Question

Arrange the following carbohydrate molecules in the increasing molecular weights :
(A) Starch
(B) Sucrose
(C) Glucose
(D) Amylopectin
Choose the correct answer from the options given below :

• A. (C), (B), (A), (D)
• B. (B), (A), (C), (D)
• C. (B), (A), (D), (C)
• D. (A), (D), (C), (B)

Hint:

To arrange carbohydrates by molecular weight, classify them by type:
Monosaccharide ($1\text{ unit}$) $<$ Disaccharide ($2\text{ units}$) $<$ Mixed Polysaccharide (Starch) $<$ Giant Branched Polysaccharide (Amylopectin).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Carbohydrates are classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides based on their degree of polymerization.
The molecular weight of a carbohydrate increases progressively with the number of monomeric sugar units linked together by glycosidic bonds.

Step 3: Detailed Explanation:
Let us analyze the molecular scale of each given carbohydrate:
1. (C) Glucose:
Glucose is a simple monosaccharide (hexose sugar) with the chemical formula $\text{C}_6\text{H}_{12}\text{O}_6$.
Its molecular weight is approximately $180\text{ g/mol}$.
This is the smallest molecule among the choices.
2. (B) Sucrose:
Sucrose is a disaccharide formed by a condensation reaction linking one glucose molecule and one fructose molecule together ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$).
Its molecular weight is approximately $342\text{ g/mol}$.
3. (A) Starch:
Starch is a mixture of two polysaccharides: linear amylose ($15\%\text{ to }20\%$) and highly branched amylopectin ($80\%\text{ to }85\%$).
The molecular weight of starch is an average of these two fractions, typically ranging from $10^5$ to $10^6\text{ g/mol}$.
4. (D) Amylopectin:
Amylopectin is a massive, highly branched polymer consisting of thousands of glucose units joined by $\alpha(1\rightarrow4)$ and $\alpha(1\rightarrow6)$ glycosidic linkages.
It is one of the largest molecules found in nature, with molecular weights ranging from $10^7$ to $10^8\text{ g/mol}$.
Its molecular weight is much higher than the average molecular weight of natural starch, which contains the smaller amylose molecules.
Therefore, the increasing order of molecular weight is: (C) $\rightarrow$ (B) $\rightarrow$ (A) $\rightarrow$ (D).

Step 4: Final Answer:
The correct increasing sequence of molecular weights is (C), (B), (A), (D).
This matches Option (A).