Although cellulose appeared 3.5 billion years ago it was not until the evolution of land plants that it became universally present in plants. Besides providing rigidity, so that cellular turgor could develop, it also provided mechanical strength due to its layered microfibril development. The tensile strength of cellulose microfibrils is over twice that of steel. The strength of cellulose is diminished, however, when it is wet. Materials embedded in the matrix of cellulose, such as lignin, hemicellulose, and pectins, add waterproofing as well as other properties. Chitin has structural properties similar to those of cellulose and could well have played a more general role in plant evolution. However, in contrast to cellulose, chitin requires reduced nitrogen, and it is thought that cellulose won the evolutionary race owing to the general nitrogen limitation in the biosphere. Organisms that utilize chitin, such as saprophytic and parasitic fungi, are found in generally nitrogen-rich environments (Duchesne and Larson, 1989). Cellulose, a polysaccharide, is one of the most abundant chemicals on earth, constituting about one-half of the earth's standing biomass.

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