Monosaccharides

Monosaccharides are the simplest carbohydrates: single sugar units that cannot be hydrolysed into smaller sugars. Their general formula is (CH₂O)_n, and they are named by the number of carbon atoms:• trioses (3C), such as glyceraldehyde, made in photosynthesis and respiration
• pentoses (5C), such as ribose (in RNA and ATP) and deoxyribose (in DNA)
• hexoses (6C), such as glucose, fructose and galactoseIn solution, pentoses and hexoses mostly form rings. Glucose, C₆H₁₂O₆, forms a six-membered ring of five carbons and one oxygen. The carbons are numbered 1 to 6, starting next to the ring oxygen.Glucose exists as two isomers. In α-glucose the –OH group on carbon 1 is below the ring; in β-glucose it is above the ring. This small difference matters: α-glucose builds starch and glycogen, while β-glucose builds cellulose.Fructose and galactose have the same formula as glucose, C₆H₁₂O₆, but a different arrangement of atoms, so they are also isomers of glucose.Monosaccharides are small, sweet and soluble, because their many –OH groups form hydrogen bonds with water. This lets them be transported easily, but it also means they lower the water potential of a cell, so they are not suitable for storage. All monosaccharides are reducing sugars.