same nucleophilic addition that makes a hemiacetal — so the chain bites its own tail and curls into a ring. Glucose forms a six-membered ring (a pyranose), fructose a five-membered one (a furanose). At equilibrium the ring forms dominate, with only a trace of the open aldehyde present at any moment.

This curling-up is not a footnote; it is why sugars behave the way they do. The ring-closing creates a brand-new chiral center (the anomeric carbon) and two ring forms, alpha and beta, that interconvert. Most importantly, the ring's anomeric -OH is the handle for joining sugars together: when it reacts with the -OH of another sugar it forms a glycosidic bond, and that single linkage, repeated, builds maltose, sucrose, starch and cellulose. Every disaccharide and polysaccharide is just monosaccharides chained through their anomeric carbons.