From Seed to Seed:
Plant Science for K-8 Educators

The cell wall

You may remember that one of the ways in which plant cells differ from animal cells is that plant cells have cell walls, and animal cells do not.

Plant cell walls are the basis for many familiar products and materials. Let's take a look at what the cell wall is, how it is constructed, and what its characteristics are. Then we'll look at some of the ways humans make use of the unique features of cell walls.

When a plant cell is first formed, the walls are composed primarily of cellulose. A molecule of cellulose consists of many glucose molecules bound end to end to form a long chain. (You'll remember that glucose is the sugar formed during photosynthesis.) These long, thin cellulose molecules are united into long threads called microfibrils. The microfibrils are, in turn, wound together like strands in a cable. These "cables," called macrofibrils, form the framework of the cell wall.

The following analogy may be helpful in explaining to your students how micro- and macro-fibrils contribute to the strength of cell walls. If you've ever had a bicycle, you've probably had a lock consisting of a steel cable with loops at both ends. And if you ever lost your key to the lock, you may have tried to cut through that cable. Steel configured in this way, as a group of wires twisted together, is incredibly tough, yet it remains flexible. This is also true of the cellulose in cells' walls. The walls are strong enough to provide structural support-yet flexible enough to allow leaves to flutter in the breeze without breaking.

As a demonstration, wrap a shoe box with string, almost, but not completely, covering all surfaces. This is a rough approximation of how the cellulose framework surrounds the cell's contents. You will notice that there are spaces between those "strings," or cellulose strands. What lies in between those strands?

Primarily, the material filling these spaces is pectin, and pectin-like substances. Those of you who have made homemade jelly will recognize pectin as a substance that helps fruit syrup gel into jelly. And those of you who have been impatient and added a bit too much pectin will know how sticky and gluey it can be. Pectin also forms a thin, sticky layer between the walls of adjoining cells, binding the cells together. This layer is called the middle lamella.

The initial layer of cellulose is called the primary wall. Young cells and actively dividing cells have only primary walls, as do cells responsible for photosynthesis and other metabolic functions. There are times, however, when more rigid support is needed. For example, a tree trunk needs to stand up to the strongest wind with no more than a little swaying. Imagine if tree trunks "fluttered in the breeze"!

In cells that need more rigidity, the protoplasm-the living contents of the cell-deposits a second wall, layer by layer, inside the primary wall. These layers may consist of more cellulose, and sometimes other hardening substances such as lignin. Thick secondary walls are commonly found in specialized cells that conduct water or provide structural support. In these cells, the protoplasm often dies after the secondary wall has been laid down.

Now let's investigate which familiar materials are composed of cellulose-or cell walls.