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Biology 2nd Baccalaureate

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2.1. Glucid concept

Glucid concept

The carbohydrates are biomolecules formed mainly by carbon (C)hydrogen (H) and oxygen (O), in a similar proportion to CnH2nOn, ie (CH2O)n.

They are also often called carbohydrates, although this name is inappropriate, since they are not hydrated carbon atoms, linked to water molecules, but carbon atoms linked to alcohol groups (-OH), called also hydroxyl radicals, and hydrogen radicals (-H). The name glucid derives from the word “glucose”, from the Greek glykys, which means “sweet”.

All carbohydrates always have a carbonyl group , that is, a carbon linked to an oxygen through a double bond . This carbonyl group can be:

  • An aldehyde group (-CHO).
  • ketone group  (-CO).


By Javier Velasco (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Depending on the carbonyl group, carbohydrates can be defined as polyhydroxyaldehydes or polyhydroxyketones. They are polyalcohols with an aldehyde or ketone group.

In addition, some carbohydrates may have nitrogen (N), sulfur (S) and phosphorus (P) atoms , but without these being essential in their constitution. The presence of these elements occurs after amination processes (incorporation of amino groups (-NH2)), and esterification with acids, such as, for example, sulfuric acid (H2SO4) or phosphoric acid (H3PO4).


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