Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE PLANT CELL WALL
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animal extracellular matrix, which is rich in protein and other nitrogen-containing
polymers, the plant cell wall is made almost entirely of polymers that contain
no nitrogen, including cellulose and lignin. For a sedentary organism that depends
on CO 2 , H 2 O, and sunlight, these two abundant biopolymers represent “cheap,”
carbon-based structural materials, helping to conserve the scarce fixed nitrogen
available in the soil that generally limits plant growth. Thus trees, for example,
make a huge investment in the cellulose and lignin that comprise the bulk of their
biomass.
In the cell walls of higher plants, the tensile fibers are made from the polysaccharide
cellulose, the most abundant organic macromolecule on Earth, tightly
linked into a network by cross-linking glycans. In primary cell walls, the matrix
in which the cross-linked cellulose network is embedded is composed of pectin,
a highly hydrated network of polysaccharides rich in galacturonic acid. Secondary
cell walls contain additional molecules to make them rigid and permanent;
lignin, in particular, forms a hard, waterproof filler in the interstices between the
other components. All of these molecules are held together by a combination of
covalent and noncovalent bonds to form a highly complex structure, whose composition,
thickness, and architecture depend on the cell type.
The plant cell wall thus has a “skeletal” role in supporting the structure of the
plant as a whole, a protective role as an enclosure for each cell individually, and
a transport role, helping to form channels for the movement of fluid in the plant.
When plant cells become specialized, they generally adopt a specific shape and
produce specially adapted types of walls, according to which the different types
of cells in a plant can be recognized and classified. We focus here, however, on
the primary cell wall and the molecular architecture that underlies its remarkable
combination of strength, resilience, and plasticity, as seen in the growing parts of
a plant.
The Tensile Strength of the Cell Wall Allows Plant Cells to Develop
Turgor Pressure
The aqueous extracellular environment of a plant cell consists of the fluid contained
in the walls that surround the cell. Although the fluid in the plant cell wall
contains more solutes than does the water in the plant’s external milieu (for example,
soil), it is still hypotonic in comparison with the cell interior. This osmotic
imbalance causes the cell to develop a large internal hydrostatic pressure, or turgor
pressure, which pushes outward on the cell wall, just as an inner tube pushes
outward on a tire. The turgor pressure increases just to the point where the cell is
in osmotic equilibrium, with no net influx of water despite the salt imbalance. The
turgor pressure generated in this way may reach 10 or more atmospheres, about
five times that in the average car tire. This pressure is vital to plants because it is the
main driving force for cell expansion during growth, and it provides much of the
mechanical rigidity of living plant tissues. Compare the wilted leaf of a dehydrated
plant, for example, with the turgid leaf of a well-watered one. It is the mechanical
strength of the cell wall that allows plant cells to sustain this internal pressure.
4
O
6 CH 2 OH
OH
5
3
O
2
O
OH
1 4
OH
1
3 2 5 O
OH 6 CH 2 OH
cellulose molecule
n
The Primary Cell Wall Is Built from Cellulose Microfibrils Interwoven
with a Network of Pectic Polysaccharides
Cellulose gives the primary cell wall tensile strength. Each cellulose molecule consists
of a linear chain of at least 500 glucose residues that are covalently linked to
one another to form a ribbonlike structure, which is stabilized by hydrogen bonds
within the chain (Figure 19–62). In addition, hydrogen bonds between adjacent
cellulose molecules cause them to stick together in overlapping parallel arrays,
forming bundles of about 40 cellulose chains, all of which have the same polarity.
These highly ordered crystalline aggregates, many micrometers long, are called
cellulose microfibrils, and they have a tensile strength comparable to that of
steel. Sets of microfibrils are arranged in layers, or lamellae, with each microfibril
about 20–40 nm from its neighbors and connected to them by long cross-linking
cellulose
microfibril
Figure 19–62 Cellulose. Cellulose
molecules are long, unbranched chains of
β1,4-linked glucose units. Each glucose
residue is inverted with respect to its
neighbors, and the resulting disaccharide
repeat occurs hundreds of times in a single
cellulose molecule. About 16 individual
cellulose molecules assemble to form
a strong, hydrogen-bonded cellulose
microfibril.