A monosaccharide is a carbohydrate that cannot be hydrolyzed into a simpler unit. It has a potential carbonyl group at the end of the carbon chain (an aldehyde group) or at an inner carbon (a ketone group). These two types of monosaccharides are therefore named aldoses and ketoses. Free monosaccharides can exist in open chain or ring forms.
Ring forms of the monosaccharides are the rule in oligosaccharides, which are branched or linear chains of monosaccharides attached to one another via glycosidic linkages (the term polysaccharide is typically reserved for large glycans that are composed of repeating oligosaccharide motifs). The ring form of a monosaccharide generates a chiral (anomeric) center (at C-1 for aldo sugars or at C-2 for keto sugars) (for details, see Chapter 2). A glycosidic linkage involves the attachment of a monosaccharide to another residue, typically via the hydroxyl group of this anomeric center, which can be α linkages or β linkages depending on the relationship of the oxygen to the anomeric carbon
Monosaccharides are the basic structural units of glycans.