186 Introduction to Renewable Biomaterials
Ca2+ bridges between
pectin molecules
Neutral pectin
molecule
Hemicellulose
molecule
Acidic pectin
molecule
Glycoprotein
Cellulose
microfibril
100 nm
The relative arrangements of molecule types in a primary cell wall.
Pit
Middle
lamella
Three-layered
secondary wall
Primary wall
Macrofibril Microfibril
Cellulose
molecule Micelle
Micelle
(d)
(b) (c)
(a)
Figure 6.4(a) Formation of micro- and macrofibrils (fibers) of cellulose and their position in the wall.
(b) Magnified view of cellulose microfibril.
abundant. Due to these differences in hemicellulose compositions, agricultural waste
products as well as hardwoods are rich in pentose sugars, such as xylose, whereas
softwoods are rich in hexose sugars (Kirk-Otmer, 2001; Jorgensenet al., 2007).
As shown in Figure 6.5, the molecule of a xylan involves 1→4 linkages of xylopyra-
nosyl units withα-(4-O)-methyl-d-glucuronopyranosyl units attached to anhydroxylose
units. The result is a branched polymer chain that is largely composed of 5-carbon sugar
monomers, xylose, and to a minor extent 6-carbon sugar monomer such as glucose.
