Disaccharides: Videos & Practice Problems

A disaccharide is formed when two monosaccharides are connected by a glycosidic linkage. There are several types of disaccharides, with the most common being maltose, cellobiose, lactose, and sucrose. Each disaccharide is characterized by the specific monosaccharides involved and the type of glycosidic linkage, which can be classified as either alpha or beta.

For instance, maltose is created from two D-glucose molecules linked by an α(1→4) bond. This disaccharide is commonly found in barley and is a key ingredient in various types of beer. In contrast, cellobiose also consists of two D-glucose units, but they are connected by a β(1→4) linkage. This disaccharide is present in trace amounts in honey, making it a familiar food source.

Lactose, known as milk sugar, is formed from D-galactose and D-glucose through a β(1→4) linkage. While humans typically struggle to digest β linkages, the enzyme lactase allows many individuals to break down lactose. Those who are lactose intolerant often have insufficient levels of lactase, leading to difficulties in digesting dairy products.

Lastly, sucrose, commonly referred to as table sugar, is produced from D-glucose and D-fructose. It features both an α(1→2) and a β(2→1) linkage, distinguishing it from other disaccharides. Understanding these disaccharides and their linkages is essential for grasping the role of carbohydrates in nutrition and metabolism.

In the context of carbohydrates, understanding the composition of disaccharides is essential. Disaccharides are formed by the linkage of two monosaccharides. In this example, we are tasked with identifying which option contains galactose as one of its sugar subunits.

First, it is important to note that monosaccharides, such as glucose and ribose, cannot serve as subunits in a disaccharide since they are single sugar units. This eliminates glucose and ribose from consideration.

This leaves us with three potential candidates: maltose, lactose, and cellobiose. Maltose and cellobiose can also be ruled out because they are both composed solely of D-glucose subunits. Specifically, maltose features an α(1→4) glycosidic linkage, while cellobiose has a β(1→4) glycosidic linkage, both involving only glucose.

Consequently, lactose emerges as the only viable option. Lactose, commonly known as milk sugar, is a disaccharide formed from one D-glucose and one D-galactose unit, linked together by a β(1→4) glycosidic bond. This unique structure is what distinguishes lactose from the other disaccharides mentioned.

In summary, lactose is the disaccharide that contains galactose as one of its sugar subunits, highlighting its significance in carbohydrate chemistry.