374
14.10 Cell Wall Constituents
The organic compounds that make up the cell wall comprise about 75–90% of the
dry weight of the leaf. They vary with the location in the leaf, by phenology (season), with environmental stressors, and by species. They also vary depending on
whether the walls are primary or secondary structures or part of the vascular bundles. Common constituents include cellulose, hemicellulose, lignin, proteins, and
pectins. They are large molecules of variable molecular composition; their –OH
units form extensive hydrogen bonds that link the molecules together, providing
both flexibility and strength. They form solid structures and are mostly insoluble.
This makes them poor candidates for spectroscopy, so they are generally measured
with other techniques.
Cellulose is the most abundant organic polymer on Earth (Klemm et al. 2005). It
is a polysaccharide forming a straight chain of many to thousands linked D-glucose
units (Fig. 14.14a) with a nonreducing end (shown on left) linked to repeating glucose units by oxygen and ending with a reducing oxygen ion. The –OH groups form
hydrogen bonds across chains, holding them together and providing structure.
Species can be identified based on the relative abundance of these molecules.
Hemicellulose refers to any of several heteropolymers that form the matrix polysaccharides in most plant cell walls (Fig. 14.14b). Hemicellulose monomers include
hexoses and fructoses such as xyloglucans, xylans, and glucans. Figure 14.15 illustrates some differences in hemicellulose subunit structures. Their chemical composition is similar to that of cellulose, so they are spectrally similar and are not
generally separately identified in leaf spectra. The structure of these molecules,
however, varies between species, suggesting that they could contribute to identifying biodiversity from RS data and possibility different species or related species.
Lignins are a widely distributed class of complex amorphous organic, branched,
and cross-linked polymers (Fig. 14.14c) of the approximate composition C 31 H 34 O 11 .
They form the structural materials of cell walls and other support tissues in leaves,
such as the walls of tracheids in the vascular system. Lignin is unusually heterogeneous in composition and lacks a defined primary molecular structure. Figure 14.14c
illustrates the cross-linked phenol units that make it very slow to decompose.
Eventually, the recalcitrant residues become a major fraction of the soil humus.
Humus is important in the global C cycle for sequestering soil C and nutrients and
retaining soil moisture. Lignins interact with soil organic matter and N turnover
rates, and this affects lignin stabilization (Thevenot et al. 2010), so they are important in biogeochemical cycling. The variety of lignin molecules found in different
plant species makes quantification difficult, but some forms are known to occur in
specific plant clades, providing a basis for estimating diversity between lower vascular plants, conifers and angiosperms (Weng and Chapple 2010) (Fig. 14.15).
Although fundamental frequencies are known for C–H, C=O, C–N, O–H, and
N–H bonds, dried and powdered leaf materials have overlapping spectral properties
of various cell wall molecules, proteins, enzymes, amino acids, sugars, starches,
waxes, and other biomolecules that make it difficult to isolate specific molecules
S. L. Ustin and S. Jacquemoud
14.10 Cell Wall Constituents
The organic compounds that make up the cell wall comprise about 75–90% of the
dry weight of the leaf. They vary with the location in the leaf, by phenology (season), with environmental stressors, and by species. They also vary depending on
whether the walls are primary or secondary structures or part of the vascular bundles. Common constituents include cellulose, hemicellulose, lignin, proteins, and
pectins. They are large molecules of variable molecular composition; their –OH
units form extensive hydrogen bonds that link the molecules together, providing
both flexibility and strength. They form solid structures and are mostly insoluble.
This makes them poor candidates for spectroscopy, so they are generally measured
with other techniques.
Cellulose is the most abundant organic polymer on Earth (Klemm et al. 2005). It
is a polysaccharide forming a straight chain of many to thousands linked D-glucose
units (Fig. 14.14a) with a nonreducing end (shown on left) linked to repeating glucose units by oxygen and ending with a reducing oxygen ion. The –OH groups form
hydrogen bonds across chains, holding them together and providing structure.
Species can be identified based on the relative abundance of these molecules.
Hemicellulose refers to any of several heteropolymers that form the matrix polysaccharides in most plant cell walls (Fig. 14.14b). Hemicellulose monomers include
hexoses and fructoses such as xyloglucans, xylans, and glucans. Figure 14.15 illustrates some differences in hemicellulose subunit structures. Their chemical composition is similar to that of cellulose, so they are spectrally similar and are not
generally separately identified in leaf spectra. The structure of these molecules,
however, varies between species, suggesting that they could contribute to identifying biodiversity from RS data and possibility different species or related species.
Lignins are a widely distributed class of complex amorphous organic, branched,
and cross-linked polymers (Fig. 14.14c) of the approximate composition C 31 H 34 O 11 .
They form the structural materials of cell walls and other support tissues in leaves,
such as the walls of tracheids in the vascular system. Lignin is unusually heterogeneous in composition and lacks a defined primary molecular structure. Figure 14.14c
illustrates the cross-linked phenol units that make it very slow to decompose.
Eventually, the recalcitrant residues become a major fraction of the soil humus.
Humus is important in the global C cycle for sequestering soil C and nutrients and
retaining soil moisture. Lignins interact with soil organic matter and N turnover
rates, and this affects lignin stabilization (Thevenot et al. 2010), so they are important in biogeochemical cycling. The variety of lignin molecules found in different
plant species makes quantification difficult, but some forms are known to occur in
specific plant clades, providing a basis for estimating diversity between lower vascular plants, conifers and angiosperms (Weng and Chapple 2010) (Fig. 14.15).
Although fundamental frequencies are known for C–H, C=O, C–N, O–H, and
N–H bonds, dried and powdered leaf materials have overlapping spectral properties
of various cell wall molecules, proteins, enzymes, amino acids, sugars, starches,
waxes, and other biomolecules that make it difficult to isolate specific molecules
S. L. Ustin and S. Jacquemoud