Key Concepts Of Section

Adhesive Interactions and Nonepithelial Cells

■ Many nonepithelial cells have integrin-containing aggregates (e.g., focal adhesions, 3D adhesions, podosomes) that physically and functionally connect cells to the extracellular matrix and facilitate inside-out and outside-in signaling.

■ Integrins exist in two conformations that differ in the affinity for ligands and interactions with cytosolic adapter proteins (see Figure 6-28).

■ Dystroglycan, an adhesion receptor expressed by muscle cells, forms a large complex with dystrophin, other adapter proteins, and signaling molecules (see Figure 6-29). This complex links the actin cytoskeleton to the surrounding matrix, providing mechanical stability to muscle. Mutations in various components of this complex cause different types of muscular dystrophy.

■ Neural cell-adhesion molecules (CAMs), which belong to the immunoglobulin (Ig) family of CAMs, mediate Ca2+-independent cell-cell adhesion, predominantly in neural tissue and muscle.

■ The combinatorial and sequential interaction of several types of CAMs (e.g., selectins, integrins, and ICAMs) is critical for the specific and tight adhesion of different types of leukocytes to endothelial cells in response to local signals induced by infection or inflammation (see Figure 6-30).

■ Gap junctions are constructed of multiple copies of con-nexin proteins, assembled into a transmembrane channel that interconnects the cytoplasm of two adjacent cells (see Figure 6-32). Small molecules and ions can pass through gap junctions, permitting metabolic and electrical coupling of adjacent cells.

Plant Tissues

We turn now to the assembly of plant cells into tissues. The overall structural organization of plants is generally simpler than that of animals. For instance, plants have only four broad types of cells, which in mature plants form four basic classes of tissue: dermal tissue interacts with the environment; vascular tissue transports water and dissolved substances (e.g., sugars, ions); spacefilling ground tissue constitutes the major sites of metabolism; and sporogenous tissue forms the reproductive organs. Plant tissues are organized into just four main organ systems: stems have support and transport functions; roots provide anchorage and absorb and store nutrients; leaves are the sites of photosynthesis; and flowers enclose the reproductive structures. Thus at the cell, tissue, and organ levels, plants are generally less complex than most animals.

Moreover, unlike animals, plants do not replace or repair old or damaged cells or tissues; they simply grow new organs. Most importantly for this chapter and in contrast with animals, few cells in plants directly contact one another through molecules incorporated into their plasma membranes. Instead, plant cells are typically surrounded by a rigid cell wall that contacts the cell walls of adjacent cells (Figure 6-33). Also in contrast with animal cells, a plant cell rarely changes its position in the organism relative to other cells. These features of plants and their organization have determined the distinctive molecular mechanisms by which their cells are incorporated into tissues. I

▲ FIGURE 6-33 Schematic representation of the cell wall of an onion. Cellulose and hemicellulose are arranged into at least three layers in a matrix of pectin polymers. The size of the polymers and their separations are drawn to scale. To simplify the diagram, most of the hemicellulose cross-links and other matrix constituents (e.g., extensin, lignin) are not shown. [Adapted from M. McCann and K. R. Roberts, 1991, in C. Lloyd, ed., The Cytoskeletal Basis of Plant Growth and Form, Academic Press, p. 126.]

The Plant Cell Wall Is a Laminate of Cellulose Fibrils in a Matrix of Glycoproteins

The plant cell wall is «0.2 |im thick and completely coats the outside of the plant cell's plasma membrane. This structure serves some of the same functions as those of the extracellular matrix produced by animal cells, even though the two structures are composed of entirely different macromolecules and have a different organization. Like the extracellular matrix, the plant cell wall connects cells into tissues, signals a plant cell to grow and divide, and controls the shape of plant organs. Just as the extracellular matrix 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. In the past, the plant cell wall was viewed as an inanimate rigid box, but it is now recognized as a dynamic structure that plays important roles in controlling the differentiation of plant cells during embryogenesis and growth.

