Lactose Ring Structure Molecule Model

Lactose, or milk sugar, is a disaccharide composed of glucose & galactose found mainly in the milk of lactating female eutherian mammalians. You can build glucose (β-D-glucose) & galactose (β-D-galactose) models separately for comparison & note that they are carbon 4 epimers.

Combine them so that the β(1→4) glycosidic bond connects anomeric carbon 1 of β-D-galactose to the hydroxyl group on carbon 4 of the β-D-glucose to form β-D-lactose. This structure results in a relatively linear arrangement of the two monosacharride sub-units as shown also in a stand Haworth projection.

Lactose is the principal carbohydrate in mammalian milk. Aside from being an essential energy source in infants it also facilitates the uptake of calcium and other minerals necessary for bone development. It has a further advantage of promoting fewer cavities in teeth than other sugars such as sucrose, glucose, and fructose.

Lactose is hydrolyzed into glucose and galactose by the enzyme lactase. Lactase is found on the brush border of  the epithelial cells in the small intestine. The enzyme cleaves the β(1→4) glycosidic bond via a hydrolysis reaction which involves the addition of a water molecule to produce free glucose and galactose molecules.

These monosaccharides are then absorbed across the intestinal epithelium into the bloodstream where they are metabolizes for energy or stored in muscle as glycogen.

Lactose intolerance (not an allergy) occurs in individuals with lactase deficiency such that unhydrolyzed lactose passes into the large intestine & is is fermented by gut bacteria causing symptoms such as gas production & bloating.