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23.2: Classification of Carbohydrates

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    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

    Carbohydrates are the most abundant class of organic compounds found in living organisms. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll.

    \[ nCO_2 + n H_2O + Energy \rightarrow C_nH_{2n}O_n + nO_2\]

    As noted here, the formulas of many carbohydrates can be written as carbon hydrates, \(C_n(H_2O)_n\), hence their name. The carbohydrates are a major source of metabolic energy, both for plants and for animals that depend on plants for food. Aside from the sugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material (cellulose), a component of the energy transport compound ATP/ADP, recognition sites on cell surfaces, and one of three essential components of DNA and RNA.

    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.


    Simple Carbohydrates

    Complex Carbohydrates
    disaccharides, oligosaccharides
    & polysaccharides


    C4 sugars

    C5 sugars

    C6 sugars

    C7 sugars


    C=O Function

    sugars having an aldehyde function or an acetal equivalent.
    sugars having a ketone function or an acetal equivalent.


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


    Contributors and Attributions

    23.2: Classification of Carbohydrates is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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