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The historical story of lignin: Why is it called the "soul of wood"?!
Lignin is a complex organic polymer that mainly forms an important structural material for plant support tissues. It is particularly important in the formation of cell walls in wood and bark, for its ability to impart rigidity and resistance to decay. By understanding lignin's structure, history and biological functions, we will explore its key role in nature and the economy.
The history of lignin
The history of lignin can be traced back to 1813, when Swiss botanist A. P. de Candolle first mentioned this substance. He described it as fibrous, odorless and insoluble in water and alcohol, but soluble in slightly alkaline solutions and precipitated by acid. So he named it "lignine", derived from the Latin "lignum", which means wood.
Lignin is one of the most abundant organic polymers on Earth, second only to cellulose and chitin.
Lignin accounts for 30% of Earth's terrestrial non-fossil organic carbon and 20% to 35% of wood dry weight. The study found that lignin also exists in red algae, suggesting that the common ancestor of plants and red algae may have had the innate ability to synthesize lignin. The discovery also suggests that lignin's original function may have been to provide structural support, as it does in the red alga Calliarthron, by supporting joints between calcified parts.
Composition and structure
The composition of lignin varies between species. For example, the composition of an aspen sample is 63.4% carbon, 5.9% hydrogen, 0.7% ash (mineral composition), and 30% oxygen (derived from the difference), which is equivalent to the chemical formula (C31H34O11)n. Lignin is a highly heterogeneous polymer cross-linked from a small number of precursors, wood alcohols. There are three main types of these cross-linked wood alcohols, all derived from phenylpropane.
Lignin has a molecular weight of over 10,000 u, and its structural diversity and degree of cross-linking have a significant impact on its properties.
Biological functions
Lignin fills the gaps in cell walls, especially with the components cellulose, hemicellulose and pectin, especially in vascular bundles and supporting tissues. Lignin plays a key role in the conduction of water and water-soluble nutrients in plant stems. The polysaccharide component of plant cell walls is highly hydrophilic and therefore highly permeable to water, whereas lignin is more hydrophobic.
The most common function of lignin is to provide structural support to plants by strengthening wood.
Economic significance
Global commercial production of lignin originates from the paper industry. In 1988, more than 220 million tons of paper were produced globally, most of which was delignified. Lignin makes up about one-third of the mass of the cellulose precursor but often needs to be removed during the papermaking process because it causes discoloration and loss of paper strength. The process of delignification is one of the core technologies of the paper industry and poses significant environmental concerns.
Biosynthesis
Lignin biosynthesis begins in the cytoplasm with the synthesis of glycated wood alcohol from the amino acid phenylalanine. This process shares the first reaction with the phenylpropanoid pathway. There are still many unanswered questions about the synthesis of lignin, including the specific role of the oxidase enzymes that catalyze the polymerization reaction.
Biodegradation
Compared with other biopolymers, lignin is somewhat resistant to degradation, which makes it more stable in the environment. While some fungi are able to efficiently degrade lignin, bacteria are limited in their ability to do so, primarily because lignin derivatives inhibit bacterial growth.
Chemistry and Analysis
In the paper industry, lignin quantification is usually performed using Klason lignin and acid-soluble lignin tests. These standardized procedures enable efficient analysis and quantification of lignin composition, furthering understanding of its properties and applications.
The history and role of lignin is not limited to its chemical structure and function. It is an element closely linked to the environment, economy and ecosystems. In the future, can we use this precious resource more efficiently and develop greener materials and improved technologies?
