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A hexoaldose monosaccharide with a chemical formula of C6H12O6, similar to glucose except that the orientations of H and OH on carbon 4 are exchanged, and does not occur naturally, but forms lactose when combined with glucose


Monosaccharides are the simplest form of carbohydrates. They may be classified based on the number of constituent carbon atoms. For instance, hexose is a monosaccharide with six carbon atoms. They may also be classified based on the functional group present: aldose and ketose. Glucose, galactose, and fructose with the same chemical formula: C6H12O6 are hexose monosaccharides. Glucose and galactose are aldoses whereas fructose is a ketose.

Galactose is similar to glucose in terms of chemical structure. However, the orientations of H and OH on carbon 4 are exchanged. Unlike glucose, galactose generally does not occur in free state. It usually is a constituent of complex biomolecules. For instance, galactose together with glucose forms lactose (milk sugar), which is a disaccharide.

Galactose is a component of cerebrosides and gangliosides, glycoproteins. Lactose, the disaccharide of milk, consists of galactose joined to glucose by a _(1-4) glycosidic link. The joining of galactose and glucose is catalyzed by the enzymes lactase and β-galactosidase. Galactose catabolism (where glucose is converted to glucose) is carried out via the Leloir pathway. In human lactation, one of the sources of lactose in breast milk is through de novo synthesis of galactose and glucose through hexoneogenesis.

In plants such as the axlewood ( Anogeissus latifolia) and acacia trees, galactose monomers link together and form a polysaccharide referred to as galactan.

Chemical formula:

  • C6H12O6

See also: