Carbohydrates: Structure and Classification

Carbohydrates

• Carbohydrates are the most abundant group of organic compounds on earth.
• They have a general elemental composition of C, H, and O.
• They contain the same proportion of H: O, which equals to 2:1 as in water.
• The general formula is Cx(H2O)y.

Monosaccharides

• Monosaccharides are the simplest form of carbohydrates.
• They have a general molecular formula of (CH2O)n.
• They are reducing sugars, water soluble, and occur in crystalline form.
• Monosaccharides are classified based on the number of carbon atoms:
  • 3C- Triose (e.g. Glyceraldehydes)
  • 4C- Tetroses (e.g. Erythrose)
  • 5C- Pentoses (e.g. Ribose, Deoxyribose, Ribulose)
  • 6C- Hexoses (e.g. Glucose, Fructose, Galactose)
• Monosaccharides are also classified based on the type of carbonyl (Keto, aldo) group:
  • Aldoses (e.g. glucose, galactose)
  • Ketoses (e.g. fructose)

Disaccharides

• Disaccharides are sugars formed by joining two monosaccharides by a glycosidic bond.
• The glycosidic bond is formed by removal of a water molecule from two adjacent monosaccharides by a condensation reaction.
• Examples of disaccharides:
  • Maltose (glucose + glucose)
  • Sucrose (glucose + fructose)
  • Lactose (glucose + galactose)
• Maltose and lactose are reducing sugars, while sucrose is a non-reducing sugar.

Polysaccharides

• Polysaccharides are macromolecules and biopolymers.
• They are made up of few hundred to a few thousand monosaccharide subunits.
• They are non-crystalline, water insoluble, and not considered as sugars.
• Polysaccharides can be classified based on their function:
  • Storage polysaccharides (e.g. Starch, Glycogen)
  • Structural polysaccharides (e.g. Cellulose, Hemicellulose, Pectin)
• Polysaccharides can also be classified based on their architecture:
  • Linear forms (e.g. Cellulose, Amylose)
  • Branched forms (e.g. Glycogen, Amylopectin, Hemicellulose)

Functions of Carbohydrates

• Monosaccharides:
  • Energy source
  • Building blocks of disaccharides and polysaccharides
  • Components of nucleotides (DNA, RNA)
• Disaccharides:
  • Storage sugar in milk (Lactose)
  • Translocation in phloem (Sucrose)
  • Storage sugar in sugarcane (Sucrose)
• Polysaccharides:
  • Storage polysaccharides: starch stores glucose as energy source in plants and chlorophytes, glycogen stores glucose as energy source in animals and fungi, and inulin stores fructose as energy source in Dahlia tubers.
  • Structural polysaccharides: Cellulose in the cell walls of plants and chlorophytes, Pectin in the middle lamella of plant tissues, Hemicellulose in cell walls of plants, Peptidoglycan in the cell walls of prokaryotes, and Chitin in the cell walls of fungi and in exoskeleton in Arthropods.