monosaccharid

Monosaccharide is the simplest form of carbohydrate and the fundamental building block for more complex carbohydrates such as disaccharides and polysaccharides. They are small, water-soluble molecules that contain multiple hydroxyl groups and a carbonyl group, and they cannot be hydrolyzed into smaller sugars.

Most monosaccharides contain between three and seven carbon atoms. They are classified by the number of carbons into triose, tetrose, pentose, hexose, and heptose, and by the position of the carbonyl group into aldoses (aldehydes) and ketoses (ketones). Many monosaccharides exist as mirror-image isomers, designated as D- and L- forms; in biological systems the D- forms are predominant.

Prominent examples include glucose (an aldohexose), fructose (a ketohexose), and galactose (an aldohexose that is an epimer of glucose). Ribose and deoxyribose are important aldopentoses in nucleic acids. In solution, monosaccharides can cyclize to form ring structures such as pyranose and furanose forms, a feature that influences their reactivity and naming.

Monosaccharides are joined by glycosidic bonds to form disaccharides (e.g., sucrose, lactose) and polysaccharides (e.g., starch, cellulose). Beyond providing energy through glycolysis and respiration, they serve as essential components of nucleotides, glycoproteins, and glycolipids, and participate in a variety of metabolic pathways through interconversion and phosphorylation.

Dietary monosaccharides are absorbed in the small intestine and circulated in the bloodstream, contributing to cellular energy supply and metabolic regulation. The term distinguishes simple sugars from larger carbohydrate polymers.