<P> Disaccharides are one of the four chemical groupings of carbohydrates (monosaccharides, disaccharides, oligosaccharides, and polysaccharides). The most common types of disaccharides--sucrose, lactose, and maltose--have twelve carbon atoms, with the general formula C H O . The differences in these disaccharides are due to atomic arrangements within the molecule . </P> <P> The joining of simple sugars into a double sugar happens by a condensation reaction, which involves the elimination of a water molecule from the functional groups only . Breaking apart a double sugar into its two simple sugars is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase . As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule . These reactions are vital in metabolism . Each disaccharide is broken down with the help of a corresponding disaccharidase (sucrase, lactase, and maltase). </P> <P> There are two functionally different classes of disaccharides: </P> <Ul> <Li> Reducing disaccharides, in which one monosaccharide, the reducing sugar of the pair, still has a free hemiacetal unit that can perform as a reducing aldehyde group; cellobiose and maltose are examples of reducing disaccharides, each with one hemiacetal unit, the other occupied by the glycosidic bond, which prevents it from acting as a reducing agent . </Li> <Li> Non-reducing disaccharides, in which the component monosaccharides bond through an acetal linkage between their anomeric centers . This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent . Sucrose and trehalose are examples of non-reducing disaccharides because their glycosidic bond is between their respective hemiacetal carbon atoms . The reduced chemical reactivity of the non-reducing sugars in comparison to reducing sugars, may be an advantage where stability in storage is important . </Li> </Ul>

The bond that joins two monosaccharides into a disaccharide is a(n)