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

1082 Chapter 19: Cell Junctions and the Extracellular Matrix
Figure 19–61 Plant cell walls. (A) Electron
micrograph of the root tip of a rush,
showing the organized pattern of cells
that results from an ordered sequence of
cell divisions in cells with relatively rigid
cell walls. In this growing tissue, the cell
walls are still relatively thin, appearing as
fine black lines between the cells in the
micrograph. (B) Section of a typical cell wall
separating two adjacent plant cells. The
two dark transverse bands correspond to
plasmodesmata that span the wall (see
Figure 19–27). (A, courtesy of C. Busby
and B. Gunning, Eur. J. Cell Biol. 21:214–
223, 1980. With permission from Elsevier;
B, courtesy of Jeremy Burgess.)
(A)
10 µm
(B)
200 nm
together to form the intact plant (Figure 19–61), are generally thicker, stronger,
and, most important of all, more rigid than the extracellular matrix produced by
animal cells. In evolving relatively rigid walls, which can be up to many micrometers
thick, early plant cells forfeited the ability to crawl about and adopted a sedentary
lifestyle that has persisted in all present-day plants.
MBoC5 m19.76/19.62
The Composition of the Cell Wall Depends on the Cell Type
All cell walls in plants have their origin in dividing cells, as the cell plate forms
during cytokinesis to create a new partition wall between the daughter cells
(discussed in Chapter 17). The new cells are usually produced in special regions
called meristems, and they are generally small in comparison with their final size.
To accommodate subsequent cell growth, the walls of the newborn cells, called
primary cell walls, are thin and extensible, although tough. Once cell growth
stops, the wall no longer needs to be extensible: sometimes the primary wall is
retained without major modification, but, more commonly, a rigid secondary cell
wall is produced by depositing new layers of matrix inside the old ones. These new
layers generally have a composition that is significantly different from that of the
primary wall. The most common additional polymer in secondary walls is lignin,
a complex network of covalently linked phenolic compounds found in the walls of
the xylem vessels and fiber cells of woody tissues.
Although the cell walls of higher plants vary in both composition and organization,
they are all constructed, like animal extracellular matrices, using a
structural principle common to all fiber-composites, including fiberglass and
reinforced concrete. One component provides tensile strength, while another,
in which the first is embedded, provides resistance to compression. While the
principle is the same in plants and animals, the chemistry is different. Unlike the