What will be formed if glucose is combined to galactose?

December 23, 2025 •

3 min read

Lactose, or milk sugar, is a naturally occurring disaccharide that makes up 2–8% of milk by mass. This vital carbohydrate is formed through a chemical process where the monosaccharides glucose and galactose are combined. Understanding what will be formed if glucose is combined to galactose reveals fundamental principles of carbohydrate chemistry and human digestion.

Quick Summary

When the monosaccharides glucose and galactose are joined through a dehydration synthesis reaction, they form the disaccharide known as lactose. This process, which creates a β-1,4-glycosidic linkage, is essential for producing milk sugar found in dairy products. Lactose can then be broken down into its constituent parts by the enzyme lactase during digestion.

Key Points

Formation of Lactose: The monosaccharides glucose and galactose combine via a condensation (dehydration synthesis) reaction to form the disaccharide lactose.
Glycosidic Bond: This combination involves the creation of a β-1,4-glycosidic linkage between the two sugar molecules.
Lactose (Milk Sugar): The resulting molecule, $C{12}H{22}O_{11}$, is found in the milk of mammals and is commonly referred to as milk sugar.
Digestion with Lactase: The body breaks down lactose back into glucose and galactose using the enzyme lactase during digestion.
Lactose Intolerance: Many adults lack sufficient lactase, which leads to digestive problems when consuming dairy products containing lactose.
Industrial Significance: Lactose is a valuable ingredient in the food and pharmaceutical industries, acting as a filler, stabilizer, and for creating lactose-free products.

Formation of Lactose: The Condensation Reaction

The combination of glucose and galactose results in the formation of lactose, a disaccharide sugar. The chemical process is a condensation reaction, also known as dehydration synthesis, where a molecule of water is removed to form a covalent bond between the two monosaccharides. The specific linkage formed in lactose is a β-1,4-glycosidic bond, connecting the beta anomer of galactose to the C4 of glucose.

The Chemical Equation for Lactose Formation

To understand the reaction, consider the simplified chemical equation:

$C6H{12}O_6 (Glucose) + C6H{12}O6 (Galactose) \to C{12}H{22}O{11} (Lactose) + H_2O$

This formula illustrates that two monosaccharides, each with the formula $C6H{12}O6$ (though isomers with different structures), combine to form a disaccharide, $C{12}H{22}O{11}$, and a water molecule. The structural difference between glucose and galactose is subtle, involving the orientation of a hydroxyl group on the fourth carbon, yet this difference is crucial for defining the resulting disaccharide's structure and properties.

Structure and Properties of Lactose

Lactose is commonly known as 'milk sugar' because it is the primary carbohydrate found in milk and dairy products. It is a white, crystalline solid with a mildly sweet taste, though it is less sweet than sucrose.

Key structural characteristics include:

Composition: One molecule of β-D-galactose linked to one molecule of D-glucose.
Glycosidic Linkage: The connection is a β-1,4-glycosidic bond, which determines how the molecule is digested.
Reducing Sugar: Unlike sucrose, lactose is a reducing sugar because one of its monosaccharide units (the glucose) has a free anomeric carbon capable of opening into an aldehyde group. This property is important in food chemistry and analytical testing.

Digestion and Lactose Intolerance

For the body to absorb the energy from lactose, it must be broken down into its constituent monosaccharides: glucose and galactose. This hydrolysis reaction is catalyzed by the enzyme lactase, which is produced in the small intestine.

However, many adults experience a natural decrease in lactase production after infancy. This condition, known as lactase non-persistence, leads to lactose intolerance. When undigested lactose reaches the large intestine, bacteria ferment it, producing gas and other byproducts that cause digestive discomfort. Lactose-free dairy products are created by adding the lactase enzyme to milk, breaking down the lactose beforehand.

Comparison of Common Disaccharides


Feature	Lactose	Sucrose	Maltose
Component Monosaccharides	Glucose + Galactose	Glucose + Fructose	Glucose + Glucose
Common Name	Milk Sugar	Table Sugar	Malt Sugar
Glycosidic Linkage	β-1,4	α-1,2	α-1,4
Reducing Sugar?	Yes	No	Yes
Primary Source	Milk, Dairy Products	Sugar Cane, Sugar Beet	Sprouting Grains

Synthesis and Industrial Applications

While naturally formed in the mammary glands of mammals, lactose can also be produced on an industrial scale. It is often isolated as a byproduct of cheese production from whey. Its properties, such as relatively low sweetness and low solubility, make it a valuable ingredient in the food and pharmaceutical industries.

Food Industry: Lactose is used as a filler, stabilizer, and texture modifier in products like baked goods and infant formulas. It provides bulk without excessive sweetness.
Pharmaceutical Industry: It serves as a filler and binding agent in tablets and capsules due to its inert nature and stability.
Industrial Hydrolysis: For lactose-free products, manufacturers use industrial processes involving the enzyme β-galactosidase to hydrolyze lactose into glucose and galactose. This makes dairy products palatable for those with lactose intolerance.

Conclusion

In summary, the combination of glucose and galactose forms the disaccharide lactose, or milk sugar, through a dehydration synthesis reaction involving a β-1,4-glycosidic linkage. This fundamental biochemical process is crucial for the nutritional value of milk for mammals, particularly infants. The resulting lactose molecule's unique chemical structure, including its reducing sugar properties, has significant implications for both human digestion and a variety of industrial applications. The inability to properly digest this molecule leads to lactose intolerance, which is addressed by the commercial hydrolysis of lactose using the lactase enzyme.

Frequently Asked Questions

The chemical reaction that forms lactose from glucose and galactose is called a condensation reaction, also known as dehydration synthesis, because a water molecule is removed in the process.

Lactose is a disaccharide, meaning it is a carbohydrate formed by the combination of two monosaccharide units: one glucose and one galactose molecule.

Glucose and galactose are linked by a β-1,4-glycosidic bond to form lactose. The specific orientation and carbon positions are critical for defining the molecule's structure and properties.

Lactose is called 'milk sugar' because it is the main type of sugar found in the milk of mammals, including humans. Its presence in milk provides a critical energy source for infants.

Lactose intolerance is the inability to break down lactose into its component parts, glucose and galactose, due to a deficiency of the enzyme lactase. Without this enzyme, the lactose remains undigested, causing discomfort.

Other common disaccharides include sucrose (table sugar), which is made from glucose and fructose, and maltose (malt sugar), which is formed from two glucose molecules.

In the food industry, lactose is used as a filler and stabilizer in baked goods and infant formulas. In pharmaceuticals, it is a common excipient used as a filler or binding agent in tablets and capsules.