Carbohydrates

Carbohydrates

Carbohydrates (saccharides) are polyhydroxy aldehydes or ketones and their derivatives. They serve as the primary energy source for living organisms. General formula: Cₙ(H₂O)ₙ.

Classification

  • Monosaccharides: Simplest units; cannot be hydrolysed further. Examples: Glucose (aldohexose), Fructose (ketohexose), Galactose, Ribose (pentose in RNA), Deoxyribose (in DNA).
  • Disaccharides: Two monosaccharides linked by a glycosidic bond. Sucrose (Glucose+Fructose, non-reducing), Lactose (Glucose+Galactose, reducing), Maltose (Glucose+Glucose, reducing).
  • Oligosaccharides: 3–10 monosaccharide units. Found on cell surface glycoproteins important for cell recognition.
  • Polysaccharides: Many units. Starch & Glycogen (storage), Cellulose & Chitin (structural).

Isomerism

Epimers differ at one carbon (Glucose & Galactose differ at C4). Anomers differ at the anomeric carbon (α-D-glucose vs β-D-glucose). Mutarotation is the spontaneous interconversion of α and β forms in solution, changing optical rotation.

Glycosaminoglycans (GAGs)

Long unbranched heteropolysaccharides with repeating disaccharide units. Examples: Hyaluronic acid (joint fluid), Heparin (anticoagulant), Chondroitin sulfate (cartilage). They are highly negatively charged and bind large amounts of water.

Reducing vs Non-Reducing Sugars

Reducing sugars have a free anomeric -OH group and give a positive Benedict's test (glucose, fructose, maltose, lactose). Sucrose is non-reducing (no free anomeric carbon).

Clinical Significance

  • Lactose intolerance: Deficiency of lactase enzyme; lactose accumulates causing osmotic diarrhea and bloating.
  • Galactosemia: Deficiency of Galactose-1-phosphate uridyltransferase; galactose accumulates causing liver damage and cataracts.
  • G6PD deficiency: Impairs HMP shunt; RBCs cannot maintain glutathione, leading to hemolytic anemia with oxidant drugs.

Key Facts for Exams

  • Starch = α-1,4 linkages (amylose) + α-1,6 branches (amylopectin)
  • Glycogen is more branched than starch; branching enzyme adds α-1,6 links every 8–12 residues
  • Cellulose has β-1,4 linkages — humans lack cellulase
  • Chitin: β-1,4 linked N-acetylglucosamine — exoskeleton of insects, fungi cell walls