Galactose and Glucose Molecules

Galactose is an isomer of glucose. This means that they both share the same formula: C₆H₁₂O₆.

The only real difference is that the -H and -OH groups on carbon 4 (labelled) face in different directions.

Both of these are shown in the α-form, with the -OH on carbon 1 projecting downwards, but they can each be found in the β-form, with the -OH pointing upwards.

These two monosaccharide sugars are (stereo)isomers, because they contain the same atoms, but they differ in the arrangement of their atoms in space.

They can also be called epimers because they represent different configurations of atoms about a single stereogenic centre - in this case carbon 4, and this is enough to give them different properties and therefore different names, but they are not called anomers like α- and β-glucose.

Lactose is a disaccharide formed by the condensation of β-galactose and glucose.