A sugar consists of a chain of carbons; one carbon is double-bonded to oxygen to form a carbonyl, and all the other carbons have has a hydroxyl group. Sugars are grouped as either aldoses or ketoses. An aldose has the carbonyl group at one of the carbon chain ends, while a ketose has the carbonyl within the carbon backbone. Sugars exist also in cyclic forms, giving rise to hemialdoses an hemiketoses; the former has only hydroxyl groups and one ethanol group attached to the ring, while the latter has hydroxyl groups, an ethanol group and an additional alcohol group. A six-membered sugar ring is a pyranose, and a five-membered ring is a furanose. The chain form of a sugar has many different enantiomers: a six-carbon chain will have sixteen different stereoisomers, with eight pairs of enantiomers (mirror-images). Thus, only half the stereoisomers are named, and the mirror images are distinguished as either D or L enantiomers.

The chiral carbon furthest from carbonyl is used to determine whether the structure is D or L. When drawn as a chain form Fischer projection, if the hydroxyl is on the right then it is D; if on the left, it is L. The ring forms are completely predicted via this D or L notation, except for the anomeric carbon, conventionally drawn as the rightmost carbon of the ring and with the oxygen atom to its left. The hydroxyl group attached to it can either point up or down; when it is pointing up, that is the β conformation; and when down, that is the α conformation. Thus, chain glucose can be either D or L, and the ring form can be α-D, α-L, β-D or β-L. However, in nature all sugar exist in their D forms except for arabinose, yet the cyclic forms are mutarotatory: they exist in solution as either α or β, interconverting by reverting to their chain form then forming a new ring until a proportional homeostasis is established. Rarely do chain forms exist stably in solution.

MonosaccharideOne sugar molecule.
DisaccharideTwo sugar molecules bonded via glycosidic linkage. For example: glucose and fructose link together to form sucrose.
OLigosaccharideSeveral sugar molecules bonded together.
PolysaccharideHundreds to thousands of sugar molecules bonded together.
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