Essential Cell Biology 5th edition

694 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
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
of the plant cell and its surroundings. Once cell growth stops and the
wall no longer needs to expand, a more rigid secondary cell wall is often
produced (see Figure 20–3A)—either by thickening of the primary wall or
by deposition of new layers with a different composition underneath the
old ones. When plant cells become specialized, they generally produce
specially adapted types of walls: waxy, waterproof walls for the surface
epidermal cells of a leaf; hard, thick, woody walls for the xylem cells of
the stem; and so on.
cellulose microfibril
Figure 20–4 A cellulose microfibril
is made from a bundle of cellulose
molecules. Cellulose molecules are long,
unbranched chains of glucose. Each
glucose subunit is inverted with respect
to its neighbors and joined to them via a
β1,4-linkage. The resulting disaccharide
repeat occurs hundreds of times in each
individual cellulose molecule. About
16 cellulose molecules are held together
via hydrogen bonds in a single cellulose
microfibril, as shown.
ECB5 m19.62/20.04
Cellulose Microfibrils Give the Plant Cell Wall Its Tensile
Strength
Like all extracellular matrices, plant cell walls derive their tensile strength
from long fibers oriented along the lines of stress. In higher plants, the
long fibers are generally made from the polysaccharide cellulose, the
most abundant organic macromolecule on Earth (Figure 20–4). These
cellulose microfibrils are interwoven with other polysaccharides and
some structural proteins, all bonded together to form a complex structure
that resists both compression and tension (Figure 20–5). In woody
tissue, a highly cross-linked network of lignin (a complex polymer built
from aromatic alcohol groups) is deposited within this matrix to make it
more rigid and waterproof.
For a plant cell to grow or change its shape, the cell wall has to stretch
or deform. Because the cellulose microfibrils resist stretching, their orientation
governs the direction in which the growing cell enlarges: if, for
example, they are arranged circumferentially as a corset, the cell will
grow more readily in length than in girth (Figure 20–6). By controlling the
way that it lays down its wall, the plant cell consequently controls its own
shape and thus the direction of growth of the tissue to which it belongs.
Cellulose is produced in a radically different way from most other
extracellular macromolecules. Instead of being made inside the cell and
then exported by exocytosis (discussed in Chapter 15), it is synthesized
on the outer surface of the cell by enzyme complexes embedded in the
plasma membrane. These complexes transport glucose monomers from
the cytosol across the plasma membrane and incorporate them into a set
of growing cellulose chains at their points of membrane attachment. The
resulting cellulose chains assemble to form a cellulose microfibril (see
Figure 20−4).
The paths followed by the membrane-embedded enzyme complexes dictate
the orientation in which cellulose is deposited in the cell wall. But
middle
lamella
pectin
Figure 20–5 A scale model shows a
portion of a primary plant cell wall.
Cellulose microfibrils (blue) provide tensile
strength. Other polysaccharides (red strands)
cross-link the cellulose microfibrils, while the
polysaccharide pectin (green strands) fills the
spaces between the microfibrils, providing
resistance to compression. The middle
lamella (yellow) is rich in pectin and is the
layer that cements one cell wall to another.
primary
cell wall
plasma
membrane
50 nm
CYTOSOL
cellulose
microfibril
cross-linking polysaccharide