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.

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.

"GAlactose = Glucose with carbon 4 Altered": a C4 epimer.

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.

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.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Metabolism of carbohydrates and carbohydrate derivatives (Reactome, 2026); Digestion of dietary carbohydrate (Reactome, 2026); beta-Lactose (CID 6134) (PubChem, NCBI, 2026) · Public domain; Sucrose (CID 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).

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Representative model, for educational purposes only.