Overview

Saccharide is a general term for organic compounds often called sugars or carbohydrates. At the simplest level a saccharide consists of a carbon chain bearing multiple hydroxyl groups and one carbonyl group; a common empirical formula is (CH2O)n, which expresses the historic name "carbohydrate." In biology and chemistry the word covers single sugar units, short oligomers and long polymeric chains.

Structure and classification

Chemically, saccharides are classified by size and functional type. Monosaccharides are single sugar units; disaccharides contain two linked monosaccharides; oligosaccharides have a few units; polysaccharides are long chains. Monosaccharides can be aldoses (with an aldehyde group) or ketoses (with a ketone). Many form five- or six-membered rings called furanoses and pyranoses. The stereochemistry at asymmetric carbons produces many isomers and gives rise to alpha and beta anomers at the anomeric carbon.

Types and examples

  • Monosaccharides: glucose, fructose, galactose.
  • Disaccharides: sucrose (table sugar), lactose (milk sugar), maltose.
  • Oligosaccharides: short chains often attached to proteins or lipids on cell surfaces.
  • Polysaccharides: starch and glycogen (energy storage), cellulose and chitin (structural).

Biological roles and uses

Saccharides are central to life. They provide immediate energy (glucose), store energy in compact forms (starch in plants, glycogen in animals), and form structural materials (cellulose in plant cell walls, chitin in arthropods and fungi). Surface-attached oligosaccharides participate in cell recognition, immune response and signaling as components of glycoproteins and glycolipids. Outside biology, saccharides are important in food, fermentation, industrial chemistry and biofuels.

History and study

The term derives from the Greek word for sugar. The molecular diversity and stereochemistry of saccharides were clarified by late 19th- and early 20th-century chemists who developed methods to determine configuration and reaction behavior. Modern analysis uses chromatography and spectroscopic techniques to identify and quantify saccharides and their linkages.

Notable distinctions and practical notes

Some saccharides are reducing sugars (capable of acting as reducing agents at the anomeric position) while others are nonreducing; this affects reactivity in chemical tests and digestion. The type of glycosidic bond between units (for example alpha-1,4 versus beta-1,4) determines whether an organism can enzymatically break a polymer down. Dietary fiber consists largely of non-digestible polysaccharides and influences nutrition and health. Synthetic and modified saccharides also have pharmacological and material applications.