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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.