By Cynthia Ruscitto
Updated Aug 30, 2022
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Taxonomy classifies living organisms into categories based on shared traits. The Linnaean system, devised by Swedish naturalist Carolus Linnaeus, divides life into seven major groups, one of which is the kingdom. Today we recognize six kingdoms—Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia—each distinguished, in part, by their cell wall characteristics. As the outermost structural layer, a cell wall shapes the cell, maintains osmotic balance, and often determines ecological niche.
Both domains of bacteria possess semi‑rigid walls, but their chemistry diverges. Eubacteria (or “true” bacteria) feature peptidoglycan—a mesh of sugars and amino acids that safeguards against lysis in hypotonic environments. Mycoplasmas, a unique eubacterial subgroup, lack a wall entirely, rendering them highly susceptible to osmotic pressure.
Archaebacteria thrive in extreme habitats such as hot springs and hydrothermal vents. Their walls are constructed from proteins or the polymer pseudomurein, which shares functional similarities with peptidoglycan but differs in composition and synthesis.
Protists encompass diverse single‑cell eukaryotes that are neither bacteria, fungi, plants, nor animals. Protozoa (e.g., amoebae, paramecia) are animal‑like and lack walls, whereas algal protists (e.g., diatoms, green algae) possess walls rich in cellulose microfibrils, often interwoven with silica, calcium carbonate, or polysaccharides. Fungus‑like protists such as water molds and slime molds display variable wall presence: water molds have cellulose‑glycan walls, while slime molds exhibit a cellulosic wall only during specific developmental stages.
Fungi, predominantly multicellular terrestrial organisms, rely on chitin—a β‑1,4‑linked N‑acetylglucosamine polymer—for their walls. Chitin provides rigidity and resilience, akin to the exoskeletons of crustaceans and insects. Yeasts and molds also contain chitin, though in differing proportions.
Plant cells are distinguished by a robust cellulose‑based wall that restricts turgor expansion and facilitates directional growth. Interwoven with cellulose are lignin, which confers mechanical strength, and suberin or cutin waxes that protect against desiccation. In contrast, animal cells lack a cell wall entirely, relying instead on an adaptable plasma membrane for shape and mechanical support.
