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

1084 Chapter 19: Cell Junctions and the Extracellular Matrix
middle
lamella
primary
cell wall
pectin
Figure 19–63 Scale model of a portion
of a primary plant cell wall showing the
two major polysaccharide networks. The
orthogonally arranged layers of cellulose
microfibrils (green) are tied into a network
by the cross-linking glycans (red) that form
hydrogen bonds with the microfibrils. This
network is coextensive with a network of
pectin polysaccharides (blue). The network
of cellulose and cross-linking glycans
provides tensile strength, while the pectin
network resists compression. Cellulose,
cross-linking glycans, and pectin are
typically present in roughly equal amounts
in a primary cell wall. The middle lamella
is especially rich in pectin, and it cements
adjacent cells together.
plasma
membrane
cellulose
microfibril
cross-linking glycan
50 nm
glycan molecules, which are attached by hydrogen bonds to the surface of the
microfibrils. The primary cell wall consists of several such lamellae arranged in a
plywoodlike network (Figure 19–63).
The cross-linking glycans are a heterogeneous group of branched polysaccharides
that bind tightly to the surface of each cellulose microfibril and thereby help
to cross-link the microfibrils into a complex network. There are many classes of
cross-linking glycans, but they all have a long linear backbone composed of one
type of sugar (glucose, xylose, MBoC6 or mannose) m19.79/19.64 from which short side chains of other
sugars protrude. It is the backbone sugar molecules that form hydrogen bonds
with the surface of cellulose microfibrils, cross-linking them in the process. Both
the backbone and the side-chain sugars vary according to the plant species and
its stage of development.
Coextensive with this network of cellulose microfibrils and cross-linking glycans
is another cross-linked polysaccharide network based on pectins (see Figure
19–63). Pectins are a heterogeneous group of branched polysaccharides that contain
many negatively charged galacturonic acid units. Because of their negative
charge, pectins are highly hydrated and associated with a cloud of cations, resembling
the glycosaminoglycans of animal cells in the large amount of space they
occupy (see Figure 19–33). When Ca 2+ is added to a solution of pectin molecules,
it cross-links them to produce a semirigid gel (it is pectin that is added to fruit
juice to make jam set). Certain pectins are particularly abundant in the middle
lamella, the specialized region that cements together the walls of adjacent cells
(see Figure 19–63); here, Ca 2+ cross-links are thought to help hold cell wall components
together. Although covalent bonds also play a part in linking the components,
very little is known about their nature. Regulated separation of cells at
the middle lamella underlies such processes as the ripening of tomatoes and the
abscission (detachment) of leaves in the fall.
In addition to the two polysaccharide-based networks that form the bulk of all
plant primary cell walls, proteins are present, contributing up to about 5% of the
wall’s dry mass. Many of these proteins are enzymes, responsible for wall turnover
and remodeling, particularly during growth. Another class of wall proteins,
like collagen, contains high levels of hydroxyproline. These proteins are thought
to strengthen the wall, and they are produced in greatly increased amounts as a
local response to attack by pathogens. From the genome sequence of Arabidopsis,
it has been estimated that more than 700 genes are required to synthesize, assemble,
and remodel the plant cell wall.