Galactose is a monosaccharide composed of six carbons and one aldehyde group. It is an aldohexose and a C-4 epimer of glucose. Galactose is a component of lactose in mammalian milk. It often exists in the brain and Nervous tissue in the form of D-galactoside, and is also an important component of some glycoproteins. Galactose is the fastest monosaccharide absorbed in the intestine.
Carbohydrates are polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses) composed of C, H and O, or form polyhydroxy aldehydes or ketones upon hydrolysis. Carbohydrates come in the form of monosaccharides, disaccharides, oligosaccharides and polysaccharides. The simplest sugars, monosaccharides and disaccharides, consist of one or two monosaccharide units. Oligosaccharides are generally classified as carbohydrates containing 3-10 monosaccharide units. Polysaccharides contain more than 10 monosaccharide units and can be quite large. Glucose is an aldose, the most important monosaccharide in the body, used by cells as fuel. Other aldohexoses are galactose and mannose, which form part of complex molecules. Fructose is a ketohexose sugar, while ribose is the most important aldose pentose sugar and a component of RNA.