Lignin

Lignin is a complex, cross-linked polymer that stiffens the cell walls of land plants and makes wood hard and waterproof. After cellulose, it is the most abundant organic polymer on Earth, and it makes up roughly 20 to 30 percent of the dry mass of wood.

Lignin forms when plant cells link together three small alcohols, called monolignols, into an irregular three-dimensional network. Unlike proteins or DNA it has no fixed repeating sequence, which makes it hard for enzymes to break down. It fills the spaces between cellulose fibers in the cell wall, much as a resin binds a composite, and it lets the xylem conduct water under negative pressure without collapsing.

Few organisms can break lignin down. White-rot fungi use oxidizing enzymes such as peroxidases and laccases to attack it, and they are the main agents of wood decay. Because lignin resists decay, it contributes to the long-lived carbon in soil and to the buildup of plant material in the carbon cycle. Some researchers link the abundance of undecayed wood in the Carboniferous period to the formation of coal, although the extent of this is still debated.

Lignin must be separated from cellulose to make paper, and it is a by-product burned for energy in pulp mills. It is being studied as a source of aromatic chemicals and materials, and its resistance to breakdown is a main obstacle for converting plant waste into fuel in biotechnology. It is also one of the compounds stored in fossil fuels such as coal.

• Cellulose — the polymer it reinforces
• Xylem — the water-conducting tissue it strengthens
• Carbon Cycle — where its persistence matters