Question

What are the exact constituent monosaccharide units and the specific structural glycosidic linkage present inside a standard molecule of Lactose (Milk Sugar)?

• A. \( \alpha \)-D-Glucose and \( \beta \)-D-Fructose linked via a \( \text{C}_1-\text{C}_2 \) glycosidic bond.
• B. Two units of \( \alpha \)-D-Glucose linked via a \( \text{C}_1-\text{C}_4 \) glycosidic bond.
• C. \( \beta \)-D-Galactose and \( \beta \)-D-Glucose linked via a \( \text{C}_1-\text{C}_4 \) glycosidic bond.
• D. Two units of \( \beta \)-D-Glucose linked via a \( \text{C}_1-\text{C}_6 \) glycosidic bond.

Hint:

Remember that Lactose is the only major disaccharide in the NCERT curriculum that contains Galactose as a building block. It uses a \(\beta\)-linkage, distinguishing it from maltose which uses an \(\alpha\)-linkage.

Answer 1

The Correct Option is C

Concept: Lactose is a reducing disaccharide sugar found naturally in milk. Its structure consists of two distinct hexose monosaccharide rings linked together through a dehydration condensation bridge known as a glycosidic bond.

Step 1: Identify the monomer sub-units of Lactose.
Lactose is formed by the condensation of one molecule of \(\beta\)-D-galactose and one molecule of \(\beta\)-D-glucose.

Step 2: Trace the carbon positions that form the glycosidic bridge.
The glycosidic linkage is formed between the anomeric Carbon-1 (\(\text{C}_1\)) of the \(\beta\)-D-galactose ring and the alcohol Carbon-4 (\(\text{C}_4\)) of the adjacent \(\beta\)-D-glucose ring. This specific arrangement is designated as a \(\beta(1\rightarrow4)\) glycosidic linkage.

Step 3: Verify why lactose acts as a reducing sugar.
Because the anomeric center of the glucose ring (Carbon-1) remains free and unbound, it can easily open into its straight-chain aldehyde form. This allows lactose to reduce Tollens' and Fehling's test reagents, making it a reducing sugar.