23.2: Classification of Carbohydrates

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Objectives

After completing this section, you should be able to

classify a specific carbohydrate as being a monosaccharide, disaccharide, trisaccharide, etc., given the structure of the carbohydrate or sufficient information about its structure.
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

Contributors and Attributions

Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University)
Prof. Steven Farmer (Sonoma State University)
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry