Carbohydrate Summary

Carbohydrates

Overview

  • Most abundant biomolecule on earth.
  • Contain a carbonyl functional group and multiple OH groups.
  • Classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
  • Monosaccharides are classified by the location of the carbonyl group (aldose or ketose) and the number of carbons in the carbon skeleton.

Monosaccharides

  • Simple sugars with carbon atoms linked by single bonds.
  • Molecular formulas are usually multiples of CH2OCH_2O (e.g., trioses-3C, tetroses-4C, pentoses-5C, hexoses-6C).
  • Glucose (C<em>6H</em>12O6C<em>6H</em>{12}O_6) is the most common monosaccharide.
  • Examples: Glucose, Fructose, Galactose, Ribose

Disaccharides

  • Formed when a dehydration reaction joins two monosaccharides, creating a glycosidic linkage/bond.
  • Maltose:
    • 2 glucose molecules joined by a (1-4)-glycosidic linkage.
    • Intermediate product of starch hydrolysis.
  • Sucrose:
    • Common table sugar (glucose + fructose) with a 1–2 glycosidic linkage.
  • Lactose:
    • Found in milk (galactose + glucose) with a (1-4)-glycosidic linkage.

Oligosaccharides

  • Small polymers of 3 to 20 monosaccharide residues covalently joined to non-sugar molecules (lipids or proteins).
  • Glycolipids: oligosaccharides linked to lipids.
  • Glycoproteins: oligosaccharides linked to proteins via O-glycosidic linkages (serine or threonine) or N-glycosidic linkages (asparagine).
  • Function: Protect cell surface from mechanical and chemical damage; involved cell-cell recognition and binding.
  • Examples: GlcNAc, Neu5Ac, GalNAc

Cell-Cell Recognition and Binding

  • Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.
  • Proteins called lectins recognize particular oligosaccharide side chains on WBC.
  • CHO layers adsorb water giving cells a slimy surface.
  • Define human blood groups (A, B, and O).

Polysaccharides

  • Carbohydrate macromolecules with 100s-1000s of monosaccharide units connected by glycosidic linkages.
  • Homopolysaccharides: one type of monosaccharide.
  • Heteropolysaccharides: two or more types of monosaccharides.
  • Storage Polysaccharides: Starch and Glycogen
  • Structural Polysaccharides: Cellulose
Starch
  • Storage polysaccharide in plants, consisting entirely of glucose monomers.
  • Two types:
    • Amylose (20-30%): unbranched, long, tight helices.
    • Amylopectin (70-80%): branched polymer of glucose subunits, branch points occur every 20-25 glucose residues, prevents helix formation.
  • Hydrolyzed by α-amylase.
Glycogen
  • Storage polysaccharide in vertebrates (liver, muscle cells).
  • Polymer of linked glucose molecules with branch points every 8-12 residues.