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 composed of two monosaccharide units joined by a glycosidic linkage.
Step 2: Key Formula or Approach:
Identify the monosaccharides and the linkage type:
Lactose = \(\beta\)-D-galactose + \(\beta\)-D-glucose.
Linkage: \(\beta\)-1,4-glycosidic bond.
Step 3: Detailed Explanation:
Lactose is composed of a \(\beta\)-D-galactose unit and a \(\beta\)-D-glucose unit. The bond is formed between the C1 of galactose and the C4 of glucose.
- Sucrose is Glucose + Fructose.
- Maltose is Glucose + Glucose.
Step 4: Final Answer:
Lactose is composed of one \(\beta\)-D-galactose and one \(\beta\)-D-glucose unit.