2.3.1 Lignocellulose as a Chemical Resource

Cellulose, hemicellulose, and lignin are the polymers that provide the structural rigid­ity in higher plants that grow vertically from a few centimeters to tens of meters — the giant redwood (Sequoiadendron giganteum) reaches up to 300 ft (90 m) in height. Although a multitude of microorganisms can elaborate enzymes to degrade cellulose and hemicelluloses, the success of the lignocellulose architecture in the global eco­system is such that it was only with the advent of Homo sapiens with flint and (later) metal axes that the domination of deciduous and coniferous forests (especially in the Northern Hemisphere) was seriously challenged.

At the cellular level, plants derive their remarkable resilience to physical and microbial weathering and attack from having evolved the means to greatly thicken their cell walls, using cellulose in linear polymers of high molecular weight (500,000-1,500,000) that are overlapped and aggregated into macroscopic fibers.15 Linear strands of cellulose have a close molecular arrangement in fibrillar bun­dles that are sufficiently regular to have X-ray diffraction patterns characteristic of “crystals.” This not only augments the structural cohesion but also limits access by water-soluble components and enzymes, and native cellulose is essentially insolu­ble in water.

Hemicelluloses are diverse in both sugar components and structure, with polymeric molecular weights below 50,000.15 The heterogeneity of lignins is even greater; any estimate of molecular weight is highly dependent on the method used for extraction and solubilization, and average molecular weight distributions may be less than 10,000.16 Lignin and hemicelluloses may form chemically linked complexes that bind water-soluble hemicelluloses into a three-dimensional array, cemented together by lignin, that sheaths the cellulose microfibrils and protects them from enzymic and chemical degradation.151718