Carbohydrates: sugars and their isomers

Living things are built mainly from four kinds of large molecules: carbohydrates, lipids, proteins and nucleic acids. Carbohydrates are, chemically, aldehyde or ketone derivatives of alcohols with many -OH groups. They give energy, store energy, build cell walls, form part of DNA and RNA, and help cells recognise each other.

By size, carbohydrates are of three classes:

• Monosaccharides: single sugar units that cannot be split further; general formula Cn(H₂O)n. By number of carbons they are trioses (3), tetroses (4), pentoses (5) or hexoses (6). By their reactive group they are aldoses (with an aldehyde group: glucose, ribose) or ketoses (with a ketone group: fructose, ribulose).
• Oligosaccharides: two to ten units joined.
• Polysaccharides: more than ten units.

A carbon atom with four different groups attached is chiral: it can exist in two mirror-image arrangements, like left and right hands. A sugar with n chiral carbons can have 2ⁿ different forms (stereoisomers); glucose has 4, so 2⁴ = 16. Whether a sugar is D or L depends on the -OH on the chiral carbon farthest from the carbonyl group. Most sugars in living things are D-sugars.

In water, glucose mostly closes into a ring. Closing the ring creates a new chiral carbon (the anomeric carbon), giving two forms, alpha and beta, called anomers. They slowly change into each other in solution (mutarotation). Epimers differ at one carbon only: mannose differs from glucose at carbon 2, and galactose at carbon 4.

Two sugars are joined by a glycosidic bond. Common disaccharides: maltose = glucose + glucose; lactose (milk sugar) = galactose + glucose; sucrose (table sugar) = glucose + fructose. Raffinose has three units (a trisaccharide). Sugars joined to proteins or lipids form glycoproteins, proteoglycans and glycolipids, found on cell surfaces.