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2.3. Monosaccharides


The monosaccharides are carbohydrates consisting of a single chain polihidroxialdehídica (aldose) or polihidroxicetónica (ketoses). They cannot be decomposed by hydrolysis. They are named by adding the ending -ose to the number of carbons, such as triose, tetrose, pentose, hexose, etc.

  • An aldose is a monosaccharide whose molecule contains an aldehyde group, that is, a carbonyl at the end of it. The simplest aldose is glyceraldehyde , which has only three carbon atoms.
  • A  ketose  is a monosaccharide with one ketone group  per molecule. The dihydroxyacetone, three carbon atoms, is the simplest and only ketose having no optical activity.

Therefore, aldoses  and ketoses have a carbonyl group (carbon double bonded to oxygen).

  • Physical properties They are crystalline solids, white in color, sweet-tasting, and soluble in water , since hydroxyl radicals (-OH) and hydrogen radicals (-H) establish hydrogen bonds with water molecules.
  • Chemical properties. The presence of the carbonyl group  also gives them reducing properties that, as will be seen later, are used to identify them.

Monosaccharides can be oxidized (or they reduce) by certain ions such as the ferric ion (Fe3+) and cupric (Cu2+) that will pass when reduced to Fe2+ or Cu+1 .The carbonyl group of monosaccharides becomes acidic when it is oxidized.

This is the basis of the Fehling reaction in which a blue solution, due to the presence of Cu2+, becomes Cu+, red in color, in the presence of reducing sugars such as glucose .

Another chemical property of carbohydrates is their ability to associate with -NHamino groups .

Fundamental Ideas About Monosaccharides

The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones are molecules having 3 to 7 carbon atoms with an alcohol group (-OH) in all except a carbon bearing the aldehyde group (-CHO) or ketone group (-CO-), but never both at the same time.

Monosaccharides can be classified according to the number of carbon atoms that compose them or according to their carbonyl group , if it is aldehyde (–CHO) or ketone (–CO–). Monosaccharides are:

  • Aldosas: monosaccharide carbohydrate whose carbonyl group is an aldehyde group (–CHO) .
  • Ketoses: monosaccharide carbohydrate whose carbonyl group is a ketone group (–CO–) .

We will see the main functions of monosaccharides when we talk about each of their types and we will also see them at the end of the topic, when we see the general functions of carbohydrates , but we anticipate that they are:

  • Structural function:
    • Formation of polysaccharides by polymerization, by joining many monosaccharides.
    • Nucleotide formation.
    • Etc
  • Energy function:
    • Cells obtain energy from the oxidation of glucose.