The paper we write on, the cotton fibers in the clothes we wear, the wood in the chairs we sit on are, in fact, the extracellular matrix of plants. In plants, the extracellular matrix forms the cell wall, and the main component of the plant cell wall is the polysaccharide cellulose (Chapters 2 and 5). Its presence in the cell wall of every plant makes cellulose the most widespread organic macromolecule on Earth.

The plant cell wall represents possibly one of the most complex examples of an extracellular matrix. It is certainly one of the most diverse in the functions it performs. Cell walls maintain the shape and turgor pressure of plant cells and act as a barrier that prevents foreign materials and pathogens from reaching the plasma membrane. In many plants, cell walls collectively serve as a skeletal support structure for the entire plant.

The plant cell wall is composed of as many as three layers: the outermost middle lamella, the primary cell wall, and the secondary cell wall, located closest to the plasma membrane (Fig. 10.15). The middle lamella is synthesized first, during the late stages of cell division. It is composed of a gluelike complex carbohydrate, and it is the main mechanism by which plant cells adhere to one another. The primary cell wall is formed next and consists mainly of cellulose, but it also contains a number of other molecules, including pectin. The primary cell wall is laid down while the cells are still growing. It is assembled by enzymes on the surface of the cell and remains thin and flexible. Once cell growth has stopped, the secondary cell wall is constructed in many, but not all, plant cells. It also is made largely of cellulose but in addition contains a substance called lignin. Lignin hardens the cell wall and makes it water resistant. In woody plants, the cell wall can be up to 25% lignin. The rigid secondary cell wall permits woody plants to grow to tremendous heights. Giant sequoia trees grow to more than 300 feet and are supported entirely by the lignin-reinforced cellulose fibers of the interconnected cell walls.

As a plant cell grows, additional cell wall components must be synthesized to expand the area of the wall. Unlike the extracellular matrix components that are secreted by animal cells, the cellulose polymer is assembled outside the cell, on the extracellular surface of the plasma membrane. Both the glucose monomers that form the polymer and the enzymes that attach them are delivered to the cell surface by arrays of microtubules. Here is yet another example of how the cytoskeleton plays an indispensable role in regulating the shape of a cell.

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