Carbohydrates are a family of chemical substances encompassing monosaccharides, combinations of monosaccharides, and their derivatives. Glucose is a small carbohydrate; cellulose is a long-chain polysaccharide. Deoxyribose illustrates why composition alone does not define the family: replacing a hydroxy group in ribose by hydrogen still leaves a carbohydrate. See IUPAC Gold Book: carbohydrates.
Many familiar monosaccharides have a molecular formula of the form , equivalently . This explains the historical name, “hydrate of carbon”, but does not mean the molecule consists of carbon mixed with intact water molecules. Nor is it a universal formula: deoxyribose has . The family is defined through chemical structure and relationships, not that ratio. See IUPAC's definition and origin of the name carbohydrate.
The useful surprise is the variety of roles obtainable from related building blocks. Glucose participates in metabolism, ribose forms part of RNA, and linked glucose units make structural cellulose in a plant cell wall. Understanding carbohydrates therefore connects cellular energy, molecular information, and material structure. See Alberts et al., Molecular Biology of the Cell: the chemical components of a cell.
