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Carbohydrates are polyhydroxy aldehydes or ketones with the general formula (CH₂O)n. They are classified by chain length (monosaccharides 3-7 carbons, oligosaccharides 2-10 units, polysaccharides >10 units) and by aldehyde vs. ketone functional groups (aldoses vs. ketoses). Carbohydrates form cyclic structures (hemiacetals and acetals) in aqueous solution, adopting six-membered pyranose or five-membered furanose rings, with multiple stereoisomeric forms (anomers and epimers).
Draw the structures of major monosaccharides (glucose, fructose, galactose) in both open-chain and cyclic forms. Recognize the α and β anomers and understand how they interconvert via mutarotation. Use Fischer and Haworth projections interchangeably.
Carbohydrates are the most abundant biomolecules on Earth, and their structural diversity arises from a surprisingly simple chemical framework. Starting from organic chemistry, you know that aldehydes and ketones are carbonyl compounds — a carbon double-bonded to oxygen. Carbohydrates are simply polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses): carbonyl compounds with hydroxyl (−OH) groups on every other carbon. The general formula (CH₂O)n reflects this: for every carbon, you have roughly one oxygen and two hydrogens, as though water molecules were strung onto a carbon chain.
The classification system follows logically from structure. Chain length gives you the prefix: trioses (3C), pentoses (5C), hexoses (6C), and so on. The position of the carbonyl gives you the suffix: *ald*ose if it's a terminal aldehyde, *ket*ose if it's an internal ketone. Combine them and you can name any monosaccharide: glucose is an aldohexose, fructose is a ketohexose. Beyond single sugar units, two monosaccharides joined by a glycosidic bond make a disaccharide; chains of many units make polysaccharides.
Here is where a critical misconception must be addressed: the open-chain structural formulas you learn first are not what glucose actually looks like in solution. In water, the C5 hydroxyl group attacks the C1 aldehyde intramolecularly, forming a ring via a hemiacetal. This produces a six-membered pyranose ring (named after pyran) that accounts for more than 99% of glucose molecules at equilibrium. The open-chain form is only a transient intermediate. Fructose and ribose prefer five-membered furanose rings. Drawing the open-chain form is a useful shorthand and a pedagogical starting point, but the cyclic form is the biochemically relevant structure.
Ring formation creates a new stereocenter at C1 — the anomeric carbon. The hydroxyl that forms there can point in two directions relative to the ring, giving α and β anomers. In α-D-glucose (the common convention), the C1 −OH is *axial* (pointing down in the Haworth projection, opposite the CH₂OH); in β-D-glucose it is *equatorial* (pointing up, same side as CH₂OH). This distinction matters enormously in biology: starch is made of α-glucose linked at C1–C4, and cellulose is made of β-glucose linked the same way. Humans can digest starch but not cellulose because our enzymes are stereospecific — they recognize α-linkages but not β-linkages.
Finally, distinguish anomers from epimers. Anomers differ only at the anomeric carbon (C1 in glucose). Epimers differ at exactly one other stereocenter in the ring. Glucose and galactose are epimers — they are identical except at C4, where the hydroxyl points in the opposite direction. This single stereochemical difference makes galactose a distinct molecule requiring different transporters, enzymes, and metabolic pathways. Stereochemistry in sugars is not a minor detail; it is the primary determinant of biological identity.