[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Carbohydrate chemistry [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # Carbohydrate chemistry Sugars differ by their carbonyl group, their number of carbons and the direction of their OH groups; glycosidic bonds join them together. Updated 2026-11-01 ## Key facts - Aldoses (aldehyde group: glucose, galactose) and ketoses (ketone group: fructose); named by carbon count: trioses, pentoses, hexoses. - D or L series depends on the OH of the chiral carbon farthest from the carbonyl; nearly all body sugars are D. - Epimers differ at one chiral carbon only. Glucose and galactose are C4 epimers; glucose and mannose are C2 epimers. - Ring closure turns the carbonyl carbon into the anomeric carbon: α (OH down in the Haworth projection) or β (up). Mutarotation interconverts them in solution. - A glycosidic bond is α or β according to the anomer that forms it: starch and glycogen use α-1,4 and α-1,6; cellulose uses β-1,4, which humans cannot digest. ## In detail - 1. A monosaccharide is a polyhydroxy aldehyde or ketone; glyceraldehyde (an aldotriose) is the reference for the D/L series. - 2. Isomers share a formula (glucose, fructose and galactose are all C₆H₁₂O₆) but differ in structure. Stereoisomers change only the spatial direction of the OH groups. - 3. Hexoses cyclise: the C5 OH attacks the carbonyl and forms a hemiacetal (pyranose, six-membered ring) or a hemiketal (furanose, the five-membered ring of fructose). - 4. The new OH on the anomeric carbon can point α or β; in water glucose sits in equilibrium, mostly β. - 5. A reducing sugar keeps its anomeric carbon free and can open its ring (glucose, lactose, maltose). Sucrose joins both anomeric carbons (α1→β2) and is non-reducing. - 6. Exam disaccharides: maltose (glucose-α1,4-glucose), lactose (galactose-β1,4-glucose) and sucrose (glucose-α1,β2-fructose). Each has its own brush-border disaccharidase. - 7. Derivatives: amino sugars (glucosamine, N-acetylgalactosamine), uronic acids (glucuronic, iduronic), deoxy sugars (deoxyribose) and sugar alcohols (sorbitol). They build glycosaminoglycans and glycoproteins. ## Memory aid "GAlactose = Glucose with carbon 4 Altered": a C4 epimer. **Also called** Isomers, anomers and glycosidic bonds ## Clinical notes - Lactose intolerance reflects loss of lactase, which cleaves the β1,4 bond of lactose; undigested sugar is fermented in the colon. - Cellulose (fibre) passes through the gut undigested because no human enzyme cleaves the β1,4 bond between glucoses. - Urine reducing-substance tests pick up galactose or fructose, useful when the glucose dipstick is negative. ## Pathways - [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/) - [Galactose metabolism](https://durgical.com/en/biochemistry/pathways/galactose-metabolism/) - [Glycogen metabolism](https://durgical.com/en/biochemistry/pathways/glycogen-metabolism/) ## Related conditions - [Lactose intolerance](https://durgical.com/en/conditions/lactose-intolerance/) - [Classic galactosaemia](https://durgical.com/en/conditions/classic-galactosaemia/) ## Related - [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/) - [Fructose](https://durgical.com/en/biochemistry/molecules/fructose/) - [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/) - [Glycogen](https://durgical.com/en/biochemistry/molecules/glycogen/) - [Hyaluronic acid (hyaluronan)](https://durgical.com/en/biochemistry/molecules/hyaluronic-acid/) - [Glycosaminoglycans (mucopolysaccharides)](https://durgical.com/en/biochemistry/topics/glycosaminoglycans/) ## Test yourself Think of the answer, then tap the question to see it. ## How are glucose and galactose related chemically? They are C4 epimers: they differ only in the direction of the OH on that carbon. ## Why is sucrose not a reducing sugar? Its bond joins the anomeric carbons of glucose and fructose, so neither is free to open the ring and act as a reducing agent. ## What distinguishes an α anomer from a β anomer? The position of the anomeric OH after ring closure: down (α) or up (β) in the Haworth projection. ## Why can humans digest starch but not cellulose? Starch has α-1,4 bonds that amylase cleaves; cellulose has β-1,4 bonds and we have no enzyme to hydrolyse them. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/carbohydrate-chemistry/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Metabolism of carbohydrates and carbohydrate derivatives](https://reactome.org/PathwayBrowser/#/R-HSA-71387) (Reactome, 2026); [Digestion of dietary carbohydrate](https://reactome.org/PathwayBrowser/#/R-HSA-189085) (Reactome, 2026); [beta-Lactose (CID 6134)](https://pubchem.ncbi.nlm.nih.gov/compound/6134) (PubChem, NCBI, 2026) · Public domain; [Sucrose (CID 5988)](https://pubchem.ncbi.nlm.nih.gov/compound/5988) (PubChem, NCBI, 2026) · Public domain; Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. 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