100 Chapter Four
Box 4.10 BiopolymersCelluloseCellulose is the most common plant polymer
forming the structural fibres of many plants.
Cellulose is constructed solely of glucose
monomers (Fig. 1a), i.e. isolated or single
units with consistent structure, in this case
based on a carbon ring. The glucose
monomers are linked by an ether bond
(C–O–C; Fig. 1a). Thus, cellulose has a simple
straight chain structure. Degradation of
cellulose occurs stepwise. Initially the large
polymer chain is cleaved into smaller unitscontaining, for example, two or three glucose
monomers (cellobiose and cellotriose,
respectively Fig. 1b) by the action of cellulase
enzymes. It is this depolymerization reaction
that is the rate-limiting step in cellulose
biodegradation. The smaller units are then
further degraded to produce glucose
monomers that are then completely
mineralized (converted to their inorganic
constituents, e.g. CO 2 and H 2 O) under aerobic
conditions (Fig. 1c) or fermented to ethanol
and CO 2 under anaerobic conditions.OOOHOHCH 2 OHOO O
CH 2 OHOHO
OHOHCH 2 OHOO O
CH 2 OHOHnOOHOHOHCH 2 OHOO O
CH 2 OHOHO
OHOHCH 2 OH
OHn nOOHOHOHCH 2 OHOO
CH 2 OHOH
OHCellulose
CellulaseCellotriose
CellotriaseCellobioseO
OHOHCH 2 OHOH OH¥ n6O 2
6CO 2 + 6H 2 OGlucose(a)(b)(c)OHOH OHOHFig. 1Steps in the aerobic degradation of cellulose.
(continued)