Most newly formed cells in a multicellular plant are produced in special regions called meristems,

41 Most newly formed cells in a multicellular plant are produced in special regions called meristems,

as explained in Chapter 21. These new cells are generally small in comparison to their final size, and to accommodate subsequent cell growth, their walls, called primary cell walls, are thin and only semirigid. Once growth stops and the wall no longer needs to be able to expand, either the primary wall is simply retained or, far more commonly, a rigid, secondary cell wall is produced, either by thickening the primary wall or by depositing new layers with a different composition underneath the old ones. In addition to a structural or "skeletal" role, the cell wall also protects the underlying cell and functions in the transport of fluid within the plant. When plant cells become specialized, they generally produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified.

Although the primary cell walls of higher plants vary greatly in both composition and organization, like all extracellular matrices they are constructed according to a common principle: they derive their tensile strength from long fibers and their resistance to compression from the matrix of protein and polysaccharide in which the fibers are embedded. In the cell walls of higher plants the fibers are generally made from the polysaccharide cellulose, the most abundant organic macromolecule on earth. The rest of the matrix is composed predominantly of two other types of polysaccharide, hemicellulose and pectin, together with structural proteins. All of these molecules are held together by a combination of covalent and noncovalent bonds to form a highly complex structure whose composition depends on the cell type.

Th e Te n s ile S t re n g t h o f t h e Ce ll W a ll Allo w s P la n t Ce lls t o

D e v e lo p Tu rg o r P re s s u re 42

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 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

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 (see Panel 11-1, p. 517). 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.

Th e Ce ll W a ll I s B u ilt fro m Ce llu lo s e Mic ro fib rils

I n t e rw o v e n w it h a N e t w o rk o f P o ly s a c c h a rid e s a n d

P ro t e in s 43

The tensile strength of the primary cell wall is provided by cellulose. A 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. In addition, intermolecular hydrogen bonds between adjacent cellulose molecules cause them to adhere strongly to one another in overlapping parallel arrays, forming a bundle of 60 to 70 cellulose chains, all of which have the same polarity. These highly ordered crystalline aggregates, many micrometers long, are called cellulose microfibrils. 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 hemicellulose molecules that are bound 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- 65).

Hemicelluloses are a heterogeneous group of branched polysaccharides that bind tightly to the surface of each cellulose microfibril as well as to one another and thereby help to cross-link microfibrils into a complex network. Their function is analogous to that of the fibril-associated collagens discussed earlier. There are many classes of hemicelluloses, but they all have a long linear backbone composed of one type of sugar, from which short side chains of other sugars protrude. Both the backbone sugar and the side-chain sugars vary according to the plant species and its stage of development. It is the sugar molecules in the backbone that form hydrogen bonds with cellulose microfibrils.

Coextensive with this network of cellulose microfibrils and hemicelluloses is another cross-linked polysaccharide network based on pectins (see Figure 19-65). These are a heterogeneous group of branched polysaccharides that contain many negatively charged galacturonic acid residues. Because of their negative charge, pectins are highly hydrated and accompanied by a cloud of cations, resembling the glycosaminoglycans of animal cells in the large amount of space they occupy. 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 jelly). Certain pectins are particularly abundant in the middle lamella, the specialized central region of the wall that cements together the walls of adjacent cells, and such Ca 2+ cross-links are thought to help hold cell wall components together. Thus many plant tissues, if treated with a Ca 2+ chelating agent, dissociate into their constituent cells. Although covalent bonds also play a part in linking the different plant cell-wall components together, very little is known about their nature.

In addition to the two polysaccharide-based networks that are present in all plant primary cell walls, there is a variable contribution from structural proteins. One class of proteins contains high levels of hydroxyproline, like collagen. These proteins are thought to strengthen the wall, and they are produced in greatly increased amounts as a local response to attack by microorganisms. During normal differentiation cells use structural proteins to modify local regions of their walls, as required to create the wide range of functionally specialized secondary walls characteristic of mature cell types (see Panel 1-1, pp. 18-19).

In order for a plant cell to grow or change its shape, the cell wall has to stretch or deform. But because of their crystalline structure, individual cellulose microfibrils are unable to stretch. Thus stretching or deformation of the cell wall must involve either the sliding of microfibrils past one another, or the separation of adjacent microfibrils, or both. As we discuss next, the direction in which the growing cell enlarges depends on the orientation of the strain-resisting cellulose microfibrils in the primary wall, which in turn depends on the orientation of microtubules in the underlying cell cortex at the time the wall was deposited.