monosacharide

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar units that cannot be hydrolyzed into smaller carbohydrates. Their general molecular formula is (CH₂O)n, where n is typically three to seven; the most common monosaccharides contain six carbon atoms (hexoses). They are classified according to the number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses) and the position of the carbonyl group, which may be an aldehyde (aldoses) or a ketone (ketoses). In solution, monosaccharides interconvert between open‑chain and cyclic forms, the latter arising from intramolecular hemiacetal or hemiketal formation, giving rise to α and β anomers.

Glucose, fructose, and galactose are the most prevalent dietary monosaccharides. Glucose serves as the primary energy source for cellular respiration, entering glycolysis and the citric acid cycle. Fructose is metabolized mainly in the liver, while galactose is converted to glucose derivatives via the Leloir pathway. Monosaccharides also act as building blocks for disaccharides, oligosaccharides, and polysaccharides such as starch, glycogen, and cellulose, and they participate in nucleic acid synthesis as ribose and deoxyribose sugars.

In physiological contexts, monosaccharide concentrations are tightly regulated; abnormalities can lead to metabolic disorders, exemplified by hyperglycemia in diabetes mellitus. Their solubility, sweetness, and involvement in cell‑cell recognition make monosaccharides central to nutrition, biochemistry, and cellular signaling.