Skip to main content
Chemistry LibreTexts

2.2: Classification of Carbohydrates

  • Page ID
    233977
  • Objectives

    After completing this section, you should be able to

    1. classify a specific carbohydrate as being a monosaccharide, disaccharide, trisaccharide, etc., given the structure of the carbohydrate or sufficient information about its structure.
    2. classify a monosaccharide according to the number of carbon atoms present and whether it contains an aldehyde or ketone group.

    Key Terms

    Make certain that you can define, and use in context, the key terms below.

    • aldose
    • disaccharide
    • ketose
    • monosaccharide (simple sugar)
    • polysaccharide

    Role of carbohydrates

    Carbohydrates are a major source of metabolic energy and energy storage, both for plants and for animals. Aside from the sugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material (cellulose in plants, tissues, and cells in animals), and as a carbon source for the synthesis of other molecules.

    Carbohydrates are called saccharides or, if they are relatively small, sugars. Several classifications of carbohydrates have proven useful, and are outlined in the following table.

    Classification according to Molecular size or Complexity

    Simple Carbohydrates
    monosaccharides (1 unit)

    Complex Carbohydrates
    disaccharides (2 units) 

    oligosaccharides (3-10 units)

    polysaccharides (hundreds or thousands of units)

    Classification according to Number of carbon atoms

    Triose
    C3 sugars

    Tetrose
    C4 sugars

    Pentose
    C5 sugars

    Hexose
    C6 sugars

     

    Classification according to Functional group

    Aldose
    sugars having an aldehyde functional group R-HC=O
    Ketose
    sugars having a ketone functional group R2-C=O

    Classification according to Reactivity in Redox Reactions

    Reducing
    sugars oxidized by Tollens' reagent (or Benedict's or Fehling's reagents).
    Non-reducing
    sugars not oxidized by Tollens' or other reagents.

    We will first center on monosaccharides, that is, carbohydrates consisting of a single polyhydroxy ketone or polyhydroxy aldehyde unit.  Monosaccharides can be classified based on the number of carbon atoms and the functional groups, for example, glucose is an aldose (it contains an aldehyde functional group) and glucose is also a hexose (it contains six carbon atoms. Therefore, glucose is considered as an aldohexose

     

    D-glucose-chain-2D-Fischer

    Glucose is classified as an aldohexose, because it contains six carbon atoms and an aldehyde functional group. Image by Ben; Yikrazuul, Public domain, via Wikimedia Commons

    Exercises

    Questions

    Q25.1.1

    Classify each of the following sugars.

    (a)

    (b)

    (c)

    (d)

    Solutions

    S25.1.1

    (a) Aldoterose

    (b) Ketopentose

    (c) Ketohexose

    (d) Aldopentose

    Contributors and Attributions

    • Was this article helpful?