3.20: The cell wall provides additional protection and support for plant cells.

Until this point, we’ve discussed organelles and structures that are common to both plants and animals. Now we’re going to look at several structures that are not found in all eukaryotic cells. Because plants are rooted in the ground, unable to move, they have several special needs beyond those of animals. In particular, they can’t outrun predators or outmaneuver their competitors to get more food. If they need more light, they have few options beyond simply growing larger or taller. And to resist plant-eating animals, they must simply reduce their edibility.

One structure that helps plants achieve these goals is the cell wall, which surrounds the plasma membrane. The cell wall is made largely from long fibers of a polysaccharide called cellulose. Note that although animal cells do not have cell walls, some non-plants such as archaea, bacteria, protists, and fungi also have cell walls; their cell walls are made of polysaccharides other than cellulose.

In plants, cell wall production can be a multistage process. Initially, when the cell is still growing, a primary cell wall is laid down, along with some glue that helps adjacent cells adhere to each other. Sometime later in life, a secondary cell wall is laid down. This secondary cell wall usually contains a complex molecule called lignin, which gives the wall much of its strength and rigidity. Oddly, this strength remains even after the plant cell dies—hence wood’s great value to humans as a construction material, among other things.

The cell wall is nearly 100 times thicker than the plasma membrane. The tremendous structural strength it confers on plant cells enables some plants to grow several hundred feet tall and provides some protection from insects and other animals that might eat the plant. Cell walls also help to make plants more water-tight—an important feature in organisms that cannot move out of the hot sun to reduce water loss through evaporation (FIGURE 3-39).

Surprisingly, despite its great strength, the cell wall does not completely seal off plant cells from one another. Rather, it is porous, allowing water and solutes to reach the plasma membrane. Additionally, the cells in most plants have anywhere from 1,000 to 100,000 microscopic tube-like channels, called plasmodesmata (sing. plasmodesma) (see Figure 3-39), connecting the cells to each other and enabling communication and transport between them. In fact, because so many of the cells are connected to one another, sharing cytoplasm and other molecules, some biologists have wondered whether we should consider a plant as just one big cell. How would you respond to that idea?