"In chemical structure, arachidonic acid is a carboxylic acid with a 20-carbon chain and four cis-double bonds; 
the first double bond is located at the sixth carbon from the omega end."
"Some chemistry sources define 'arachidonic acid' to designate any of the eicosatetraenoic acids. However, almost all writings in biology, medicine and nutrition limit the term to all-cis-5,8,11,14-eicosatetraenoic acid."
"Arachidonic acid is not one of the essential fatty acids. However, it does become essential if there is a deficiency in linoleic acid or if there is an inability to convert linoleic acid to arachidonic acid, which is required by most mammals. Some mammals lack the ability to—or have a very limited capacity to—convert linoleic acid into arachidonic acid, making it an essential part of their diet. Since little or no arachidonic acid is found in common plants, such animals are obligate carnivores; the cat is a common example.[4][5] A commercial source of arachidonic acid has been derived, however, from the fungus Mortierella alpina."