Disaccharides

Free monosaccharides are actually rare in nature — most of the sugar found in living organisms exists combined as disaccharides. A disaccharide is formed when two monosaccharide units react by condensation (a reaction that releases a molecule of water and consumes energy), forming a glycosidic bond that links the two sugar units together:

C₁₂H₂₂O₁₁ + H₂O → (hydrolysis) C₆H₁₂O₆ + C₆H₁₂O₆

(disaccharide + water → monosaccharide + monosaccharide; the reverse reaction, condensation, runs the other way and releases water instead)

The three common disaccharides are sucrose (cane sugar; hydrolyses to glucose + fructose), lactose (milk sugar; hydrolyses to glucose + galactose) and maltose (malt sugar; hydrolyses to two units of glucose). Sucrose is a non-reducing sugar, because both of its component sugars' aldehyde/ketone groups are used up in forming the glycosidic bond, leaving no free reducing group; lactose and maltose, by contrast, are reducing sugars (lactose also exists in a beta form, made of β-galactose and β-glucose). …