The cell wall is a box-like layer of material synthesized by plant cells outside the plasma membrane. Hooke saw the cell walls of cork cells when he first looked through his microscope. Cellulose fibers form the basic skeletal foundations of cell walls. Each molecules of cellulose is a polymer of glucose monomers about 8,000 to 12,000 units long. These cellulose polymers are aggregated into bundles of about 30 to 50 chains each, held together by hydrogen bonds. The cellulose fibers are embedded in a matrix of other polysaccharides and can build up to sizes easily seen under the electron microscope.
Cell walls give plants rigidity and shape, and their chemical composition ensures both flexibility and mechanical strength. They also guard cells against the problems of osmosis. If a plant cell absorbs water, its contents swell just like an animal cell does. However, the plant cell will swell only until the cell membrane is pushed up against the tough cell wall, at which point the cell can expand no further and no more water can be taken up by osmosis.
A simple experiment demonstrates this. Place small pieces of Elodea (a pond weed) leaf in salty water for an hour, and then observe the contents of the cells under a microscope. The contents of each cell should have shrunk back away from the cell wall as water was drawn out of the cell by osmosis. Wash away the salt and watch as water is again absorbed by the plant cells. The contents expand, but, unlike the animal cells the plant cells never burst.