The most useful carbohydrate classification scheme divides the carbohydrates into groups according to the number of individual simple sugar units. Monosaccharides contain a single unit; disaccharides contain two sugar units; and polysaccharides contain many sugar units as in polymers - most contain glucose as the monosaccharide unit.
Lactose or milk sugar occurs in the milk of mammals - 4-6% in cow's milk and 5-8% in human milk. It is also a by product in the the manufacture of cheese. Lactose is made from galactose and glucose units. The galactose and glucose units are joined by an acetal oxygen bridge in the beta orientation. To recognize galactose look for the upward projection of the -OH on carbon # 4.
Sucrose or table sugar is obtained from sugar cane or sugar beets. Sucrose is made from glucose and fructose units. The glucose and fructose units are joined by an acetal oxygen bridge in the alpha orientation. The structure is easy to recognize because it contains the six member ring of glucose and the five member ring of fructose.
Thumbnail Ball-and-stick model of the α-lactose molecule. (Public Domain; Ben Mills).