A cell wall is present in organisms in the kingdoms Archaebacteria, Eubacteria, Protista, Fungi, and Plantae (bacteria, protists, fungi, and plants). Animals are the only organisms that do not have a cell wall. (For more about the classification of organisms, see the chapter “Basics of Biology.”)

In fact, the cell wall is one of the features of all the other organisms’ cells that distinguish them from animal cells. For example, plant walls protect the plant cell; they also help maintain its shape and consist mainly of cellulose in a matrix of protein and sugar polymers. The cell walls in prokaryotes (for example, bacteria) also define the cell’s shape and give rigidity to the cell. Though unlike plant cell walls, bacterial cell walls consist mainly of peptidoglycans—(polysaccharide chains [amino sugars] cross-linked by small peptides)—and not cellulose.