Lactose is a disaccharide with a chemical formula C12H22O11. It is constituted of one molecule of glucose and one molecule of galactose.
The disaccharide lactose is found primarily in milk and products made from milk. Lactase is an essential enzyme required for the digestion of this sugar in milk. In the absence of lactase the chemical reaction responsible to convert lactase to the constituent simple sugars cannot be carried out and this leads to lactose intolerance.
The catalytic action of lactase results in the enzymatic hydrolysis of the disaccharide lactose producing one molecule of glucose and one molecule of galactose.