**2. Structure of lignin**

As known from centuries, that lignin is found in the cell wall of the plants and is the major component to provide rigidity to cell walls. The cell wall is basically constituted of cellulose, hemicellulose, and lignin part. The utilization of cellulose and hemicellulose can be seen in various forms in literature, but lignin owing to its complex polymeric structure not being utilized much. As mentioned above, lignin is composed of various aromatic units along with methoxy and aldehyde groups as their functionality. The typical composition of these monomer units is different in different plant species dependent on their growth, their environmental condition, type of biomass, etc. [3, 4]. Despite seeing their variation in composition, it is generally considered that every cell wall on an average contains 15–25% of lignin along with 30–40% of cellulose and 15–30% hemicellulose, and few % of other components like starch, pectin, protein, carbohydrate, minerals, etc., exist in the cell wall [5–7]. Most of the natural polymers are made up of single monomer; on contrary, lignin is composed of 3-D copolymer interconnected via ether linkages of phenylpropanoid units (**Figure 1**).

**Figure 1.** *Structure of lignocellulosic biomass.*
