Question

Which among the followings is not a carbhohydrate?

• A. Lignin
• B. Cellulose
• C. Hemicellulose
• D. Starch

Hint:

While lignin is part of the dietary fiber fraction of plants, it is chemically distinct from carbohydrates because it is composed of phenolic (aromatic) rings rather than sugar monomers.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen, typically with a hydrogen-to-oxygen ratio of 2:
They include sugars, starches, and structural plant polysaccharides.

Step 2: Detailed Explanation:
Let us analyze the chemical structure of each option:
- Starch: A plant storage polysaccharide made of glucose monomers linked by $\alpha$-1,4 and $\alpha$-1,6 glycosidic bonds. It is a carbohydrate.
- Cellulose: A structural plant polysaccharide made of linear chains of glucose monomers linked by $\beta$-1,4 glycosidic bonds. It is a carbohydrate.
- Hemicellulose: A heterogeneous structural plant polysaccharide made of various sugar monomers (such as xylose, mannose, and galactose). It is a carbohydrate.
- Lignin: A complex, cross-linked organic polymer found in the secondary cell walls of plants.
Lignin is not made of sugar monomers; instead, it is synthesized from phenolic precursors (such as coniferyl, sinapyl, and p-coumaryl alcohols).
Therefore, chemically, lignin is a non-carbohydrate phenolic polymer that associates with cell-wall carbohydrates to provide structural rigidity.

Step 3: Final Answer:
Lignin is a complex phenolic polymer and is not classified as a carbohydrate.