Carbohydrates are the most abundant biomolecules on Earth, serving as energy sources, structural components, and recognition elements in living organisms. They are composed of carbon, hydrogen, and oxygen, typically with the empirical formula (CH2O)n.
Monosaccharides
Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into smaller sugars. They are classified by the number of carbon atoms: trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C). Glucose, fructose, and galactose are important hexoses, while ribose and deoxyribose are essential pentoses found in nucleic acids.
Monosaccharides exhibit stereoisomerism due to chiral carbon atoms. The D and L notation, also used for amino acids, refers to the configuration of the highest-numbered chiral carbon. Naturally occurring sugars are predominantly in the D-configuration. In solution, monosaccharides cyclize to form hemiacetals, creating alpha and beta anomers through the anomeric carbon.
Disaccharides and Oligosaccharides
Disaccharides consist of two monosaccharide units linked by a glycosidic bond. Important examples include sucrose (glucose-alpha-1,2-fructose), lactose (galactose-beta-1,4-glucose), and maltose (glucose-alpha-1,4-glucose). The type of glycosidic bond determines whether the disaccharide is reducing or non-reducing.
Oligosaccharides contain three to ten monosaccharide units. They are often found attached to proteins and lipids on cell surfaces, where they function as recognition markers. ABO blood group antigens are classic examples of oligosaccharide-based identity markers.
Polysaccharides
Polysaccharides are polymers of monosaccharide units linked by glycosidic bonds. They can be classified as storage or structural polysaccharides.
Storage polysaccharides: Starch is the main storage polysaccharide in plants, composed of amylose (linear alpha-1,4-linked glucose) and amylopectin (branched alpha-1,4 and alpha-1,6 links). Glycogen is the animal equivalent, more highly branched than amylopectin, found primarily in liver and muscle tissue.
Structural polysaccharides: Cellulose is a linear polymer of beta-1,4-linked glucose units, forming rigid fibrils that provide structural support in plant cell walls. Chitin, composed of N-acetylglucosamine units with beta-1,4 linkages, forms the exoskeleton of arthropods. Glycosaminoglycans such as hyaluronic acid and chondroitin sulfate are key components of the extracellular matrix.
Glycosidic Bonds and Reducing Sugars
A glycosidic bond forms between the anomeric carbon of one monosaccharide and a hydroxyl group of another. The bond is designated as either alpha or beta depending on the configuration at the anomeric carbon. A sugar with a free anomeric carbon can act as a reducing agent, hence the term reducing sugar. When both anomeric carbons are involved in the glycosidic bond, as in sucrose, the disaccharide is non-reducing.
Modified Carbohydrates
Carbohydrates can be modified in various ways to alter their properties. Phosphorylation traps sugars inside cells and activates them for metabolism. Amino sugars such as glucosamine and N-acetylglucosamine are components of glycoproteins and bacterial cell walls. Deoxy sugars like deoxyribose and fucose lack hydroxyl groups at specific positions, affecting their chemical reactivity and biological function.
