An Introduction to Environmental Chemistry

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