Key Questions

  • Answer:

    There are several methods of classification, but the most common method is based on the number of sugar units they contain.


    Number of Sugar Units

    • Monosaccharides — 1 unit, e.g. glucose
    • Oligosaccharides —2 to 9 units
      o Disaccharides — 2 units, e.g. sucrose
    • Polysaccharides — ≥ 10 units, e.g., starch

    Other methods of classification include:

    Number of Carbon Atoms

    • 5 = pentose, e.g. ribose
    • 6 = hexose, e.g., glucose

    Functional Groups

    • Aldoses — contain an aldehyde group, e.g. glucose
    • Ketoses — contain a ketone group, e.g. fructose

    Chemical Properties

    • Reducing — react with Fehling's solution or Tollens' reagent, e.g. glucose, lactose
    • Non-reducing — no reaction with Fehling's solution or Tollens' reagent, e.g. sucrose, all polysaccharides

    Polysaccharides have other classifications.


    • Linear chains, e.g. amylose
    • Branched chains, e.g. glycogen


    • Storage, e.g. starch, glycogen
    • Structural, e.g. starch


    • Homopolysaccharides — contain only single monosaccharide units, e.g. starch
    • Heteropolysaccharides — contain different kinds of monosaccharides, e.g. hyaluronic acid (N-acetylglucosamine and glucuronic acid)