A long-chain polymer that can be considered nature's building block, as it is the basic structural material of all trees and plants. The chemical structure of cellulose (whose basic chemical formula is C6H10O5) comprises a long, repeating chain of carbon-hydrogen-oxygen units. Cellulose fiber is the most basic and important constituent of paper, and the size and quality of cellulose fibers are important considerations in the papermaking process. The greater the cellulose content of a paper pulp, the higher the quality of the paper. Cotton is nearly pure cellulose, and consequently produces the best paper; woods tend to range from 50:90% cellulose, which also makes them ideally suited to papermaking, although the cellulose content of trees varies from tree to tree, or even within the same tree. The size and shape of the cellulose fibers also differ from tree to tree. The two botanical classifications of trees—coniferous and deciduous—each have different cellulose fiber characteristics, which can dramatically affect the quality of paper that is ultimately produced.

In its natural state, cellulose fibers are bound together with another substance called lignin, the removal of which by chemical or mechanical means greatly improves paper quality. Wood and plant material also contains other carbohydrate compounds (called hemicelluloses, since for many years chemists had difficulty distinguishing them from true cellulose) which also must be removed by the paper pulping process.

Cellulose is indigestible by all living organisms (with the exception of bacteria) and is also used to make some types of plastic—such as cellulose acetate, cellulose acetate butyrate, cellophane, and ethyl cellulose—used as resins, film formers, and substrates in gravure, flexography, and other printing processes.

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