What is the function of lignin


A lignified trunk or branches give a tree stability and resistance to mechanical loads. We have already seen in the chapter vascular bundles that tracheids and trachea have reinforced (lignified) cell walls in order to achieve greater strength. Wood fibers are very woody. But what makes the cell walls so stable? How does lignification happen? To do this, we have to name a substance lignin turn to, which is also called wood pulp because of its function.
But first let's get a brief overview of the plant cell wall. It consists not only of one layer that encloses the interior of the cell, but of several layers that are formed one after the other. The primary cell wall consists mainly of hemicelluloses (?) And protopectin (?) And is quite thin. Between the primary cell walls of cells that are adjacent to each other lies the middle lamella, a thin layer made up of pectins. When the cell is fully grown, a secondary one, which is much thicker and contains a lot of cellulose, is layered on the primary cell wall.

Simplified representation of a plant cell. Pits are not shown


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Lignin is stored in this secondary cell wall during the lignification process. One speaks of an encrusted cell wall and lignin is called an encrust. This makes the cell wall stable and very solid, which gives the desired resistance.
How is lignin structured? It consists of three alcohols that polymerize to form a complex network. These three alcohols (coumaric alcohol, coniferyl alcohol and sinapyl alcohol) are shown here:

Coniferyl alcohol:

Structural formula

3D structure
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Cumaryl alcohol:

Structural formula

3D structure
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Structural formula

3D structure
Click to enlarge

 

Lignin itself is not always composed in the same way. While coniferyl alcohol is the main component in coniferous trees, coniferyl and sinapyl alcohol are present in roughly equal proportions in deciduous trees. Cumaryalcohol occurs only in monocotyledonous (?) Plants, e.g. in large quantities in palm trees.
The structure of lignin can be seen in the following figure:

 

As described in more detail in the chapter on cellulose, this substance forms so-called fibrils, which run parallel through the secondary cell wall like wires. The gaps are filled with lignin. Think of it like reinforced concrete: cellulose is steel and lignin is concrete.
After cellulose, lignin is the most common organic compound on earth and accounts for up to 30% of wood.



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