The evolution of plants and animals has resulted in forms of life whose chemistry makes generous use of glucose, a molecule composed of six carbon atoms. Glucose is one of the essential components in life processes: when connected together in repeating units, glucose forms long macromolecules, known as polymers, which are used to build the living cells of humans and animals, plants and trees. Polymer properties change considerably with the structure of these long molecular chains, and that’s where tree stems and branches—what we commonly call wood—produce a unique resistance to rotting.
Rotting, or more technically, biodegradation, occurs when microbes break down organic substances. The rate of this decomposition is “controlled by chemical and physical properties,” explains Professor of Biology Anthony Sinskey. Microbes are tiny organisms that “are bags of enzymes,” he says, using water to rapidly break down a “biomass” and claim its glucose. Processed wood, however, is very dry, and has a lower water content than, say, tomatoes. In addition, a dense crystalline glucose polymer known as cellulose forms a fibrous frame for the growth of wood. Packed between these fibers is a dark-colored polymer called lignin, which, along with cellulose provides the rigidity and mechanical strength for trees to reach soaring heights. Cellulose and lignin together form a dense physical barrier that blocks most microbes. By contrast, most fruits and vegetables are made up (in part) of starch, a loosely packed polymer of glucose that is more open to microbial penetration, and thus degradation.
This combination of low water content and dense structure makes wood highly resistant to decomposition, but by no means invulnerable. “Trees will degrade in wet soil,” notes Sinskey. Conversely, lowering “the water activity” in fruits and vegetables increases their resistance to microbial decomposition (think of sun-dried tomatoes).
Other decay processes, such as oxidation, also play a role in the aging of organic matter. Oxidation results in the rapid browning of a freshly cut banana or the slow yellowing of some types of paper. Lower-grade paper used in newsprint and common paperbacks is formed from ground-up wood containing lignin, and oxidation of lignin results in that familiar darkening. In contrast, the materials in acid-free paper are treated with a chemical solvent process that removes the lignin, leaving behind densely packed cellulose that doesn’t oxidize as readily.
The high density of cellulose makes for a challenging material, one that natural “microbiology hasn’t worked around to decompose quickly,” says Sinskey. It is part of the reason trees have such long lives—and why our books and furniture are so long-lasting.—Sajan Saini