Cellulose

Cellulose is a long-chain polysaccharide of glucose units — the most abundant organic polymer on Earth. It is the primary structural component of plant cell walls, providing the rigidity and tensile strength that allow plants to grow upright. Hemicellulose is a related family of branched polysaccharides that cross-links cellulose fibers and fills the matrix between them. Together with lignin, cellulose and hemicellulose form lignocellulose — the composite material that constitutes wood, straw, leaf litter, and virtually all dead plant matter.

Cellulose matters for mycology because it is a primary food source for saprotrophic fungi. Breaking cellulose down into its component glucose molecules requires cellulase enzymes — a suite of proteins that hydrolyze the bonds between glucose units. Many fungi produce cellulases, but the challenge of accessing cellulose in nature is that it is typically embedded in a matrix of hemicellulose and lignin. Lignin physically shields cellulose from enzymatic attack. This is why lignin degradation is so ecologically important: until the lignin barrier is breached, the cellulose energy locked inside wood remains inaccessible. White-rot fungi solve this by degrading lignin first (using lignin peroxidase, manganese peroxidase, and laccase), then attacking the exposed cellulose. Brown-rot fungi use a different strategy — generating hydroxyl radicals through Fenton chemistry to attack cellulose directly, partially disrupting the lignocellulose matrix without fully degrading the lignin (see extracellular digestion).

The distinction between cellulose and chitin marks a fundamental boundary between kingdoms. Plant cell walls are built from cellulose. Fungal cell walls are built from chitin. Both are structural polysaccharides — long chains of sugar units — but their chemistry is different (cellulose is poly-glucose; chitin is poly-N-acetylglucosamine), and this difference reflects the deep evolutionary divergence between plants and fungi. Fungi do not make cellulose; they eat it.

Related terms

• Lignin — the polymer that protects cellulose in wood, forming the lignocellulose matrix
• Chitin — the fungal cell wall polymer, structurally analogous but chemically distinct
• Extracellular Digestion — the enzymatic process that breaks cellulose down
• Saprotroph — organisms that feed on cellulose-containing dead matter
• Substrate — the cellulose-rich materials fungi colonize
• Decomposition as Relation — the ecological process of cellulose breakdown