Monosaccharide sugars built on a five-carbon skeleton, a family that includes the aldoses ribose, arabinose, and xylose alongside the ketoses xylulose and ribulose. Ribose is the most biologically famous member, forming the backbone of RNA and of energy carriers such as ATP, while xylose and arabinose are major building blocks of hemicellulose in plant cell walls, making them abundant in agricultural residues. In food processing, pentoses participate readily in Maillard browning reactions with amino acids, contributing color and roasted flavors to baked goods, and xylose is the industrial feedstock for xylitol production by hydrogenation. Although humans digest pentoses poorly compared with glucose, they appear in small amounts in fruits and vegetables and play outsized roles in biotechnology, flavor chemistry, and plant biology.