Plant cell wall material is composed of three important constituents: cellulose, lignin, and hemicellulose. Lignin[local] is the second most abundant naturally occurring polymer. It is formed by oxidative coupling of phenolic monomers. These reactions are not reversible. Lignin is mainly found in plant cell walls.

There are many possible monomers of lignin, and the types and proportions depend on the source in nature.
	A possible monomeric unit of lignin.