You may remember that one of the ways plant cells differ from animal cells is that plant
cells have cell walls, and animal cells don’t.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’s constructed, and its characteristics. 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" are called
macrofibrils; form the framework
of the cell wall.
If you’ve ever had a bicycle, you 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 remains flexible. This is also
true of the cellulose in cell's walls. The walls are strong enough to provide structural
support—yet flexible enough to allow leaves to flutter in the breeze without
breaking.
Now imagine a shoebox, with string wrapped round and
round, almost, but not completely covering all surfaces. This is a rough approximation of
how the cellulose frameworks surrounds the cell's contents. You can imagine 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, like me, 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. Cells with
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.
