Plant cell walls are complex structures composed mostly of lignocellulose—the most abundant organic material on Earth—which is a matrix of cross-linked polysaccharide networks, glycosylated proteins, and lignin. This matrix has three main components: cellulose (38–50%), hemicellulose (17–32%), and lignin (15–30%).
Cellulose is a polysaccharide consisting of a linear chain of several hundred to more than 10,000 d-glucose units linked by β-1,4 bonds. This bonding motif differs from the α-1,4 glucose linkage of starch, such as cornstarch that comes from corn kernels.
This structural difference proves to be quite significant. Cellulose chains are linear and somewhat rigid, but starch takes on a coiled chain structure. That makes the cellulose chains amenable to forming numerous hydrogen bonds, which, unlike starch, leads the cellulose chains to assemble into cablelike bundles of crystalline fibrils that have high tensile strength and are resistant to hydrolysis to glucose.
Hemicellulose is also a polysaccharide, but it is typically made up of chains of xylose interspersed with side chains containing arabinose, galactose, mannose, glucose, acetyl, and other sugar groups, depending on plant type. Hemicellulose contains 500 to 3,000 sugar units and includes a small amount of pectin, another polysaccharide, with which it forms a cross-linked network.
Lignin is a cross-linked macromolecule composed of three types of substituted phenols (phenylpropanoids). It fills the spaces in the cell wall between cellulose, hemicellulose, and pectin and is covalently linked to hemicellulose. Lignin resembles a kind of phenol-formaldehyde resin that acts like glue to hold the lignocellulose matrix together. Lignin helps provide additional strength to cell walls and resistance to insects and diseases.