monosaccharide

Meaning: A monosaccharide is a single sugar unit: a polyhydroxy aldehyde (aldose) or ketone (ketose) with usually three to seven carbons. Glucose and galactose are aldohexoses, fructose is a ketohexose, and ribose is an aldopentose. In water, five- and six-carbon sugars mostly cyclize when a hydroxyl attacks the carbonyl, forming a hemiacetal (aldoses) or hemiketal (ketoses). This creates a new stereocenter, the anomeric carbon, giving α and β anomers. Monosaccharides join through glycosidic bonds to form disaccharides and polysaccharides.

Equation: (CH₂O)n, where n is the number of carbons (a count, no unit); glucose has n = 6, so C₆H₁₂O₆.

On the MCAT: Expect questions that classify a sugar as an aldose or ketose, assign D or L from the stereocenter farthest from the carbonyl, or spot epimers (glucose and galactose differ only at C4). You may also convert a Fischer projection to a Haworth ring, locate the anomeric carbon, or decide whether a sugar is reducing (free anomeric carbon, positive Benedict or Tollens test).

Common confusion: D and L describe configuration, not the direction light is rotated; that is shown by (+) and (−). D-glucose is (+), but D-fructose is (−). Also, anomers differ only at the anomeric carbon of the ring form, while epimers such as glucose and galactose differ at one other stereocenter.