Chapter 8: Carbohydrates

Beginning with monosaccharides as the fundamental units, the chapter establishes how these simple sugars are classified as either aldoses or ketoses based on their carbonyl group type, and how their three-dimensional stereochemistry, depicted through Fischer projections, determines their biological activity with cells preferentially synthesizing D-enantiomers. The discussion progresses to cyclization, explaining how monosaccharides spontaneously form ring structures in aqueous environments, creating either six-membered pyranoses or five-membered furanoses with a new chiral center at the anomeric carbon that yields alpha and beta anomers. Various monosaccharide modifications are presented, including phosphorylated sugars essential for metabolic pathways, deoxy sugars like ribose in nucleic acids, amino sugars in cell walls, and oxidized or reduced derivatives. The chapter then addresses how monosaccharides polymerize through glycosidic bonds, acetal linkages formed between anomeric carbons and hydroxyl groups, creating disaccharides with distinct properties such as reducing versus nonreducing character depending on whether the anomeric carbon remains reactive. Polysaccharides are examined as large polymers serving either energy storage or structural roles, with homoglycans including starch and glycogen for metabolism and cellulose and chitin for structural support, while heteroglycans form the basis of glycoconjugates. The final section addresses glycoconjugates where carbohydrates covalently attach to proteins or lipids, including proteoglycans in the extracellular matrix providing tissue elasticity, peptidoglycans forming bacterial cell walls where penicillin disrupts cross-linking, and glycoproteins modified with O-linked or N-linked oligosaccharides that regulate protein trafficking, stability, and cellular recognition including blood group antigen systems.