The plant cell wall consists of a multilayer structure in which cellulose and hemicellulose are intimately associated with lignin. Lignin provides strength and impermeability, and serves as a barrier against microbial attack across the cell wall. In contrast to other natural polymers composed of regularly interlinked, repetitive monomers, lignin is an amorphous polymer made of randomly distributed phenylpropanoid monomers. Lignin production is estimated at 20 × 10⁶ megatons annually; lignin therefore plays a central role in the global renewable carbon cycle. Its structure imposes unusual restrictions on its biodegradability.