Lactose (milk sugar) is a disaccharide - formula C12
- consisting of one
and one glucose unit
The two sugars are linked via a glycosidic bond - a beta 1-4 bond
between galactose and glucose.
This bond - effectively an oxygen bridge - is formed as a result of a condensation reaction
bonding carbon atom numbers.
Note that one of these ring-shaped sections (glucose) is in equilibrium with an
open-chain form in which Carbon 1 has a CHO aldehyde group
which gives it reducing properties
, so that it reacts with reagents such as Benedict's.