Two monosaccharides joined by a glycosidic bond, such as sucrose, lactose and maltose.
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Disaccharides form when two monosaccharides join through a glycosidic bond, a covalent link formed by condensation that releases one molecule of water. Sucrose combines glucose and fructose, lactose combines galactose and glucose, and maltose combines two glucose units. Because the anomeric carbon of sucrose is tied up in the bond, sucrose does not reduce Benedict's reagent, whereas maltose and lactose do. The bond must be broken by hydrolysis before the body can absorb the sugars, which is why sucrase, lactase and maltase are needed in the small intestine. Disaccharides are a major source of dietary sugar in fruit, milk and table sugar.
Key ideas: Two monosaccharides join by a glycosidic bond with loss of water. Sucrose is glucose plus fructose (alpha-1,2 link, non-reducing), maltose is glucose plus glucose (alpha-1,4, reducing), lactose is galactose plus glucose (beta-1,4, reducing). Hydrolysis with dilute acid or the right enzyme gives back the monomers. Sucrose hydrolysis gives invert sugar, a mixture that rotates light the opposite way.
Common mistakes and connections: Students think sucrose is a reducing sugar, but both anomeric carbons are tied up in the bond so it gives no Tollens or Fehling positive. Lactose intolerance is lack of lactase, which cuts the beta link. Links to enzymes specificity and polysaccharides.
Relative sweetness of the three common disaccharides, each formed by a glycosidic bond that releases one water molecule when two monosaccharides join. The interactive lesson shows this as a live animation or chart.
Table sugar is sucrose, and sucrase hydrolysis releases one glucose and one fructose molecule.
Two monosaccharides joined by a glycosidic bond, such as sucrose, lactose and maltose.
Disaccharides is a beginner topic in Biochemistry, Environment & Industry. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Biochemistry, Environment & Industry first: Biomolecules, Monosaccharides.
The next topics in Biochemistry, Environment & Industry are Polysaccharides and Starch.
Yes. The interactive lesson includes a self-check question on Disaccharides, and the Biochemistry, Environment & Industry quiz draws on every topic in this area.
Disaccharides is topic 3 of 127 in Biochemistry, Environment & Industry.
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