Environmental Engineering Reference
In-Depth Information
5.2
Plant cell Wall
5.2.1
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The cell wall provides a plant cell with protection, structure, and shape. Physiological functions of
the cell wall are imperative in facilitating the normal growth and development of a plant cell; for
example, they are significant in cell-to-cell interactions, generation of biologically active signaling
molecules, regulation of diffusion of materials through apoplast, storage of carbohydrate reserves,
and protection from biotic and abiotic stresses. Therefore, research on the understanding of cell wall
biosynthesis is an important aspect of basic plant research. The understanding of plant cell walls is
also imperative for improving human life because it is a key source for dietary fibers, the textile/
paper industries, and the growing interest in the future biofuel industry. Our understanding of plant
cell wall structure and biosynthesis is becoming clearer with the onset of technological advance-
ments (Roberts 2001). Generally, plant cell walls are classified as type I and type II cell walls. Type I
plant cell walls are characteristic of dicots and noncommelinoid monocots and are mainly composed
of significant proportions of pectic polysaccharides and hemicellulosic polysaccharide, xyloglucans
(Pauly and Keegstra 2008). In contrast, arabinoxylan is the main hemicellulose in type II walls (i.e.,
the walls of the Poales, which include grasses) (Carpita 1996). In addition, type II walls have an
increased proportion of cellulose and only lesser amounts of pectins and proteins (Carpita 1996).
From a structural and functional point of view, there are two classes of plant cell walls: primary
(essentially present in growing and dividing cells) and secondary cell walls (essentially present in
woody tissues). As a part of growing and dividing cells, primary cell walls have to be flexible and
plastic to contain the cell growth (Cosgrove 2003). The first well-defined model for the structure of
primary cell walls came from the work of Peter Albersheim and his colleagues (Keegstra et al. 1973).
Progress in research since that first model has allowed us to reach a much more detailed understand-
ing of the macromolecular framework that makes up the plant cell wall, although many details are still
missing. The primary plant cell wall contains numerous interconnected matrices composed of diverse
polysaccharides and (glyco) proteins. The polysaccharides in these matrices include cellulose microfi-
brils, hemicelluloses (e.g., xyloglucan, xylan, and galactomannans), and pectins. Cellulose microfibrils
are noncovalently cross-linked by hemicelluloses. This intricate network of cellulose and associated
hemicelluloses are embedded in the pectic matrix, which is formed of several kinds of pectins. Pectins
are yet another important class of plant cell wall molecules that are primarily composed of homogalac-
turonan, rhamnogalacturonan I (RG-I), and rhamnogalacturonan II (RG-II) (O'Neill and York 2003).
Mature plant cells develop secondary walls. This wall is much stronger and accounts for most of
the carbohydrate in biomass. The secondary walls also contain primary wall components such as
cellulose, hemicelluloses, and pectins. However, their relative proportion may vary. For instance,
along with increased cellulose content, xylan also accounts for up to 30% of the mass in secondary
walls of wood and grasses. It is this increased cellulose content that makes secondary walls a good
target for the biofuel industry. In secondary walls, lignin largely replaces water, thus making them
nearly inaccessible to solutes and degrading enzymes. Lignin is an important component of the cell
wall and the main contributor for cell wall recalcitrance against degradation. Once incorporated,
lignin further strengthens the secondary walls in most tissues. In brief, secondary walls are com-
posed of cellulose microfibrils embedded in cross-linking hemicelluloses that are strengthened by
lignin. The major hurdle for producing bioethanol out of plant cell walls is making the cellulose
impregnated in xylan and lignin accessible to degrading enzymes such as cellulases.
5.2.2
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Much of our increased understanding of plant cell wall structure and biosynthesis has benefited from
the complete sequencing of the genomes of several plants, but most notably of
Arabidopsis thaliana
.
