carbohydrate

Carbohydrates are the sugars and starches — the table sugar in your tea, the starch in a potato, the cellulose in wood and cotton, the glucose your blood carries to feed your cells. They are nature's most abundant organic compounds, and at heart they are nothing more exotic than a chain of carbons each wearing a hydroxyl group, with one carbon turned into a carbonyl. From an organic-chemistry point of view, a sugar is just a small molecule with a lot of -OH groups and one C=O.

The name is a fossil of an old observation: many of them have the formula Cn(H2O)m — they look like carbon with water added, hence carbo-hydrate, even though they contain no actual water molecules. More usefully, a carbohydrate is a polyhydroxy aldehyde or ketone (or something that gives one on hydrolysis). Glucose, for example, is a six-carbon chain with five hydroxyls and an aldehyde at the end: that aldehyde and a nearby hydroxyl are exactly the functional groups you already know, and they react in exactly the ways you already learned — the aldehyde and an -OH on the same molecule close up into a ring, a cyclic hemiacetal.

Carbohydrates matter because everything you learned about carbonyls, hydroxyls, hemiacetals and acetals comes alive here in the molecules of life. The sweetness of sugar, the way starch stores energy and cellulose builds plant walls, the blood-group markers on your cells, all trace back to which sugars are linked and how. They are the first of the four great biomolecule families, and the cleanest example of ordinary organic functional groups doing biology's work.