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Biology LibreTexts

2.1: Organic Molecules

Functional groups

The various functional groups include:

  • hydroxyl group (-OH)
  • carboxyl group [-COOH]
  • carbonyl group (-C=O)
  • amino group -NH2

Organic Molecules

Various organic molecules formed by these groups are as follows:


Organic molecules with a hydroxyl group (-OH).

Methanol [CH3OH] and ethanol (beverage alcohol)[CH3CH2OH] are common examples.

Sugars are also alcohols.

Carboxylic Acids

Contain one or more carboxyl groups [-COOH].

Many of the intermediates in the breakdown of foodstuffs by cellular respiration are carboxylic acids.


Contain a carbon atom to which is attached one hydrogen atom and — by a double bond — one oxygen atom.

Formaldehyde [HCHO] is a powerful disinfectant and preservative (it denatures proteins).

Acetaldehyde is produced during the conversion of pyruvic acid to ethanol when yeast ferment sugars. The converse is also true — acetaldehyde is produced in the liver as it metabolizes ingested ethanol (and may be the prime culprit in a "hangover").

Phosphoglyceraldehyde is an intermediate in glycolysis and the "dark reaction" of photosynthesis


Formed when two carbon atoms are linked by an oxygen atom.

Diethyl ether is a commonly-used anesthetic.


The removal of a molecule of water between the -OH group of an alcohol and the -OH group of a

  • carboxylic acid (-COOH) [shown in the diagram] or
  • phosphoric acid

produces an ester.

  • Fats are triesters of three fatty acids and glycerol (the alcohol).
  • Phospholipids are also esters.
  • Nucleotides are esters of nucleosides and phosphoric acid.
  • The nucleotides of DNA and RNA are linked by a double ester linkage called a phosphodiester bond.


Organic molecules with a carbonyl group (-C=O) between two hydrocarbon portions.

Ketones are synthesized in the liver, usually from fatty acids.

When glucose metabolism is suppressed, during starvation or in diabetics, fatty acids are used as a source of energy. But instead of entering the citric acid cycle, the acetyl-CoA produced from them is converted into the ketone acetoacetate. Some of this is then converted into acetone (which can be smelled on the breath of patients whose diabetes is out of control).


Organic molecules with an amino group, -NH2. Some examples:

  • all the amino acids (lysine has two of them).
  • the thyroid hormones thyroxine (T4) and triiodothyronine (T3)
  • Many neurotransmitters:
    • adrenaline and noradrenaline
    • dopamine
    • serotonin (5-hydroxytryptamine)
    • histamine


Amides are organic molecules containing a carbonyl group (-C=O) attached to a nitrogen atom. The peptide bond between the amino acids linked in a polypeptide is also called an amide bond.


John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation.