All disaccharides are formed by glycosidic bonds (acetal linkages) between two monosaccharides with loss of H2O.

Sucrose: alpha-D-glucopyranosyl-(1→2)-beta-D-fructofuranoside. Both anomeric C involved → non-reducing. Hydrolyzed by sucrase/invertase or dilute acid.

Maltose: alpha-D-glucopyranosyl-(1→4)-D-glucopyranose. One anomeric C free → reducing. Product of starch digestion by amylase.

Lactose: beta-D-galactopyranosyl-(1→4)-D-glucopyranose. Note: galactose unit has BETA linkage (unlike alpha in maltose). One anomeric C free → reducing. Lactose intolerance = deficiency of beta-galactosidase enzyme.

The alpha/beta designation of the glycosidic bond refers to the configuration at the anomeric carbon of the sugar providing that carbon. Alpha: OH was axial before bond formation. Beta: OH was equatorial.