CellWalls and the ExtracellularMatrix

Although cell boundaries are defined by the plasma membrane, many cells are surrounded by an insoluble array of secreted macromolecules. Cells of bacteria, fungi, algae, and higher plants are surrounded by rigid cellwalls, which are an integral part of the cell. Although not encased in cellwalls, animal cells in tissues are closely associated with an extracellularmatrix composed of proteins and polysaccharides. The extracellularmatrix not only provides structural support to cells and tissues, but also plays important roles in regulating the behavior of cells in multicellular organisms.

Bacterial CellWalls

The rigid cellwalls of bacteria determine cell shape and prevent the cell from bursting as a result of osmotic pressure. The structure of their cellwalls divides bacteria into two broad classes that can be distinguished by a staining procedure known as the Gram stain, developed by Christian Gram in 1884 (Figure 12.44).
Figure 12.44. Bacterial cell walls The plasma membrane of Gram-negative bacteria is surrounded by a thin cell wall beneath the outer membrane. Gram-positive bacteria lack outer membranes and have thick cell walls.

As described earlier in this chapter, Gram-negative bacteria (such as E. coli) have a dual membrane system, in which the plasma membrane is surrounded by a permeable outer membrane. These bacteria have thin cellwalls located between their inner and outer membranes. In contrast, Gram-positive bacteria (such as the common human pathogen Staphylococcus aureus) have only a single plasma membrane, which is surrounded by a much thicker cell wall.

Despite these structural differences, the principal component of the cellwalls of both Gram-positive and Gram-negative bacteria is a peptidoglycan (Figure 12.45)
Figure 12.45. The peptidoglycan of E.coli Polysaccharide chains consist of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues joined by (14) glycosidic bonds. Parallel chains are crosslinked by tetrapeptides attached to the NAM residues. The amino acids forming the tetrapeptides vary in different species of bacteria.

consisting of linear polysaccharide chains crosslinked by short peptides. Because of this crosslinked structure, the peptidoglycan forms a strong covalent shell around the entire bacterial cell. Interestingly, the unique structure of their cellwalls also makes bacteria vulnerable to some antibiotics. Penicillin, for example, inhibits the enzyme responsible for forming cross-links between different strands of the peptidoglycan, thereby interfering with cell wall synthesis and blocking bacterial growth.

Plant CellWalls

In contrast to bacteria, the cellwalls of eukaryotes (including fungi, algae, and higher plants) are composed principally of polysaccharides (Figure 12.46).
Figure 12.46. Polysaccharides of fungal and plant cell walls (A) Chitin (the principal polysaccharide of fungal cell walls) is a linear polymer of N-acetylglucosamine residues, whereas cellulose is a linear polymer of glucose. The carbohydrate monomers are joined by (14) linkages, allowing the polysaccharides to form long, straight chains. (B) Parallel chains of cellulose associate to form microfibrils.

The basic structural polysaccharide of fungal cellwalls is chitin (a polymer of N-acetylglucosamine residues), which also forms the exoskeleton of arthropods (e.g., the shells of crabs). The cellwalls of most algae and higher plants are composed principally of cellulose, which is the single most abundant polymer on Earth. Cellulose is a linear polymer of glucose residues, often containing more than 10,000 glucose monomers. The glucose residues are joined by (14) linkages, which allow the polysaccharide to form long straight chains. Several dozen such chains then associate in parallel with one another to form cellulose microfibrils, which can extend for many micrometers in length.

Within the cell wall, cellulose microfibrils are embedded in a matrix consisting of proteins and two other types of polysaccharides: hemicelluloses and pectins (Figure 12.47).
Figure 12.47. Model of a plant cell wall (A) Structures of a representative hemicellulose (xyloglucan) and pectin (rhamnogalacturonan). Xyloglucan consists of a backbone of glucose (Glc) residues with side chains of xylose (Xyl), galactose (Gal), and fucose (Fuc). The backbone of rhamnogalacturonan contains galacturonic acid (GalA) and rhamnose (Rha) residues, to which numerous side chains are also attached. (B) Hemicelluloses bind to the surface of cellulose microfibrils, forming a fibrous network that is embedded in a gel-like matrix of pectins.

