A disaccharide is the sugar that forms when two monosaccharides or simple sugars join via a glycosidic bond. Common examples of disaccharides are sucrose, maltose, and lactose. The general chemical formula of a disaccharide is C12H22O11. Here is a look at how disaccharides form, examples, and properties.
How Disaccharides Form – Glycosidic Bond
A disaccharide forms when two monosaccharides react in a dehydration reaction. Joining two monosaccharides releases one molecule of water, while hydroxyl groups (-OH) of the sugars connect via a glycosidic bond. The hydrogen atom from the hydroxyl of one monosaccharide combines with the hydroxyl from the other monosaccharide, forming water. Meanwhile, the remaining oxygen connects the two residues. A glycosidic bond is a specific form of a covalent bond.
Each sugar may contain multiple hydroxyl groups, so two identical subunits can form different combinations of glycosidic bonds with different stereochemistry. In other words, any two subunits form multiple disaccharides. For example, both maltose and trehalose form from two glucose subunits, but the hydroxyl groups participating in the bond differ.
Disaccharide examples include sucrose, lactose, and maltose, which are all found in food. Other common disaccharides include lactulose, trehalose, and cellobiose.
Sucrose or table sugar consists of a glucose and a fructose joined by an α(1→2)β linkage. Purifying juice from sugar cane or sugar beets yields sucrose.
Lactose is a disaccharide in milk formed by a galactose and glucose with a β(1→4) glycosidic linkage. It an isomer of sucrose.
Maltose forms when two glucose residues join with an α(1→4) bond. The sugar comes from starch digestion. Its sweetness makes it a popular in candies and cereals. Commercially, it is produced from barley and other grains.
Lactulose is a man-made sugar formed by combining a galactose and fructose via a β(1→4) bond. While the sugar is not absorbed during digestion, bacteria in the colon break it down. Its products absorb water, so it is a stool-softener. Lactulose also absorbs ammonia, so it lowers high blood ammonia levels found in persons with liver disease.
Tehalose consists of two α-glucose subunits joined by an α(1→1)α glycosidic bond. It is also called mycose or tremalose. Trehalose is a natural sugar that retains water extremely well. In nature, animals and plants use it to help them survive long periods without water.
Cellobiose consists of two β-glucose subunits joined by a β(1→4) linkage. This disaccharide is a produce of cellulose hydrolysis. For example, it forms from hydrolysis of cellulose-rich paper or cotton.
Table of Common Disaccharides
This table indicates the subunits and linkages of common disaccharides.
|Dissacharide||First Unit||Second Unit||Bond|
Of course, there are many other disaccharides. Here are less common examples:
- Isomaltose (two glucose subunits)
- Turanose (one glucose, one fructose)
- Melibiose (one galactose, one glucose)
- Leucrose (one glucose, one fructose)
- Xylobiose (two xylopyranose subunits)
- Sophorose (two glucose subunits)
- Mannobiose (two mannose subunits)
There are exceptions, but many disaccharides share the following properties:
There are two broad classes of disaccharides:
- Reducing disaccharides: One of the monosaccharides is the reducing sugar because it retains a free hemiacetal. Examples of reducing disaccharides are maltose, lactose, and cellobiose.
- Non-reducing disaccharides: In a non-reducing disaccharide, the acetal linkage is such that neither subunit has a free hemiacetal. For example, sucrose and trehalose are non-reducing sugars because the glycosidic bond forms between the hemiacetal carbon atoms. Non-reducing disaccharides are typically more stable and easily stored than reducing disaccharides.
The two main functions of disaccharides in living organisms are as energy carriers and transport molecules for delivering monosaccharides where they are needed.
Here are some specific uses of disaccharides:
- Humans and other animals digest sucrose (table sugar) for quick energy. Excess sucrose gets converted into a lipid (fat) for energy storage.
- Plants use disaccharides to transport glucose, galactose, and fructose between cells.
- Lactose or milk sugar occurs in milk. It is an energy source for infants. While humans and other mammals are born with the enzyme lactase, the body produces less of it over time. So, many adult mammals (including people) are lactose intolerant.
- Maltose is a sweet-tasting sugar that does not serve a specific function in humans, so it is incompletely digested and absorbed. This means maltose does not increase blood sugar as much as sucrose.
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- Kwan, Lam Peng (2000). Biology – A course for O Level. ISBN 9810190964.
- Whitney, Ellie; Sharon Rady Rolfes (2011). Peggy Williams (ed.). Understanding Nutrition (12th ed.). California: Wadsworth, Cengage Learning. ISBN 978-0-538-73465-3.