What is the Difference Between Cellulose and Lignin

The main difference between cellulose and lignin is that cellulose is a polysaccharide made up of glucose subunits, whereas lignin is a highly non-regular organic polymer of phenol sub-units. Furthermore, cellulose makes up the cell wall of plant cells while lignin makes up cell wall thickenings of cells in the vascular tissue. In addition to these, cellulose takes part in the primary growth while lignin takes part in secondary growth. 

Cellulose and lignin are twp types of polymers abundant in plants. Generally, both of them make up cell wall structures, aiding in support and protection. 

Key Areas Covered 

1. What is Cellulose
     – Definition, Structure, Function
2. What is Lignin
     – Definition, Structure, Function
3. What are the Similarities Between Cellulose and Lignin
     – Outline of Common Features
4. What is the Difference Between Cellulose and Lignin
     – Comparison of Key Differences

Key Terms 

Cellulose, Cell Wall, D-Glucose, Lignin, Primary Growth, Secondary Growth

What is Cellulose 

Cellulose is the main constituent of the plant cell walls as well as many forms of algae and oomycetes. It is also the most abundant organic polymer on earth. Generally, cellulose is a polysaccharide containing linear chains of many thousands of β(1→4) linked D-glucose units. In addition to this, hydroxyl groups of one linear chain form hydrogen bonds with the hydroxyl groups of the second linear chain of cellulose. Thus, this firmly holds each chain together, forming microfibrils with high tensile strength.

Figure 1: Cellulose

Moreover, the digestive system of humans is unable to digest cellulose in plant-based food. However, the ruminant digestive system can digest cellulose with the help of gut microbes. Furthermore, the main sources of cellulose for industrial purposes are wood pulp and cotton. In general, cotton fibers make up 90% of cellulose. Cellulose is also important for making paper and paperboards.  

What is Lignin 

Lignin is the polymer responsible for the secondary cell wall thickening of cells of the vascular tissue of plants including fibers and vessels. It is also present in some algae. Generally, lignin is an irregular phenolic polymer. Although the composition of lignin may vary with the type of plant, the three main monolignol monomers which make up lignin are paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. They are incorporated into lignin in the form of the phenylpropanoids.

Figure 2: Lignin 

Moreover, the main function of lignin in plant cells is to fill spaces in the cell wall, occurring between other cell wall components, including cellulose, hemicellulose, and pectin. Although the polysaccharide component of the plant cell wall is hydrophilic, lignin is hydrophobic. Therefore, crosslinking the primary plant cell wall with lignin becomes an obstacle for the absorption of water into the cell wall, increasing the conducting efficiency of water. On the other hand, it provides tensile strength to the plant. 

Similarities Between Cellulose and Lignin 

• Cellulose and lignin are two types of cross-linked polymers abundant in plants. 
• They are organic compounds insoluble in water and most other solvents. 
• Both are components of the cell wall. 
• They perform a structural function, providing support to the plant. 
• Also, they are responsible for the different growth stages of plants. 
• Both are the most abundant renewable carbon sources on Earth.  

Difference Between Cellulose and Lignin 

Definition 

Cellulose refers to the main substance, which makes up the cell walls and fibers of plants while lignin refers to an organic substance, which acts as a binder for the cellulose fibers, adding strength and stiffness to the plant cell wall. 

Monomer Unit 

The monomer of cellulose is D-glucose while the three main types lignols, which are the monomers of lignin are paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. 

Type of Polymer 

Cellulose is a polysaccharide while lignin is a phenolic polymer. 

Structure 

Moreover, cellulose is a straight-chain polymer, while lignin is an irregular polymer. 

In Plant Cells 

While cellulose is the main component of the primary plant cell wall, lignin occurs as thickenings of secondary cell walls of fibers and vessels of vascular tissue. 

Function 

Cellulose makes up cell walls of plant cells while lignin provides rigidity to plants. 

Occurrence 

Furthermore, cellulose constitutes 90% cotton fiber, 40-50% of wood, and 57% of dried hemp while lignin mainly occurs in wood and bark. 

Digestibility 

Only the gut of ruminant animals can digest cellulose while lignin is totally indigestible. 

Conclusion 

Cellulose is the main component of the primary plant cell wall. Therefore, it is a structural component of plants, providing strength to the plant. Also, it is a highly regular, straight-chain polysaccharide of D-glucose monomers. In contrast, lignin is an irregular, phenolic polymer responsible for the secondary cell wall thickenings in vascular tissue. Therefore, it is important for providing rigidity to the plant stem. Hence, the main difference between cellulose and lignin is their structure and function. 

References:

1. Shapley, Patricia. “Cellulose and Lignocellulose.” University of Illinois, 2012, Available Here. 

Image Courtesy:

1. “Cellulose spacefilling model” By CeresVesta (talk) (Uploads) – Own work (Public Domain) via Commons Wikimedia   
2. “Lignin” By Smokefoot – Own work (CC BY-SA 3.0) via Commons Wikimedia