Difference Between Lignin and Cellulose

Key Difference – Lignin vs Cellulose
 

The plant cell wall is composed of primary and secondary walls. The primary cell wall is composed of multiple layers of cellulose, a glucose polysaccharide. Cellulose is the most common organic compound on earth. 33% out of all plant matter is composed of cellulose. It is a commercially important compound used in the production of different material such as paper, pharmaceuticals, and textiles. Lignin is the second most abundant compound on earth, surpassed only by cellulose; it is present mainly in woody plants. The key difference between lignin and cellulose is that cellulose is a polymer of carbohydrate whilst lignin is a non-carbohydrate aromatic polymer.

CONTENTS

1. Overview and Key Difference
2. What is Lignin
3. What is Cellulose
4. Similarities Between Lignin and Cellulose
5. Side by Side Comparison – Lignin vs Cellulose in Tabular Form
6. Summary

What is Lignin?

The general term lignin describes a large group of aromatic polymers assembled as a result of the oxidative coupling of 4-hydroxyphenylpropanoids. They are organic polymers present as structural compounds in vascular plants and some algae. In vascular plants, lignin is an important structural compound during secondary thickening and formation of the secondary cell wall. This provides rigidity to the bark and wood of the stem and imparts decay resistance by protecting cell wall polysaccharides from microbial degradation.

Lignin plays an important role in the process of conducting water in vascular plant stems. Polysaccharide polymers like cellulose present in the plant cell wall are permeable to water due to their hydrophilic nature. Because of its aromatic nature, lignin is more hydrophobic and creates an obstacle for the absorption of water into the cell wall by forming cross links between polysaccharides. This provides an efficient pathway for vascular tissue of the plant to conduct water without any obstacles.

Figure 01: Lignin Structure

Apart from being a structural compound, lignin is an important compound that drives the carbon cycle and acts as a slow decomposing agent of dead vegetation. It is a major limiting factor in the conversion of plant biomass into biofuels.

In a commercial aspect, removal of lignin from plant biomass is a complicated and costly process. Therefore, many research studies are conducted on this prospect in order to create plants with less lignin deposition and develop a form of lignin that is more susceptible to effortless chemical digestion.

What is Cellulose?

Cellulose is a polymer composed of β glucose and is the most abundant organic molecule on earth. Cellulose is mainly found in plants, and 40% of the plant cell wall is made up of cellulose. It is arranged in different layers in the plant cell wall, differentiated into primary and secondary walls. The structure of cellulose is composed of linear β glucose chains linked together by β 1-4 glyosidic bonds. The presence of hydroxyl groups (-OH) projecting out from each chain in all directions, increase the linkage between adjacent β glucose chains. Due to this cross linkage, the tensile strength of the cellulose structure increases. This high tensile strength prevents the cell from bursting when water enters the cell through osmosis. The shape of the cell is determined according to the arrangement of cellulose bundles.

Figure 02: Chemical Structure of Cellulose

Apart from its primary function as a structural compound, cellulose serves as an important source of food for some animals, bacteria, and fungi. Cellulose is catabolized into glucose by the enzyme cellulase. Even though cellulose is a good source of glucose, humans cannot utilize it since they lack the enzyme cellulase in their systems. Mammals like cows digest cellulose by their gut microorganisms which have the ability to catabolize cellulose. In a commercial aspect, cellulose is an important compound in the industries of paper, textile, and pharmaceuticals.

What are the Similarities Between Lignin and Cellulose?

• Lignin and cellulose are found in all plants.
• They are the main components of the cell wall of the plant cells.
• Both compounds involve in providing structural rigidity to the plant.
• They are organic compounds.

What is the Difference Between Lignin and Cellulose?

Summary – Lignin vs Cellulose

Cellulose and lignin are important structural components of the plant cell wall. Cellulose is a polymer of β glucose and is present in the primary cell wall. Lignin, an aromatic polymer, assists the secondary thickening and basically present in the secondary cell wall. This is the difference between lignin and cellulose. Due to their different chemical and physical properties, they are involved in many different functions inside the system of vascular plants.

References:

1.”Cellulose in Plants: Function & Structure”. Study.com, n.d. Web. Available here. 03 Aug. 2017.
2. Vanholme, Ruben, Brecht Demedts, Kris Morreel, John Ralph, and Wout Boerjan. “Lignin Biosynthesis and Structure.” Plant Physiology. American Society of Plant Biologists, July 2010. Web. Available here. 03 Aug. 2017.

Image Courtesy:

1.”Cellulose strand” By Karol Głąbpl.- own work from: Glazer, A. W., and Nikaido, H. (1995). Microbial Biotechnology: fundamentals of applied microbiology. San Francisco: W. H. Freeman, p. 340. ISBN 0-71672608-4 (CC BY-SA 3.0) via Commons Wikimedia
2. “Lignin structure” By Laghi.l  (CC BY-SA 3.0) via Commons Wikimedia

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