The chemical makeup of any fiber dictates its physical characteristics, determining how it reacts to water and heat. The distinction between the major classes of organic compounds—carbohydrates and proteins—explains why fibers derived from plants behave so differently from those sourced from animals. By examining the molecular structure of cotton, we can definitively establish its chemical identity.
The Definitive Answer: Cotton is Cellulose
Cotton fiber is composed almost entirely of cellulose, a complex carbohydrate known as a polysaccharide. Raw cotton typically contains 88 to 96 percent cellulose, with the remainder consisting of waxes, pectins, and a small amount of protein. After industrial cleaning processes, the fiber can reach up to 99 percent pure cellulose.
Cellulose is a macromolecule, a long-chain polymer built from repeating units of the simple sugar glucose. Each glucose unit is linked by a specific glycosidic bond, forming a linear, unbranched chain. This highly organized structure results in a nearly flat chain, which allows for the close packing required in a textile fiber.
This arrangement defines cellulose as a carbohydrate, distinguishing it chemically from a protein. Carbohydrates are polymers of sugars, composed only of carbon, hydrogen, and oxygen. Proteins are polymers built from amino acids and always contain nitrogen in addition to carbon, hydrogen, and oxygen. The long, linear cellulose chains in cotton can contain thousands of glucose units, making it a robust and structurally strong component.
How Cellulose Defines Cotton’s Properties
The molecular architecture of cellulose translates directly into the familiar physical properties of cotton fabric. Each glucose unit contains multiple hydroxyl (-OH) groups, which are highly attractive to water molecules. This feature makes cotton fibers naturally hydrophilic, or water-loving, responsible for the fabric’s excellent absorbency and wicking capabilities. Cotton can absorb significant moisture before feeling damp, making it comfortable to wear in warm climates.
The durability and strength of cotton result from its internal cellulose structure. The linear polymer chains pack tightly in parallel bundles, forming crystalline regions that account for 65 to 70 percent of the fiber. These tight bundles are held securely by extensive networks of hydrogen bonds between the hydroxyl groups on adjacent cellulose chains. This strong internal bonding network gives the fiber high tensile strength, making cotton textiles durable and resistant to breaking.
The strength of cotton is unique among many fibers because it increases by about 10 to 20 percent when the fabric is wet. This resilience when saturated is a direct benefit of the hydrogen bonding network, as the water molecules further stabilize the structure rather than weakening it. The rigid, crystalline nature of the cellulose also contributes to cotton’s resistance to many common solvents and chemicals.
Why Cotton is Not Protein
The fundamental difference between cotton and protein fibers lies in their chemical building blocks. Cotton is built from glucose molecules, while protein fibers are polymers constructed from amino acids. This distinction is most clearly seen in the elemental composition, as cellulose is solely composed of carbon, hydrogen, and oxygen. Protein fibers, such as wool and silk, are defined by the presence of nitrogen, which is a core component of the amino acid structure.
Protein fibers are typically derived from animals, serving biological purposes like protection. Wool is composed mainly of the protein keratin, which is rich in sulfur-containing amino acids. Silk is also a protein fiber, primarily made of fibroin, extruded by silkworms. These animal fibers are fundamentally different from cotton, a plant-based fiber that functions as the structural component of the cotton plant’s cell walls.
The two classes of fibers react differently to certain chemicals and conditions because of their distinct chemistries. The presence of amine groups in protein fibers allows them to be readily dyed with acid dyes, a process ineffective on cellulose. Cellulose reacts well with fiber-reactive dyes due to its numerous hydroxyl groups. The difference between cotton and protein fibers is a deep-seated chemical one, based on their respective polymeric backbones.