Carbohydrates: Classification and Function

Carbohydrates are polyhydroxy aldehydes or ketones, broadly following the general formula (CH2O)n, and serve chiefly as the immediate and stored source of chemical energy for the cell, in addition to important structural roles. They are classified into three groups according to the number of sugar units they contain.

Monosaccharides are the simplest carbohydrates, single sugar units that cannot be broken down into smaller sugars by hydrolysis. They are further classified by the number of carbon atoms they contain -- trioses (3 carbons), tetroses (4), pentoses (5, such as ribose and deoxyribose, the sugars of RNA and DNA) and hexoses (6, such as glucose, fructose and galactose) -- and by whether their carbonyl group is an aldehyde (aldoses, such as glucose) or a ketone (ketoses, such as fructose). Glucose, with the molecular formula C6H12O6, is the single most important monosaccharide, serving as the universal fuel oxidised during cellular respiration.

Oligosaccharides contain a small number, typically two to ten, of monosaccharide units joined together by glycosidic bonds. The most familiar oligosaccharides are the disaccharides, formed from just two monosaccharide units: sucrose (table sugar), formed from one glucose and one fructose unit; maltose, formed from two glucose units; and lactose (milk sugar), formed from one glucose and one galactose unit. …