Wood, a widely used natural material, possesses a complex composition. It is a sophisticated biological composite, intricately structured at both chemical and cellular levels. Its strength and durability stem from the precise arrangement of its constituent parts.
Primary Structural Components
Wood primarily consists of three major organic polymers: cellulose, hemicellulose, and lignin. Cellulose, the most abundant organic polymer on Earth, forms the main structural framework of wood. It is comprised of long, linear chains of glucose units, which assemble into strong, rigid microfibrils that provide tensile strength and stability to wood. Cellulose accounts for 40% to 50% of wood’s dry weight.
Hemicellulose, a group of heterogeneous polysaccharides, acts as a branching polymer that connects to both cellulose and lignin, effectively serving as a binding agent. These shorter, more branched chains are composed of various sugar units, differing from the uniform glucose chains of cellulose. Hemicelluloses contribute to the mechanical properties of wood and constitute 15% to 35% of wood, varying between hardwoods and softwoods.
Lignin is a complex, amorphous polymer that provides wood with rigidity, compression strength, and protection against degradation. It acts as a natural glue, cementing the cellulose fibers together within the cell walls. Lignin is resistant to biological decay and makes up 17% to 35% of the dry mass of wood, depending on the species.
Minor Chemical Constituents
Beyond the primary structural polymers, wood contains minor chemical components. Extractives are non-structural organic compounds that can be removed from wood using solvents. These include a diverse range of substances such as resins, fats, waxes, tannins, essential oils, and alkaloids. Extractives impart specific characteristics to wood, including its color, odor, and natural resistance to decay and insects. Their content can vary significantly, usually ranging from 1% to 10% of wood’s dry weight, but sometimes exceeding 30% in certain species.
Wood also contains inorganic minerals, often referred to as ash content. These are elements like calcium, potassium, and magnesium that remain after wood is completely combusted. While these minerals are present, their contribution to the overall composition of wood is minor, less than 0.3% of the dry weight.
Cellular Arrangement and Structure
The chemical components of wood are organized into cells, forming its macroscopic structure. Wood is composed of elongated, tube-like cells, primarily tracheids in softwoods and vessel elements and fibers in hardwoods. Additionally, parenchyma cells are present, primarily for food storage. These cells are mostly oriented parallel to the trunk’s long axis, contributing to the grain of the timber.
The strength of wood originates from its cell walls, which are multi-layered structures. These layers, including the primary wall and a thicker secondary wall (often divided into S1, S2, and S3 layers), are primarily composed of cellulose microfibrils. These microfibrils are embedded within a matrix of hemicellulose and lignin, forming a composite material. Lignin is particularly concentrated in the middle lamella, the layer that cements adjacent cells together. This layered organization and composite nature provide wood with its characteristic strength and rigidity, while the arrangement of cells in patterns like annual growth rings also contributes to its macroscopic properties and appearance.