The Chemistry of Wood


A native forest typically consists of many different botanical species, and forests like the Boreal forest comprises both softwood (conifers, such as spruce, tamarack) and hardwood (aspen, birch, poplar) trees. When processed for lumber this produces wood (for pulp and paper, or timber for construction and furniture purposes, or for energy through burning) and waste residues (bark and branches, leaves) that can also be used as an energy source, or to extract valuable compounds such as essential oils or bioactive chemicals.

The forest, and plant biomass in general, consists of what is commonly referred to as lignocellulose. Lignocellulosic materials comprise 3 main chemical components: cellulose (a carbohydrate biopolymer made up of repeating residues of glucose, and makes up ~50 % of the total weight of plant biomass), the hemicelluloses (an ambiguously defined group of carbohydrate biopolymers that exist in close association with cellulose in the plant cell wall and makes up between 20-30 % depending upon the plant species. There are 2 predominant types of hemicellulose polysaccharides; the xylans, chiefly found in hardwoods and to a minor extent in softwoods, and the galactoglucomannans of softwoods.), and lignin (a natural aromatic (85 %) and phenolic (15 %) polymer found in both the primary and secondary cell wall layers, and is present to the extent of 20-30 % and is species dependent. Lignin shields the carbohydrate polymers from microbial and enzymatic attack, and also confers mechanical strength and conduit of water in the tree).

The chemistry with respect to the hemicelluloses and lignins differs greatly between softwood & hardwood tree species. Other polysaccharides, usually of mixed constitution, are also present but in lesser amounts. Trees also produce a host of other compounds of varying chemical nature in minor and trace amounts, and some of high-value finds commercial applications. For example, wood bark, which comprises about 13 to 21 % of wood on a dry weight basis, contains a variety of chemicals some being of economic importance when extracted, and esp., as pharmaceuticals. Well-known examples of bark extractives include taxol (paclitaxel, for cancer chemotherapy) from Pacific yew tree, quinine (antimalarial agent from the Cinchona tree), aspirin (analgesic from willow tree), and curare (anaesthetic from Strychnus toxifera). Conifer leaves contain essential oils, and this too is a valued product. A host of many other natural products (phytochemicals) are used in pharmaceuticals, or as the basis of organic synthesis to produce more efficacious medicines.

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