Because a major function of a plant cell wall is to withstand the osmotic turgor pressure of the cell, the cell wall is built for lateral strength. It is arranged into layers of cellulose microfibrils—bundles of long, linear, extensively hydrogen-bonded polymers of glucose in p glycosidic linkages. The cellulose microfibrils are embedded in a matrix composed of pectin, a polymer of d-galacturonic acid and other mono-saccharides, 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 (see Figure 6-33). 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 as cellulose is formed, in directions determined by the underlying microtubule cytoskeleton.

Unlike cellulose, pectin and hemicellulose are synthesized in the Golgi apparatus 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 hy-droxyproline 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 on the matrix.

The cell wall is a selective filter whose permeability is controlled largely by pectins in the wall matrix. 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 account for 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.

Loosening of the Cell Wall Permits Elongation of Plant Cells

Because the cell wall surrounding a plant cell prevents the cell from expanding, its structure must be loosened when the cell grows. The amount, type, and direction of plant cell growth are regulated by small-molecule hormones (e.g., in-doleacetic acid) called auxins. The auxin-induced weakening of the cell wall permits the expansion of the intracellular vacuole by uptake of water, leading to elongation of the cell. We can grasp the magnitude of this phenomenon by considering that, if all cells in a redwood tree were reduced to the size of a typical liver cell, the tree would have a maximum height of only 1 meter.

The cell wall undergoes its greatest changes at the meristem of a root or shoot tip. These sites are where cells divide and expand. Young meristematic cells are connected by thin primary cell walls, which can be loosened and stretched to allow subsequent cell elongation. After cell elongation ceases, the cell wall is generally thickened, either by the secretion of additional macromolecules into the primary wall or, more usually, by the formation of a secondary cell wall composed of several layers. Most of the cell eventually degenerates, leaving only the cell wall in mature tissues such as the xylem—the tubes that conduct salts and water from the roots through the stems to the leaves (see Figure 8-45). The unique properties of wood and of plant fibers such as cotton are due to the molecular properties of the cell walls in the tissues of origin.

Plasmodesmata Directly Connect the Cytosols of Adjacent Cells in Higher Plants

Plant cells can communicate directly through specialized cell-cell junctions called plasmodesmata, which extend through the cell wall. Like gap junctions, plasmodesmata are open channels that connect the cytosol of a cell with that of an adjacent cell. The diameter of the cytosol-filled channel is about 30-60 nm, and plasmodesmata can traverse cell walls as much as 90 nm thick. The density of plasmodesmata varies depending on the plant and cell type, and even the smallest meristematic cells have more than 1000 interconnections with their neighbors.

Molecules smaller than about 1000 Da, including a variety of metabolic and signaling compounds, generally can diffuse through plasmodesmata. However, the size of the channel through which molecules pass is highly regulated. In some circumstances, the channel is clamped shut; in others, it is dilated sufficiently to permit the passage of molecules larger than 10,000 Da. Among the factors that affect the permeability of plasmodesmata is the cytosolic Ca2 + concentration, with an increase in cytosolic Ca2+ reversibly inhibiting movement of molecules through these structures.

Although plasmodesmata and gap junctions resemble each other functionally, their structures differ in two significant ways (Figure 6-34). The plasma membranes of the adjacent plant cells merge to form a continuous channel, the annulus, at each plasmodesma, whereas the membranes of cells at a gap junction are not continuous with each other. In addition, an extension of the endoplas-mic reticulum called a desmotubule passes through the annulus, which connects the cytosols of adjacent plant cells. Many types of molecules spread from cell to cell through plasmodesmata, including proteins, nucleic acids, metabolic products, and plant viruses. Soluble molecules pass through the cytosolic annulus, whereas membrane-bound molecules can pass from cell to cell through the desmotubule.

Plasma membrane

Cell wall

Endoplasmic reticulum

Cell 1

Cell 2

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