Biologists divide living organisms into kingdoms. Two of these kingdoms share the distinguishing characteristic of having walled cells while a third kingdom has a mix of organisms with and without cell walls. A cell wall is a rigid structure composed of polysaccharides. Cell walls give plants, fungi and some species of protists a firm, supported shape without the need for skeletal structures.
Robert Hooke coined the term "cell" in the latter half of the 1600s. The tiny chambers he saw in the thin sample of cork were actually the cell walls that remained after the other material in the cells had decayed. Plant cells' primary cell walls contain cellulose, an insoluble polysaccharide that imparts structure to the cell. More specialized plant cells may contain lignin, a hard and durable polysaccharide, in a secondary cell wall that develops as the cell matures. Lignin is responsible for wood's strength and rigidity. Both cellulose and lignin played an important role in helping plants colonize land masses early in Earth's history, as their rigidity formed the basis of plants' vascular systems and other specialized tissues.
Although botanists and taxonomists class some algae species as plants, other types of algae are actually colonies of single-celled organisms that properly fall into Kingdom Protista. Scientists assign diatoms to a phylum of algae. All forms of algae, whether plant or protist, have cell walls; they differ in these walls' structure, though. Tiny diatoms contain silica in their cell walls, giving them durable cell walls that leave hardy fossils. Diatomaceous earth, a mildly abrasive powder that doubles as a nontoxic insecticide, derives its effectiveness from the skeletal remains of diatoms' cell walls. Colonial algae forms have cell walls of cellulose and various glycoproteins. Taxonomists use these varied glycoprotein components to categorize algae species.
Once classed as plants, fungi now have their own kingdom. These organisms maintain their structure with cell walls of chitin, a polysaccharide also found in insects' and crustaceans' exoskeletons. Mushrooms look and feel distinctly different from vascular plants in part because of their unique chitinous cell walls. A combination pizza laden with mushrooms and green peppers provides a macroscopic example of how different microscopic structures affect rigidity; sample each topping and see how different the textures are from one another.
Almost all bacteria have cell walls, but these walls have very different structural components. Peptidoglycans, a special class of glycoprotein, make up the majority of bacterial cell walls. Microbiologists and medical personnel classify bacteria based on how their peptidoglycan cell walls absorb dye for the microscope. Bacteria with a thick peptidoglycan layer absorb the dye readily; these are Gram-positive bacteria. Gram-negative bacteria have a double-layered cell wall with various carbohydrates and lipids in their outer layer. The classification system gets its name from the researcher who first stained bacteria to see them more clearly and noted how different strains accepted or rejected the dye.
- Rice University: Five Kingdom Classification System; David R. Caprette; Aug. 19, 2007
- Florida State University: Molecular Expressions; Plant Cell Wall; Michael W. Davidson; Jan. 7, 2005
- Michigan Tech University: The Algal Cell Wall and Extracellular Polymers; Michael R. Gretz; June 2008
- Washington State University: Bacterial Architecture: The Virtual Bacterium; Dr. R. E. Hurlbert; 1999
- University of California at Berkeley: More On Bacterial Morphology; Ben Waggoner; Oct. 15, 1996