electrons is harnessed in the production of adenosine triphosphate (ATP), the
cell’s energy-storing compound. Terminal electron acceptors (e.g. oxygen) are the
final place the electrons arrive in the electron transport chain. Although oxygen
is not the only electron acceptor, it is the most thermodynamically favoured
102 Chapter Four
O
OH
OCH 3
HO
O
O
OH
H 3 CO O
O
OCH 3
O
O
H 3 CO
HO O
HO
OH
O
OCH 3
O OH
O
OCH 3
Fig. 3Partial structure of lignin. After Killops and Killops (1993).
aromatic) that is converted into three basic
aromatic (see Section 2.7) monomers,
coumaryl alcohol, coniferyl alcohol and
sinapyl alcohol. These monomers react
together and with their precursors to
produce lignin. Thus, lignin is a tremendously
complex polymer with a random structure
(Fig. 3). Lignin has a condensed structure that
is highly aromatic, making it the most
resistant component of plant tissues to
degradation. Only a few soil organisms are
capable of degrading lignin. These organisms
belong to a group of fungi known as white
rot fungi or basidiomycetes. They produce
powerful non-specific extracellular enzymes
called peroxidases which, with the aid of
H 2 O 2 , O 2 - and^1 O 2 , cause the depolymerization
of lignin. Carbon–carbon bonds and ether
(C–O–C) bonds are cleaved initially resulting
in the formation of monomeric phenols,
aromatic acids and alcohols; these are, in
turn, mineralized to CO 2 and H 2 O.
Table 4.6Relative proportions of biopolymers in plant-derived soil organic matter.
Plant residues Percentage in soils
Cellulose 50
Hemicelluloses 20
Lignin 15
Protein 5
Carbohydrates and amino acids 5
Pectin 1
Waxes and pigments 1