Name 3 Disaccharides

Disaccharides are sugars or carbohydrates made by linking two monosaccharides. This occurs via a dehydration reaction and a molecule of water is removed for each linkage. A glycosidic bond can form between any hydroxyl group on the monosaccharide, so even if the two subunits are the same sugar, there are many different combinations of bonds and stereochemistry, producing disaccharides with unique properties.

Depending on the component sugars, disaccharides may be sweet, sticky, water-soluble, or crystalline. Both natural and artificial disaccharides are known.

Here is a list of some disaccharides, including the monosaccharides they are made from and foods containing them. Sucrose, maltose, and lactose are the most familiar disaccharides, but there are others.

Sucrose (saccharose)

glucose + fructose
Sucrose is table sugar. It is purified from sugar cane or sugar beets.

Maltose

glucose + glucose
Maltose is a sugar found in some cereals and candies. It is a product of starch digestions and may be purified from barley and other grains.

Lactose

galactose + glucose
Lactose is a disaccharide found in milk. It has the formula C₁₂H₂₂O₁₁ and is an isomer of sucrose.

Lactulose

galactose + fructose
Lactulose is a synthetic (man-made) sugar that is not absorbed by the body but is broken down in the colon into products that absorb water into the colon, thus softening stools.

Its primary use is to treat constipation. It is also used to reduce blood ammonia levels in persons with liver disease since lactulose absorbs ammonia into the colon (removing it from the body).

Trehalose

glucose + glucose
Trehalose is also known as tremalose or mycose. It is a natural alpha-linked disaccharide with extremely high water retention properties.

In nature, it helps plants and animals reduce long periods without water.

Cellobiose

glucose + glucose
Cellobiose is a hydrolysis product of cellulose or cellulose-rich materials, such as paper or cotton. It is formed by linking two beta-glucose molecules by a β(1→4) bond.

Table of Common Disaccharides

Here's a quick summary of the subunits of common disaccharides and how they are linked to each other.

There are many other disaccharides, although they are not as common, including isomaltose (2 glucose monomers), turanose (a glucose and a fructose monomer), melibiose (a galactose and a glucose monomer), xylobiose (two xylopyranose monomers), sophorose (2 glucose monomers), and mannobiose (2 mannose monomers).

Bonds and Properties

Note multiple disaccharides are possible when monosaccharides bond to each other, since a glycosidic bond can form between any hydroxyl group on the component sugars. For example, two glucose molecules can join to form maltose, trehalose, or cellobiose.

Even though these disaccharides are made from the same component sugars, they are distinct molecules with different chemical and physical properties from each other.

Uses of Disaccharides

Disaccharides are used as energy carriers and to efficiently transport monosaccharides. Specific examples of uses include:

• In the human body and in other animals, sucrose is digested and broken into its component simple sugars for quick energy. Excess sucrose can be converted from a carbohydrate into a lipid for storage as fat. Sucrose has a sweet flavor.
• Lactose (milk sugar) is found in human breast milk, where it serves as a chemical energy source for infants. Lactose, like sucrose, has a sweet flavor. As humans age, lactose becomes less-tolerated. This is because lactose digestion requires the enyzme lactase. People who are lactose intolerant can take a lactase supplement to reduce bloating, cramping, nausea, and diarrhea.

• Plants use disaccharides to transport fructose, glucose, and galactose from one cell to another.
• Maltose, unlike some other disaccharides, does not serve a specific purpose in the human body. The sugar alcohol form of maltose is maltitol, which is used in sugar-free foods. Of course, maltose is a sugar, but it is incompletely digested and absorbed by the body (50 to 60 percent).

Key Points

• A disaccharide is a sugar (a type of carbohydrate) made by linking together two monosaccharides.
• A dehydration reaction forms a disaccharide. One molecule of water is removed for each linkage formed between the monosaccharide subunits.
• Both natural and artificial disaccharides are known.
• Examples of common disaccharides include sucrose, maltose, and lactose.

References

• IUPAC, "Disaccharides." Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997).
• Whitney, Ellie; Sharon Rady Rolfes (2011). Peggy Williams, ed. Understanding Nutrition (Twelfth ed.). California: Wadsworth, Cengage Learning. p. 100.