Unit two. The Living Cell
4.9. Outside the Plasma Membrane
Cell Walls Offer Protection and Support
Plants, fungi, and many protists cells share a characteristic with bacteria that is not shared with animal cells—that is, they have cell walls, which protect and support their cells. Eukaryotic cell walls are chemically and structurally different from bacterial cell walls. In plants, cell walls are composed of fibers of the polysaccharide cellulose, while in fungi they are composed of chitin. The primary walls of plant cells are laid down when the cell is still growing. These are the thinner, outer walls of the cells shown below in figure 4.18. Between the walls of adjacent cells is a sticky substance called the middle lamella, which glues the cells together. Some plant cells produce strong secondary walls, which are deposited inside the primary walls. As you can see in the photo, the secondary cell walls are very thick compared to the primary walls and therefore are not deposited until the cell has finished increasing in size.
Figure 4.18. Cell walls in plants.
Plant cell walls are thick, strong, and rigid. Primary cell walls are laid down when the cell is young. Thicker secondary cell walls may be added later when the cell is fully grown. The middle lamella lies between the walls of adjacent cells and glues the cells together.
An Extracellular Matrix Surrounds Animal Cells
As we discussed, many types of eukaryotic cells possess a cell wall exterior to the plasma membrane. The wall acts to protect the cell, maintain its shape, and prevent excessive water uptake. Animal cells are the great exception, lacking the cell walls that encase the cells of plants, fungi, and most protists. Animal cells secrete an elaborate mixture of glycoproteins (proteins with short chains of sugars attached to them) into the space around them, forming the extracellular matrix (ECM), which performs a function different than cell walls.
The fibrous protein collagen, the same protein in cartilage and ligaments, is abundant in the ECM. Figure 4.19 shows how these fibers of collagen and another fibrous protein, elastin, are embedded within a complex web of other glycoproteins called proteoglycans, which form a protective layer over the cell surface.
Figure 4.19. The extracellular matrix.
Animal cells are surrounded by an extracellular matrix (ECM) composed of various glycoproteins. The ECM carries out a variety of functions that influence cell behavior, including cell migration, gene expression, and the coordination of signaling between cells.
The ECM is attached to the plasma membrane by a third kind of glycoprotein, fibronectin. As you can see in the figure, fibronectin molecules bind not only to ECM glycoproteins but also to proteins called integrins, which are an integral part of the plasma membrane. Integrins extend into the cytoplasm, where they are attached to the microfilaments of the cyto- skeleton. Linking ECM and cytoskeleton, integrins allow the ECM to influence cell behavior in important ways, altering gene expression and cell migration patterns by a combination of mechanical and chemical signaling pathways. In this way, the ECM can help coordinate the behavior of all the cells in a particular tissue.
Key Learning Outcome 4.9. Plant and protist cells encase themselves within a strong cell wall. In animal cells, which lack a cell wall, the cytoskeleton is linked by integrin proteins to a web of glycoproteins called the extracellular matrix.