Hemicelluloses are highly branched polysaccharides that are hydrogen-bonded to the surface of cellulose microfibrils. This crosslinks the cellulose microfibrils into a network of tough, fibrous molecules, which is responsible for the mechanical strength of plant cellwalls. Pectins are branched polysaccharides containing a large number of negatively charged galacturonic acid residues. Because of these multiple negative charges, pectins bind positively charged ions (such as Ca2+) and trap water molecules to form gels. An illustration of their gel-forming properties is provided by the fact that jams and jellies are produced by the addition of pectins to fruit juice. In the cell wall, the pectins form a gel-like network that is interlocked with the crosslinked cellulose microfibrils. In addition, cellwalls contain a variety of glycoproteins that are incorporated into the matrix and are thought to provide further structural support.

Both the structure and function of cellwalls change as plant cells develop. The walls of growing plant cells (called primary cellwalls) are relatively thin and flexible, allowing the cell to expand in size. Once cells have ceased growth, they frequently lay down secondary cellwalls between the plasma membrane and the primary cell wall (Figure 12.48).
Figure 12.48. Primary and secondary cell walls Secondary cell walls are laid down between the primary cell wall and the plasma membrane. Secondary walls frequently consist of three layers, which differ in the orientation of their cellulose microfibrils

Such secondary cellwalls, which are both thicker and more rigid than primary walls, are particularly important in cell types responsible for conducting water and providing mechanical strength to the plant.

Primary and secondary cellwalls differ in composition as well as in thickness. Primary cellwalls contain approximately equal amounts of cellulose, hemicelluloses, and pectins. In contrast, the more rigid secondary walls generally lack pectin and contain 50 to 80% cellulose. Many secondary walls are further strengthened by lignin, a complex polymer of phenolic residues that is responsible for much of the strength and density of wood. The orientation of cellulose microfibrils also differs in primary and secondary cellwalls. The cellulose fibers of primary walls appear to be randomly arranged, whereas those of secondary walls are highly ordered (see Figure 12.48). Secondary walls are frequently laid down in layers in which the cellulose fibers differ in orientation, forming a laminated structure that greatly increases cell wall strength.

One of the critical functions of plant cellwalls is to prevent cell swelling as a result of osmotic pressure. In contrast to animal cells, plant cells do not maintain an osmotic balance between their cytosol and extracellular fluids. Consequently, osmotic pressure continually drives the flow of water into the cell. This water influx is tolerated by plant cells because their rigid cellwalls prevent swelling and bursting. Instead, an internal hydrostatic pressure (called turgor pressure) builds up within the cell, eventually equalizing the osmotic pressure and preventing the further influx of water.

Turgor pressure is responsible for much of the rigidity of plant tissues, as is readily apparent from examination of a dehydrated, wilted plant. In addition, turgor pressure provides the basis for a form of cell growth that is unique to plants. In particular, plant cells frequently expand by taking up water without synthesizing new cytoplasmic components (Figure 12.49).
Figure 12.49. Expansion of plant cells Turgor pressure drives the expansion of plant cells by the uptake of water, which is accumulated in a large central vacuole.

Cell expansion by this mechanism is signaled by plant hormones (auxins) that weaken a region of the cell wall, allowing turgor pressure to drive the expansion of the cell in that direction. As this occurs, the water that flows into the cell accumulates within a large central vacuole, so the cell expands without increasing the volume of its cytosol. Such expansion can result in a 10- to 100-fold increase in the size of plant cells during development.

As cells expand, new components of the cell wall are deposited outside the plasma membrane. Matrix components, including hemicelluloses and pectins, are synthesized in the Golgi apparatus and secreted. Cellulose, however, is synthesized by a plasma membrane enzyme complex (cellulose synthase). In expanding cells, the newly synthesized cellulose microfibrils are deposited at right angles to the direction of cell elongationan orientation that is thought to play an important role in determining the direction of further cell expansion (Figure 12.50).
Figure 12.50. Cellulose synthesis during cell elongation New cellulose microfibrils, synthesized by a plasma membrane enzyme complex (cellulose synthase), are laid down at right angles to the direction of cell elongation. The direction of cellulose synthesis is parallel to microtubules beneath the plasma membrane.

