disaccharide

Meaning: A disaccharide is two monosaccharides linked by a glycosidic bond. The bond forms when an anomeric carbon reacts with a hydroxyl group of the other sugar, releasing water. Linkages are named by α or β and the carbons joined. Examples: maltose = glucose + glucose, α-1,4 lactose = galactose + glucose, β-1,4 sucrose = glucose + fructose, α-1,β-2 Enzymes (maltase, lactase, sucrase) hydrolyze these bonds; the intestine absorbs the monosaccharides.

Equation: 2 C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O (two hexoses condense to a disaccharide). One water is released per glycosidic bond formed, and hydrolysis reverses the reaction.

On the MCAT: Expect a drawn structure and a question on the linkage (α or β, which carbons) or on whether the sugar is reducing (positive Benedict's or Tollens' test). A reducing sugar has a free anomeric carbon (hemiacetal) that opens to an aldehyde, as in maltose and lactose. Enzymes are specific to α or β linkages, so passages may ask which enzyme cleaves a given bond.

Common confusion: Students often assume every disaccharide is reducing. Sucrose's bond joins the anomeric carbons of both glucose and fructose, leaving no free hemiacetal, so it is non-reducing. Check for an anomeric OH, not OR.