Disaccharides: Types, Linkages, and Examples

Carbohydrates: Disaccharides

Types of Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined by glycosidic linkages. They are important energy sources and structural components in living organisms. The most common disaccharides include maltose, cellobiose, lactose, and sucrose.

• Disaccharide: A carbohydrate formed by the combination of two monosaccharides.

• Glycosidic Linkage: The covalent bond that joins two monosaccharides.

• Common Disaccharides: Maltose, Cellobiose, Lactose, Sucrose.

Table: Types of Disaccharides

Monosaccharide 1	Monosaccharide 2	Linkage Type	Name	Example
D-Glucose	D-Glucose	α-1,4 Linkage	Maltose
D-Glucose	D-Glucose	β-1,4 Linkage	Cellobiose
D-Glucose	D-Galactose	β-1,4 Linkage	Lactose
D-Glucose	D-Fructose	α-1,2 Linkage	Sucrose

Additional info: Images referenced in the table are placeholders for the actual chemical structures. In a textbook, these would be shown as line structures.

Glycosidic Linkages

The type of glycosidic linkage determines the properties and digestibility of the disaccharide. The linkage is named according to the carbon atoms of the monosaccharides involved and the configuration (α or β) of the anomeric carbon.

• α-1,4 Linkage: Found in maltose; easily digested by humans.

• β-1,4 Linkage: Found in cellobiose and lactose; cellobiose is not digestible by humans, but lactose is (with lactase enzyme).

• α-1,2 Linkage: Found in sucrose; links glucose and fructose.

Examples and Practice Questions

• Example: Which of the following contains galactose as one of the sugar subunits?

  • Lactose contains D-galactose and D-glucose.

• Practice: Which disaccharide forms a β-1,4 glycosidic linkage?

  • Lactose and Cellobiose both have β-1,4 linkages.

• Practice: What is the identity of the disaccharide below?

  • The structure shows two glucose units joined by a β-1,4 linkage, which is cellobiose.

Key Disaccharide Structures

• Maltose: $\text{D-Glucose} + \text{D-Glucose}$ via $\alpha$-1,4 linkage

• Cellobiose: $\text{D-Glucose} + \text{D-Glucose}$ via $\beta$-1,4 linkage

• Lactose: $\text{D-Galactose} + \text{D-Glucose}$ via $\beta$-1,4 linkage

• Sucrose: $\text{D-Glucose} + \text{D-Fructose}$ via $\alpha$-1,2 linkage

Summary Table: Disaccharide Composition and Linkage

Name	Monosaccharides	Linkage	Digestibility
Maltose	Glucose + Glucose	α-1,4	Digestible
Cellobiose	Glucose + Glucose	β-1,4	Not digestible by humans
Lactose	Galactose + Glucose	β-1,4	Digestible (with lactase)
Sucrose	Glucose + Fructose	α-1,2	Digestible

Additional info: Disaccharides are important in nutrition and metabolism. Lactose intolerance results from the inability to digest lactose due to lack of lactase enzyme.