202
Chapter 11 · The “Wood-Stuff” - Lignin
11
lignin. Several microfibrils (c) form macrofibrils
(b), and the interspaces are also filled with lignin.
Finally, the cell walls consist of several macrofibrils (a).
The percentage of lignin in different plants
varies considerably. In principle, the percentage
of lignin is about 20–30% by weight. Softwoods
(e.g. spruce) contain slightly more lignin (27–
33%) than hardwoods such as beech (18–25%)
and grasses (17–24%). Marine plants, such as
algae, do not contain lignin, as they maintain
their stability by buoyancy in water. Even lower
plants which do not form vessels for the transport of water and nutrients, such as mosses and
lichens, do not contain lignin. In . Table 11.1,
typical compositions of selected trees and plant
parts are summarized.
Lignin has the following functions in plants:
5 Lignification of plant parts through the formation of a composite structure. Lignin thus
ensures the compressive strength of wood
and plays a decisive role in its stability.
5 Lignin reduces the transition of water
through the cell wall due to its hydrophobic
character. This is particularly important for
the water transport in the plant vessels and
thus also for nutrients and metabolites.
5 Protection against UV light by the aromatic
character.
5 Protection against mechanical penetration of
pests.
5 Inhibition of the growth of microorganisms,
as lignin cannot be decomposed by most
fungi and bacteria.
11.2 Structure of Lignin
It was mentioned at the beginning that lignin
is a phenolic (from phenol = hydroxybenzene)
macromolecule. This clearly shows that lignin
cannot be classified in any of the natural substance categories already discussed in this book;
it is neither a carbohydrate nor a fat or a terpene. Lignin thus opens its own class. It is the
only primary plant ingredient that has aromatic
(more precisely: phenolic) structural characteristics. In addition, lignin is a highly branched and
thus three-dimensional polymer. In . Fig. 11.2,
a section of the structure of a lignin molecule of
spruce is shown as an example.
Chapter Timetable
5 In this chapter, we will take a closer look
at the occurrence and significance of
lignin for plants.
5 The special chemical structure of lignin
will be examined in more detail using
some binding principles.
5 We will look at the processes in which
lignin is produced and how it can be
obtained in a highly pure form.
5 Finally, the most important applications
of lignin will be discussed.
11.1 Occurrence of Lignin
Lignin (from lat. lignum, wood) is a phenolic
macromolecule and besides cellulose and hemicelluloses an important component of vascular plants, especially in wood. Lignin is, along
with cellulose and chitin, one of the three most
common organic compounds on earth. It is
estimated that lignin quantities of the order of
2 × 10 10 tons are formed annually.
Lignin does not have a precisely defined
chemical structure. Rather, it is a collective term
for various macromolecules with similar structural characteristics and properties. Correctly,
one should actually speak of lignins (in plural).
Lignin is stored in the cell wall of plants
where it forms a solid matrix. The formation of
a composite structure with cellulose and hemicelluloses leads to lignification of the plant parts
(7 Chap. 7). With this function, lignin plays a
decisive role in the evolution of large terrestrial plants: Only the ability to lignify allows the
formation and stability of plant parts of corresponding size known to us today. Since lignin, in
comparison to hemicelluloses, cannot be easily
separated from cellulose, it is partially covalently
bound to cellulose, and the name lignocellulose
is often used for plant substances of wood origin.
We have already learned about the spatial
arrangement of the cellulose fibers in relation to
each other in 7 Chap. 7. Lignin functions in the
composite material wood as putty, which fills the
gaps and spaces (. Fig. 11.1): Cellulose fibers are
surrounded by hemicelluloses (. Fig. 11.1e) and
thus form a micelle (d). Several micelles form a
microfibril, the spaces between being filled with
Chapter 11 · The “Wood-Stuff” - Lignin
11
lignin. Several microfibrils (c) form macrofibrils
(b), and the interspaces are also filled with lignin.
Finally, the cell walls consist of several macrofibrils (a).
The percentage of lignin in different plants
varies considerably. In principle, the percentage
of lignin is about 20–30% by weight. Softwoods
(e.g. spruce) contain slightly more lignin (27–
33%) than hardwoods such as beech (18–25%)
and grasses (17–24%). Marine plants, such as
algae, do not contain lignin, as they maintain
their stability by buoyancy in water. Even lower
plants which do not form vessels for the transport of water and nutrients, such as mosses and
lichens, do not contain lignin. In . Table 11.1,
typical compositions of selected trees and plant
parts are summarized.
Lignin has the following functions in plants:
5 Lignification of plant parts through the formation of a composite structure. Lignin thus
ensures the compressive strength of wood
and plays a decisive role in its stability.
5 Lignin reduces the transition of water
through the cell wall due to its hydrophobic
character. This is particularly important for
the water transport in the plant vessels and
thus also for nutrients and metabolites.
5 Protection against UV light by the aromatic
character.
5 Protection against mechanical penetration of
pests.
5 Inhibition of the growth of microorganisms,
as lignin cannot be decomposed by most
fungi and bacteria.
11.2 Structure of Lignin
It was mentioned at the beginning that lignin
is a phenolic (from phenol = hydroxybenzene)
macromolecule. This clearly shows that lignin
cannot be classified in any of the natural substance categories already discussed in this book;
it is neither a carbohydrate nor a fat or a terpene. Lignin thus opens its own class. It is the
only primary plant ingredient that has aromatic
(more precisely: phenolic) structural characteristics. In addition, lignin is a highly branched and
thus three-dimensional polymer. In . Fig. 11.2,
a section of the structure of a lignin molecule of
spruce is shown as an example.
Chapter Timetable
5 In this chapter, we will take a closer look
at the occurrence and significance of
lignin for plants.
5 The special chemical structure of lignin
will be examined in more detail using
some binding principles.
5 We will look at the processes in which
lignin is produced and how it can be
obtained in a highly pure form.
5 Finally, the most important applications
of lignin will be discussed.
11.1 Occurrence of Lignin
Lignin (from lat. lignum, wood) is a phenolic
macromolecule and besides cellulose and hemicelluloses an important component of vascular plants, especially in wood. Lignin is, along
with cellulose and chitin, one of the three most
common organic compounds on earth. It is
estimated that lignin quantities of the order of
2 × 10 10 tons are formed annually.
Lignin does not have a precisely defined
chemical structure. Rather, it is a collective term
for various macromolecules with similar structural characteristics and properties. Correctly,
one should actually speak of lignins (in plural).
Lignin is stored in the cell wall of plants
where it forms a solid matrix. The formation of
a composite structure with cellulose and hemicelluloses leads to lignification of the plant parts
(7 Chap. 7). With this function, lignin plays a
decisive role in the evolution of large terrestrial plants: Only the ability to lignify allows the
formation and stability of plant parts of corresponding size known to us today. Since lignin, in
comparison to hemicelluloses, cannot be easily
separated from cellulose, it is partially covalently
bound to cellulose, and the name lignocellulose
is often used for plant substances of wood origin.
We have already learned about the spatial
arrangement of the cellulose fibers in relation to
each other in 7 Chap. 7. Lignin functions in the
composite material wood as putty, which fills the
gaps and spaces (. Fig. 11.1): Cellulose fibers are
surrounded by hemicelluloses (. Fig. 11.1e) and
thus form a micelle (d). Several micelles form a
microfibril, the spaces between being filled with