The plant cell wall, an extracellular matrix that is mainly composed of polysaccharides and is about 0.2 µm thick, completely coats the outside of the plant cell’s plasma membrane. This structure serves some of the same functions as the ECM produced by animal cells, even though the two structures are composed of entirely different macromolecules and have a different organization. About 1000 genes in the plant Arabidopsis, a small flowering plant also called “thale cress” (see Chapters 1 and 8), are devoted to the synthesis and functioning of its cell wall, including approximately 414 glycosyltransferase genes and more than 316 glycosyl hydrolase genes. Similar to animal ECMs, the plant cell wall organizes cells into tissues, signals a plant cell to grow and divide, and controls the shapes of plant organs. It is a dynamic structure that plays important roles in controlling the differentiation of plant cells during embryogenesis and growth, and it provides a barrier to protect against pathogen infection. Just as the ECM helps define the shapes of animal cells, the cell wall defines the shapes of plant cells. When the cell wall is digested away from plant cells by hydrolytic enzymes, spherical cells enclosed by a plasma membrane are left.

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Because a major function of the plant cell wall is to withstand the turgor pressure of the cell (between 14.5 and 435 pounds per square inch; see Chapter 11), the cell wall is built for lateral strength. It is arranged into layers of cellulose microfibrils: bundles of 30–36 parallel chains of extensively hydrogen-bonded, long (as much as 7 µm or greater), linear polymers of glucose in β glycosidic linkages. The cellulose microfibrils are embedded in a matrix composed of pectin, a negatively charged polymer of D-galacturonic acid and other monosaccharides, and hemicellulose, a short, highly branched polymer of several five- and six-carbon monosaccharides. The mechanical strength of the cell wall depends on cross-linking of the microfibrils by hemicellulose chains (Figure 20-41b, c). The layers of microfibrils prevent the cell wall from stretching laterally. Cellulose microfibrils are synthesized on the exoplasmic face of the plasma membrane from UDP-glucose and ADP-glucose formed in the cytosol. The polymerizing enzyme, called cellulose synthase, moves within the plane of the plasma membrane along tracks of intracellular microtubules as cellulose is formed, providing a distinctive mechanism for intracellular-extracellular communication and ensuring that the cellulose microfibrils are oriented properly to permit cell-wall, and thus whole-cell, growth.

Unlike cellulose, pectin and hemicellulose are synthesized in the Golgi complex and transported to the cell surface, where they form an interlinked network that helps bind the walls of adjacent cells to one another and cushions them. When purified, pectin binds water and forms a gel in the presence of Ca²⁺ and borate ions—hence the use of pectins in many processed foods. As much as 15 percent of the cell wall may be composed of extensin, a glycoprotein that contains abundant hydroxyproline and serine. Most of the hydroxyproline residues are linked to short chains of arabinose (a five-carbon monosaccharide), and the serine residues are linked to galactose. Carbohydrate accounts for about 65 percent of extensin by weight, and its protein backbone forms an extended rodlike helix with the hydroxyl or O-linked carbohydrates protruding outward. Lignin—a complex, insoluble polymer of phenolic residues—associates with cellulose and is a strengthening material. Like cartilage proteoglycans, lignin resists compression forces.

The cell wall is a selective filter whose permeability is controlled largely by pectins. Whereas water and ions diffuse freely across cell walls, the diffusion of large molecules, including proteins larger than 20 kDa, is limited. This limitation may explain why many plant hormones are small, water-soluble molecules, which can diffuse across the cell wall and interact with receptors in the plasma membrane of plant cells.