Wood is a complex natural composite material produced by trees, primarily serving a structural role to support the plant’s massive weight and facilitate water transport. Its unique combination of strength, flexibility, and light weight stems from the precise arrangement of its chemical components. This internal blueprint determines everything from the wood’s density and durability to its ability to swell or shrink when exposed to moisture.
The Primary Chemical Building Blocks
The bulk of wood is composed of three major organic polymers: cellulose, hemicellulose, and lignin. Cellulose is the most abundant component, typically accounting for 40 to 55 percent of the wood’s dry weight. This long-chain polysaccharide is a linear polymer of glucose units, organized into crystalline bundles called microfibrils. These microfibrils provide the wood with its high tensile strength.
Lignin is the second most common component, making up approximately 20 to 30 percent of the wood, with slightly higher proportions found in hardwoods than softwoods. This complex, amorphous phenolic polymer acts as the matrix or binder, encrusting the cellulose microfibrils and filling the spaces between them. Lignin imparts rigidity and compressive strength to the structure, while also contributing to the wood’s natural water resistance.
Hemicellulose comprises 20 to 30 percent of the wood structure. Unlike cellulose, hemicellulose is a diverse group of branched polymers made up of various sugars, such as xylose and mannose. These shorter chains function as an intermediate filler and stabilizer, chemically linking the cellulose and lignin to create a cohesive cell wall structure.
The Microscopic Architecture
These chemical components are organized into hollow, elongated cells, such as tracheids in softwoods or fibers and vessel elements in hardwoods, which primarily align along the trunk’s axis. These cells are essentially microscopic tubes with multilayered walls. Their arrangement determines the wood’s macroscopic properties, including its grain pattern and immense strength.
The outermost layer of the cell wall is the compound middle lamella, a lignin-rich region that acts as the glue cementing adjacent cells together. Inside this layer is the secondary cell wall, which is divided into three distinct layers: S1, S2, and S3. The S2 layer is the thickest and is the primary determinant of the wood’s mechanical properties.
The strength of the S2 layer comes from the orientation of the helically wound cellulose microfibrils, which are embedded within the lignin and hemicellulose matrix. The angle at which these fibrils are oriented relative to the cell’s long axis is known as the microfibril angle. This angle is small in the S2 layer, maximizing the cell’s strength along the grain. The varying orientation of microfibrils across all cell wall layers provides resistance to forces from multiple directions.
Water Content and Minor Components
Water is a non-structural yet highly influential component of wood, categorized into two forms. Free water is liquid water held within the large central cavity of the wood cells, known as the lumen. Bound water, by contrast, is held by intermolecular attraction within the sub-microscopic spaces of the cell walls.
Wood does not begin to shrink or swell until the moisture content drops below or rises above the fiber saturation point (FSP). The FSP averages about 30 percent moisture content, where cell walls are saturated with bound water but no free water remains in the lumens. The removal of bound water below this point causes the cell walls to contract, leading to dimensional changes such as shrinking and warping.
In addition to water and the main polymers, wood contains a small percentage of compounds called extractives, which are organic chemicals like resins, oils, fats, and tannins. These minor components contribute significantly to a wood species’ unique characteristics. Extractives influence the wood’s color, odor, and natural resistance to decay and insect attack.