Editing Essential fatty acid (section)

==Nomenclature and terminology==
{{Main|Fatty acid#Nomenclature}}
Fatty acids comprise an [[aliphatic]] [[hydrocarbon]] chain plus a [[carboxyl group]] (–COOH) at one end, and terminated by a [[methyl group]] (–CH<sub>3</sub>) at the other end. They are almost always straight-chained. The carbon next to the carboxylate is known as α, the next carbon β, and so forth. Since biological fatty acids can be of diverse lengths, the last position is often labelled as "[[omega|ω]]", the last letter in the [[Greek alphabet]]. In the expression ''ω−x'', the minus symbol represents subtraction, indicating how many carbons away from the terminal end (ω) of the chain that the first unsaturated carbon-carbon bond appears. Typically, the number of carbons and the number of double bonds are also listed in short descriptions of unsaturated fatty acids. For instance, ω−3 18:4, or 18:4 ω−3, or 18:4 n−3 indicate [[stearidonic acid]], an 18-carbon chain with 4 double bonds, and with a double bond between the third and fourth carbon atoms from the CH<sub>3</sub> end. Double bonds are [[geometric isomerism|''cis'']] and separated by a single methylene (CH<sub>2</sub>) group unless otherwise noted. In free fatty acid form, the chemical structure of stearidonic acid is:
:[[Image:Fatty acid carbon numbering.svg|500px|Chemical structure of [[stearidonic acid]] showing physiological (red) and chemical (blue) numbering conventions]]{{clear left}}

===Examples===
[[Polyunsaturated fatty acid]]s with 16- and 18-carbon chains are sometimes classified as '''short chain polyunsaturated fatty acids''' ('''SC-PUFA'''), as opposed to '''long-chain polyunsaturated fatty acids''' ('''LC-PUFA'''), which have more than 18 carbon atoms.<ref name=PUFA>{{cite journal |vauthors=Buckley MT et al. |title=Selection in Europeans on Fatty Acid Desaturases Associated with Dietary Changes |journal=Mol Biol Evol |volume=34 |issue=6 |pages=1307–1318 |year=2017 |pmid=28333262 |pmc=5435082 |doi=10.1093/molbev/msx103}}</ref>

Both the essential fatty acids are SC-PUFA with an 18-carbon chain:
* [[omega-3 fatty acid|ω−3 fatty acid]]:
** [[alpha-linolenic acid|α-linolenic acid]] or ALA (18:3n−3)
* [[Omega-6 fatty acid|ω−6 fatty acid]]:
** [[linoleic acid]] or LA (18:2n−6)
These two fatty acids cannot be [[Biosynthesis|synthesized]] by humans because humans lack the [[desaturase]] [[enzymes]] required for their production.

They form the starting point for the creation of more desaturated fatty acids, most of which also have a longer carbon chain:
* [[omega-3 fatty acid|ω−3 fatty acid]]s:
** [[eicosapentaenoic acid]] or EPA (20:5n−3)
** [[docosahexaenoic acid]] or DHA (22:6n−3)
* [[Omega-6 fatty acid|ω−6 fatty acids]]:
** [[gamma-linolenic acid]] or GLA (18:3n−6)
** [[dihomo-gamma-linolenic acid]] or DGLA (20:3n−6)
** [[arachidonic acid]] or AA (20:4n−6)
Except for GLA, which has a short 18-carbon chain, these fatty acids have more than 18 carbon atoms and are typically classified as LC-PUFA.<ref name=PUFA />

[[Omega-9 fatty acid|ω−9 fatty acids]] are not essential in humans because they can be synthesized from carbohydrates or other fatty acids.