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25.1 Classification of Carbohydrates

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    91042

    • 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

    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.

    Complexity

    Simple Carbohydrates
    monosaccharides

    Complex Carbohydrates
    disaccharides, oligosaccharides
    & polysaccharides

    Size

    Tetrose
    C4 sugars

    Pentose
    C5 sugars

    Hexose
    C6 sugars

    Heptose
    C7 sugars


    etc.

    C=O Function

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

    Reactivity

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

    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

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