Interestingly, the cellulose microfibrils in elongating cellwalls are laid down in parallel to cortical microtubules underlying the plasma membrane. These microtubules appear to define the orientation of newly synthesized cellulose microfibrils, possibly by determining the direction of movement of the cellulose synthase complexes in the membrane. The cortical microtubules thus define the direction of cell wall growth, which in turn determines the direction of cell expansion and ultimately the shape of the entire plant.

The ExtracellularMatrix

Although animal cells are not surrounded by cellwalls, many of the cells in tissues of multicellular organisms are embedded in an extracellularmatrix consisting of secreted proteins and polysaccharides. The extracellularmatrix fills the spaces between cells and binds cells and tissues together. One type of extracellularmatrix is exemplified by the thin, sheetlike basal laminae, or basement membranes, upon which layers of epithelial cells rest (Figure 12.51).
Figure 12.51. Examples of extracellular matrix Sheets of epithelial cells rest on a thin layer of extracellular matrix called a basal lamina. Beneath the basal lamina is loose connective tissue, which consists largely of extracellular matrix secreted by fibroblasts. The extracellular matrix contains fibrous structural proteins embedded in a gel-like polysaccharide ground substance.

In addition to supporting sheets of epithelial cells, basal laminae surround muscle cells, adipose cells, and peripheral nerves. Extracellularmatrix, however, is most abundant in connective tissues. For example, the loose connective tissue beneath epithelial cell layers consists predominantly of an extracellularmatrix in which fibroblasts are distributed. Other types of connective tissue, such as bone, tendon, and cartilage, similarly consist largely of extracellularmatrix, which is principally responsible for their structure and function.

Extracellular matrices are composed of tough fibrous proteins embedded in a gel-like polysaccharide ground substancea design basically similar to that of plant cellwalls. In addition to fibrous structural proteins and polysaccharides, the extracellularmatrix contains adhesion proteins that link components of the matrix both to one another and to attached cells. The differences between the various types of extracellularmatrix result from variations on this general theme. For example, tendons contain a high proportion of fibrous proteins, whereas cartilage contains a high concentration of polysaccharides that form a firm compression-resistant gel. In bone, the extracellularmatrix is hardened by deposition of calcium phosphate crystals. The sheetlike structure of basal laminae also results from the utilization of matrix components that differ from those found in connective tissues.

The major structural protein of the extracellularmatrix is collagen, which is the single most abundant protein in animal tissues. The collagens are a large family of proteins, containing at least 19 different members. They are characterized by the formation of triple helices in which three polypeptide chains are wound tightly around one another in a ropelike structure (Figure 12.52).
Figure 12.52. Structure of collagen (A) Three polypeptide chains coil around one another in a characteristic triple helix structure. (B) The amino acid sequence of a collagen triple helix domain consists of Gly-X-Y repeats, in which X is frequently proline and Y is frequently hydroxyproline (Hyp).

The triple helix domains of the collagens consist of repeats of the amino acid sequence Gly-X-Y. A glycine (the smallest amino acid, with a side chain consisting only of a hydrogen) is required in every third position in order for the polypeptide chains to pack together close enough to form the collagen triple helix. Proline is frequently found in the X position and hydroxyproline in the Y position; because of their ring structure, these amino acids stabilize the helical conformations of the polypeptide chains. The unusual amino acid hydroxyproline is formed within the endoplasmic reticulum by modification of proline residues that have already been incorporated into collagen polypeptide chains (Figure 12.53).
Figure 12.53. Formation of hydroxyproline Prolyl hydroxylase converts proline residues in collagen to hydroxyproline

Lysine residues in collagen are also frequently converted to hydroxylysines. The hydroxyl groups of these modified amino acids are thought to stabilize the collagen triple helix by forming hydrogen bonds between polypeptide chains. These amino acids are rarely found in other proteins, although hydroxyproline is also common in some of the glycoproteins of plant cellwalls.

