Question

Lactose, the milk sugar is composed of

• A. two $\beta$-D-glucose units
• B. one $\beta$-D-glucose and one $\alpha$-D-glucose units
• C. two $\alpha$-D-glucose units
• D. one $\alpha$-D-galactose and one $\beta$-D-glucose units
• E. one $\beta$-D-galactose and one $\beta$-D-glucose units

Hint:

Remember the pairs:
Sucrose = Glucose + Fructose ($\alpha$-1, $\beta$-2)
Maltose = Glucose + Glucose ($\alpha$-1, 4)
Lactose = Galactose + Glucose ($\beta$-1, 4)

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
Lactose is a disaccharide found in milk. Disaccharides are formed when two monosaccharides join via a glycosidic linkage through the loss of a water molecule.

Step 2: Detailed Explanation:
1. Components: Lactose is composed of one molecule of $\beta$-D-galactose and one molecule of $\beta$-D-glucose.
2. Linkage: These units are joined by a $\beta$-1,4-glycosidic linkage. This means the glycosidic bond forms between $C_1$ of galactose and $C_4$ of glucose.
3. Since the $C_1$ of glucose (the hemiacetal carbon) is free, lactose acts as a reducing sugar.

Step 3: Final Answer:
Lactose is composed of one $\beta$-D-galactose and one $\beta$-D-glucose unit.