Fig. 2.

Schematic representations of the extracellular matrix and cell walls of animal, plant, and fungal cells. A The animal extracellular matrix is less densely packed than the plant and fungal cell walls and composed of trimeric collagen helices and proteoglycans. These constituents are linked to fibronectin dimers, which are in turn associated with plasma membrane-localized integrin dimers. Gel-forming polysaccharides fill the gaps between collagen and proteoglycans. B Primary plant cell wall composition. Primary plant cell walls are composed of densely packed cellulose fibrils (organized in bundles), hemicellulose and pectin. Cellulose molecules are synthesized by plasma membrane-localized cellulose synthase rosette complexes that move along cytoplasmic microtubule tracks. Few cell wall proteins are present in the mainly carbohydrate polymer-based meshwork. Secondary cell walls contain in addition lignin (not shown). C Fungal cell walls are composed of a densely packed chitin layer, typically localized below a thicker, densely packed glucan layer, primarily composed of β-1,3 glucans. Chitin- and β-1,3 glucan synthesizing enzymes reside in the plasma membrane. Mannose-rich glycoproteins (mannoproteins), which are in part attached to the outer leaflet of the plasma membrane by glycosylphosphatidylinositol (GPI) membrane anchors, are embedded into the glucan layer. The N-acetyl-glucosamine building blocks of chitin enable more inter-chain hydrogen bonds than the glucose subunits of cellulose in the plant cell wall, yielding an overall higher rigidity of the fungal cell wall. Note that the three schemes represent simplified prototypical arrangements. Figure created with BioRender.com