Cell Walls

With a few notable exceptions, plant cells are encased in a complex polymeric wall that is synthesized and assembled by the cell during its growth and differentiation. Cell walls function as the major mechanical restraint that determines plant cell size and morphology . They enable cells to generate high turgor pressure and thus are important for the water relations of plants. Cell walls also act as a physical and chemical barrier to slow the invasion of bacteria, fungi, and other plant pests, and they also take part in a sophisticated signaling and defense system that helps plants sense pathogen invasion by detecting breakdown products from wall polysaccharides. Finally, cell walls glue plant cells together and provide the mechanical support necessary for large structures (the largest trees may reach 100 meters in height, generating tremendous compression forces due to their own weight).

Cell walls vary greatly in appearance, composition, and physical properties. In growing cells, such as those found in shoot and root apical meristems , cell walls are pliant and extensible (that is, they can extend in response to the expansive forces generated by cell turgor pressure). Such walls are called primary cell walls. After cells cease growth, they sometimes continue to synthesize one or more additional cell wall layers that are referred to as the secondary cell wall. Secondary cell walls are generally inextensible and may be thick and lignified , as in the xylem cells that make up wood.

Composition and Molecular Architecture

The primary cell wall contains three major classes of polysaccharides: cellulose, hemicellulose, and pectin. Hemicellulose and pectin collectively constitute the matrix polysaccharides of the cell wall. Cellulose is present in the form of thin microfibrils, about 5 nanometers in thickness and indefinite length. The cellulose microfibril is made up of many parallel chains of 1,4--glucan, which is a linear polymer of glucose molecules linked end-toend through the carbon atoms numbered 1 and 4. These chains form a crystalline ribbon that makes cellulose very strong and relatively inert and indigestible.

Hemicellulose refers to various polysaccharides that are tightly associated with the surface of the cellulose microfibril. They are chemically similar to cellulose, except they contain short side branches or kinks that prevent close packing into a microfibril. The backbone of hemicelluloses is typically made up of long chains of glucose or xylose residues linked end to end, often ornamented with short side chains. The most abundant hemicelluloses are xyloglucans and xylans. By adhering to the surface of cellulose microfibrils, hemicelluloses prevent direct contact between microfibrils, but may link them together in a cohesive network.

Pectin constitutes the third class of wall polysaccharide. It forms a gellike phase in between the cellulose microfibrils. Unlike cellulose and hemicellulose, pectin may be solubilized with relatively mild treatments such as boiling water or mildly acidic solutions. Pectin includes relatively simple polysaccharides such as polygalacturonic acid, a long chain of the acidic sugar galacturonic acid. This pectin readily forms gels in which calcium ions link adjacent chains together. Other pectin polysaccharides are more complex, with backbones made of alternating sugar residues such as galacturonic acid and rhamnose, and long side chains made up of other sugars such as arabinose or galactose. In the cell wall, pectins probably form very large aggregates of indefinite size.

In addition to these polysaccharides, primary cell walls also contain a small amount of structural proteins, such as hydroxyproline-rich glycoproteins and glycine-rich proteins.

Secondary cell walls are like primary walls except they contain more cellulose and less pectin than primary walls, and often contain hemicellulose polymers of differing composition. Many, but not all, secondary walls also contain lignin, which is a complex and irregular phenolic polymer that acts like epoxy to glue the wall polysaccharides together. Lignification greatly increases the mechanical strength of cell walls and makes them highly resistant to degradation.

Cell Wall Synthesis

The components of the cell wall are synthesized via distinct pathways and then assembled at the cell surface. Cellulose is synthesized by a large, cellulose synthase enzyme complex embedded in the plasma membrane. This complex is large enough to be seen in the electronic microscope and looks like a hexagonal array, called a particle rosette because of its appearance. The cellulose-synthesizing complex synthesizes thirty to forty glucan chains in parallel, using substrates from the cytoplasm. The growing chains are extruded to the outside of the cell via a pore in the complex and the chains then crystallize into a microfibril at the surface of the cell. In some algae the cellulose synthase complexes assume other configurations and this is associated with differing sizes and structures of the microfibril. Genes encoding plant cellulose synthases were first identified in the late 1990s, and the molecular details of how these proteins synthesize cellulose is still being studied.

The matrix polysaccharides (hemicellulose and pectin) are synthesized in the Golgi apparatus by enzymes called glycosyl transferases. They are transported to the cell wall via vesicles that fuse with the plasma membrane and dump their contents into the wall space. The matrix polymers and newly extruded cellulose microfibrils then assemble into an organized cell wall, probably by spontaneous self-organization between the different classes of polymers.

Wall enzymes may also aid assembly by forming cross-links between wall components. For example, some enzymes remove side chains from the hemicelluloses. This enables the hemicelluloses to stick more readily to cellulose. Other enzymes cut and link polysaccharides together, forming a more intricate weave of matrix polysaccharides.

Economic Importance of the Cell Wall

The cell wall is unmatched in the diversity and versatility of its economic uses. Lumber, charcoal, and other wood products are obvious examples. Textiles such as cotton and linen are derived from the walls of unusually long and strong fiber cells. Paper is likewise a product of long fiber cell walls that are extracted, beaten, and dried as a uniform sheet. Cellulose can be dissolved and regenerated as a manmade fiber called rayon or in sheets called cellophane. Chemically modified cellulose is used to make plastics, membranes, coatings, adhesives, and thickeners found in a vast array of products, from photographic film to paint, nail polish to explosives. In agriculture, cell walls are important as animal fodder, whereas in the human diet, cell walls are important as dietary fiber or roughage. Pectin is used as a gelling agent in jellies, yogurt, low-fat margarines, and other foods, while powered cellulose is similarly used as a thickener in foods and as an inert filler in medicinal tablets.

see also Anatomy of Plants; Carbohydrates; Cells; Cellulose.

Daniel J. Cosgrove

Bibliography

Brett, C. T., and K. Waldron. Physiology and Biochemistry of Plant Cell Walls, 2nd ed. London: Chapman and Hall, 1996.

Cosgrove, Daniel J. "Cell Walls: Structure, Biogenesis, and Expansion." In Plant Physiology, 2nd ed. Lincoln Taiz and Eduardo Zeiger, eds. Sunderland, MA: Sinauer Associates, 1998.

Lapasin, Romano, and Sabrina Pricl. The Rheology of Industrial Polysaccharides: Theory and Applications. London: Blackie Academic & Professional, 1995.