The most abundant type of collagen (type I collagen) is one of the fibril-forming collagens that are the basic structural components of connective tissues (Table 12.2).

Table 12.2. Representative Members of the Collagen Family

Collagen class / Types / Tissue distribution
Fibril-forming / I / Most connective tissues
II / Cartilage and vitreous humor
III / Extensible connective tissues (e.g., skin and lung)
V / Tissues containing collagen I
XI / Tissues containing collagen II
Fibril-associated / IX / Tissues containing collagen II
XII / Tissues containing collagen I
XIV / Tissues containing collagen I
XVI / Many tissues
Network-forming / IV / Basal laminae
Anchoring filaments / VII / Attachments of basal laminae to underlying connective tissue

The polypeptide chains of these collagens consist of approximately a thousand amino acids or 330 Gly-X-Y repeats. After being secreted from the cell, these collagens assemble into collagen fibrils in which the triple helical molecules are associated in regular staggered arrays (Figure 12.54).
Figure 12.54. Collagen fibrils (A) Collagen molecules assemble in a regular staggered array to form fibrils. The molecules overlap by one-fourth of their length, and there is a short gap between the N terminus of one molecule and the C terminus of the next. The assembly is strengthened by covalent cross-links between side chains of lysine or hydroxylysine residues, primarily at the ends of the molecules.

These fibrils do not form within the cell, because the fibril-forming collagens are synthesized as soluble precursors (procollagens) that contain nonhelical segments at both ends of the polypeptide chain. Procollagen is cleaved to collagen after its secretion, so the assembly of collagen into fibrils takes place only outside the cell. The association of collagen molecules in fibrils is further strengthened by the formation of covalent cross-links between the side chains of lysine and hydroxylysine residues. Frequently, the fibrils further associate with one another to form collagen fibers, which can be several micrometers in diameter.

Several other types of collagen do not form fibrils but play distinct roles in various kinds of extracellular matrices. In addition to the fibril-forming collagens, connective tissues contain fibril-associated collagens, which bind to the surface of collagen fibrils and link them both to one another and to other matrix components. Basal laminae form from a different type of collagen (type IV collagen), which is a network-forming collagen (Figure 12.55).
Figure 12.55. Type IV collagen (A) The Gly-X-Y repeat structure of type IV collagen (yellow) is interrupted by multiple nonhelical sequences (bars).

The Gly-X-Y repeats of these collagens are frequently interrupted by short nonhelical sequences. Because of these interruptions, the network-forming collagens are more flexible than the fibril-forming collagens. Consequently, they assemble into two-dimensional crosslinked networks instead of fibrils. Yet another type of collagen forms anchoring fibrils, which link some basal laminae to underlying connective tissues.

Connective tissues also contain elastic fibers, which are particularly abundant in organs that regularly stretch and then return to their original shape. The lungs, for example, stretch each time a breath is inhaled and return to their original shape with each exhalation. Elastic fibers are composed principally of a protein called elastin, which is crosslinked into a network by covalent bonds formed between the side chains of lysine residues (similar to those found in collagen). This network of crosslinked elastin chains behaves like a rubber band, stretching under tension and then snapping back when the tension is released.

The fibrous structural proteins of the extracellularmatrix are embedded in gels formed from polysaccharides called glycosaminoglycans, or GAGs, which consist of repeating units of disaccharides (Figure 12.56).
Figure 12.56. Major types of glycosaminoglycans Glycosaminoglycans consist of repeating disaccharide units. With the exception of hyaluronan, the sugars frequently contain sulfate. Heparan sulfate is similar to heparin except that it contains fewer sulfate groups.

One sugar of the disaccharide is either N-acetylglucosamine or N-acetylgalactosamine and the second is usually acidic (either glucuronic acid or iduronic acid). With the exception of hyaluronan, these sugars are modified by the addition of sulfate groups. Consequently, GAGs are highly negatively charged. Like the pectins of plant cellwalls, they bind positively charged ions and trap water molecules to form hydrated gels, thereby providing mechanical support to the extracellularmatrix.