What are the three common types of disaccharides?

November 24, 2025 •

3 min read

Disaccharides, often called double sugars, are carbohydrates formed when two monosaccharides, or simple sugars, are joined by a glycosidic linkage. The three common types of disaccharides you will encounter most frequently are sucrose, lactose, and maltose.

Quick Summary

The three most common disaccharides are sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar), each with distinct monosaccharide components and sources.

Key Points

Sucrose: The most abundant disaccharide, made from glucose and fructose, and commonly known as table sugar.
Lactose: The primary sugar found in milk, consisting of galactose and glucose units.
Maltose: Often called malt sugar, it is derived from the breakdown of starches and is composed of two glucose molecules.
Glycosidic Bond: Disaccharides are formed when two monosaccharides are linked together by a covalent glycosidic bond.
Digestion: The body breaks down each disaccharide into its simple sugar components using specific enzymes, like sucrase, lactase, and maltase.
Energy Source: All three common disaccharides function as a vital source of energy for the body once broken down into monosaccharides.
Structure: Despite sharing a similar molecular formula ($C{12}H{22}O_{11}$), the different arrangements and constituent sugars give each disaccharide unique properties.

Understanding Disaccharides

Disaccharides are an important class of carbohydrates. They are formed through a dehydration synthesis reaction, where a molecule of water is removed to form a glycosidic bond between two monosaccharide units. While there are many possible disaccharide combinations, three stand out for their prevalence in human diets and biological importance: sucrose, lactose, and maltose. A deeper understanding of these carbohydrates helps explain their roles in nutrition and metabolism.

Sucrose: The Table Sugar

Sucrose is a non-reducing disaccharide composed of one molecule of glucose and one molecule of fructose, joined by an α-1,β-2-glycosidic linkage. Because both of the reducing groups on its constituent monosaccharides are involved in this bond, sucrose does not have a free reducing end.

Sources: Found naturally in fruits and vegetables, sucrose is most famously extracted and refined from sugarcane and sugar beets to produce common table sugar.
Function: As the primary carbohydrate transported in plants, it serves as a crucial energy source. When consumed, the enzyme sucrase hydrolyzes it in the small intestine, releasing glucose and fructose for the body to use as fuel.

Lactose: The Milk Sugar

Lactose is the disaccharide found exclusively in the milk of mammals, and it is composed of a molecule of galactose and a molecule of glucose, connected by a β-1,4-glycosidic bond. Unlike sucrose, lactose is a reducing sugar because one of its monosaccharide units retains a free hemiacetal unit.

Sources: Lactose is found in dairy products like milk, yogurt, and cheese. It is particularly vital for infants, as breast milk provides this sugar as an important source of energy.
Function: For lactose to be digested, the body requires the enzyme lactase to break the glycosidic bond. Individuals with lactose intolerance have insufficient levels of this enzyme, leading to digestive issues when they consume dairy.

Maltose: The Malt Sugar

Maltose, or malt sugar, consists of two glucose molecules joined by an α-1,4-glycosidic bond. It is a product of the enzymatic breakdown of starch and glycogen. Like lactose, maltose is also a reducing sugar.

Sources: While less common in its free form, maltose is present in germinating grains like barley and is used commercially in the brewing industry and in some candies and syrups.
Function: During human digestion, the enzyme amylase starts the breakdown of complex starches into maltose, which is then further hydrolyzed into individual glucose units by maltase in the small intestine.

A Comparison of Common Disaccharides


Feature	Sucrose	Lactose	Maltose
Monosaccharide Components	Glucose + Fructose	Galactose + Glucose	Glucose + Glucose
Common Name	Table sugar	Milk sugar	Malt sugar
Natural Sources	Sugarcane, beets, fruits	Mammalian milk, dairy products	Germinating grains, brewing
Type	Non-reducing sugar	Reducing sugar	Reducing sugar
Digestion Enzyme	Sucrase	Lactase	Maltase
Solubility in Water	Very soluble	Slightly soluble	Fairly soluble

The Formation of Disaccharides: A Condensation Reaction

The formation of these double sugars involves a process known as a condensation reaction, or dehydration synthesis. This is a fundamental biochemical process where a glycosidic bond is formed. For example, when forming sucrose, the hydroxyl group (-OH) from a glucose molecule combines with a hydrogen atom (-H) from a fructose molecule, resulting in the release of a water molecule ($H_2O$). The reverse reaction, called hydrolysis, breaks this glycosidic bond with the addition of a water molecule, releasing the two constituent monosaccharides.

Conclusion

The three common types of disaccharides—sucrose, lactose, and maltose—are all important carbohydrates that serve as energy sources. While they all share the general molecular formula $C{12}H{22}O_{11}$, their distinct properties, sources, and biological roles are determined by their specific monosaccharide components and the type of glycosidic linkage joining them. From the sweetness of table sugar to the fermentable energy of malt, these double sugars are integral to both our diet and metabolic functions.

Note: For an even deeper dive into the specific chemical structures and reactions of these sugars, consult resources like the Chemistry LibreTexts library, which offers extensive information on the structures of sucrose, lactose, and maltose.

Frequently Asked Questions

The primary function of disaccharides in the human diet is to serve as an energy source. The body breaks them down into simpler monosaccharides (simple sugars) in the small intestine, which can then be absorbed and used for cellular energy.

Disaccharides are formed from two monosaccharides through a chemical reaction called dehydration synthesis, or a condensation reaction. During this process, a glycosidic bond is created between the two monosaccharides, and a water molecule is released.

Sucrose is a non-reducing sugar. This is because the glycosidic bond forms between the anomeric carbons of both the glucose and fructose molecules, meaning neither monosaccharide has a free hemiacetal unit available to act as a reducing agent.

A person with lactose intolerance lacks or has low levels of the enzyme lactase, which is needed to break down lactose. As a result, the undigested lactose travels to the large intestine, where gut bacteria ferment it, causing uncomfortable symptoms such as bloating, cramping, and gas.

Sucrose is found in table sugar, fruits, and maple syrup. Lactose is found in milk and dairy products. Maltose is found in sprouted grains, such as malted barley used for brewing beer, and can be used as a sweetener in foods.

Maltose and lactose are both reducing sugars, while sucrose is a non-reducing sugar. Their monosaccharide composition also differs significantly: maltose contains two glucose units, lactose has a glucose and a galactose unit, and sucrose is made of glucose and fructose.

The enzyme responsible for breaking down maltose into its two glucose units is maltase. This process occurs in the small intestine during digestion.