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sorted the example table according to omega number |
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; ''trans'' : A ''trans'' configuration, by contrast, means that the adjacent two hydrogen atoms lie on ''opposite'' sides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.
In most naturally occurring unsaturated fatty acids, each double bond has three ([[omega-3 fatty acid|
The geometric differences between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).
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! Common name || Chemical structure || Δ{{sup|''x''}}{{efn|Each double bond in the fatty acid is indicated by Δx, where the double bond is located on the xth carbon–carbon bond, counting from the carboxylic acid end.}} || ''C'':''D''{{efn|name=c:d|1="C:D" is the numerical symbol: total amount of (C)arbon atoms of the fatty acid, and the number of (D)ouble (''unsaturated'') bonds in it; if D > 1 it is assumed that the double bonds are separated by one or more [[methylene bridge]](s).}} || IUPAC<ref name="IUPAClipid"/> || ''n''−''x''{{efn|name=omega-x|1=In ''n minus x'' (also ω−x or omega-x) nomenclature a double bond of the fatty acid is located on the xth carbon–carbon bond, counting from the terminal methyl carbon (designated as n or ω) toward the carbonyl carbon.}}
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| colspan="6" |'''
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|[[Eicosapentaenoic acid]]
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|[[omega-3 fatty acid|''n''−3]]
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| colspan="6" |'''
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|[[Arachidonic acid]]
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|[[omega-6 fatty acid|''n''−6]]
|-
| colspan="6" |'''
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|[[Oleic acid]]|| CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''cis''-Δ{{sup|9}}|| 18:1 || 18:1(9) || [[omega-9 fatty acid|''n''−9]]
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|[[Erucic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|7}}'''CH=CH'''(CH{{sub|2}}){{sub|11}}COOH || ''cis''-Δ{{sup|13}}|| 22:1 || 22:1(13) || [[omega-9 fatty acid|''n''−9]]
|-
| colspan="6" |'''
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|[[Myristoleic acid]] || CH{{sub|3}}(CH{{sub|2}}){{sub|3}}'''CH=CH'''(CH{{sub|2}}){{sub|7}}COOH || ''cis''-Δ{{sup|9}} || 14:1 || 14:1(9) || ''n''−5
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!''n''−''x'' <br />(or ω−''x'')
|[[Omega-3 fatty acid|''n''−3]]<br />(or [[Omega-3 fatty acid|ω−3]])
|'''''n''−''x''''' ('''''n'' minus ''x'''''; also '''ω−''x''''' or '''
|-
!Lipid numbers
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In animals, fatty acids are formed from carbohydrates predominantly in the [[liver]], [[adipose tissue]], and the [[mammary gland]]s during lactation.<ref name=stryer>{{cite book |last1= Stryer |first1= Lubert | title=Biochemistry |chapter= Fatty acid metabolism. |edition= 4th |location= New York |publisher= W. H. Freeman and Company|date= 1995 |pages= 603–628 |isbn= 978-0-7167-2009-6 }}</ref>
Carbohydrates are converted into [[Pyruvic acid|pyruvate]] by [[glycolysis]] as the first important step in the conversion of carbohydrates into fatty acids.<ref name=stryer /> Pyruvate is then decarboxylated to form [[acetyl-CoA]] in the [[mitochondrion]]. However, this acetyl CoA needs to be transported into [[cytosol]] where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, [[Citric acid|citrate]] (produced by the condensation of acetyl-CoA with [[Oxaloacetic acid|oxaloacetate]]) is removed from the [[citric acid cycle]] and carried across the inner mitochondrial membrane into the cytosol.<ref name=stryer /> There it is cleaved by [[ATP citrate lyase]] into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as [[malate]].<ref name= ferre>{{cite journal | doi = 10.1159/000100426 | title = SREBP-1c Transcription Factor and Lipid Homeostasis: Clinical Perspective | journal = Hormone Research | year = 2007 | first1 = P. | last1 = Ferre |first2=F. |last2=Foufelle | volume = 68 | issue = 2 | pages = 72–82| doi-broken-date = 1 November 2024 | pmid = 17344645 | quote = this process is outlined graphically in page 73| doi-access = free }}</ref> The cytosolic acetyl-CoA is carboxylated by [[acetyl-CoA carboxylase]] into [[malonyl-CoA]], the first committed step in the synthesis of fatty acids.<ref name= ferre /><ref name=Voet>{{cite book |last1=Voet |first1=Donald |first2=Judith G. |last2=Voet |first3=Charlotte W. |last3=Pratt |title=Fundamentals of Biochemistry |edition=2nd |publisher=John Wiley and Sons |year=2006 |pages=[https://archive.org/details/fundamentalsofbi00voet_0/page/547 547, 556] |isbn=978-0-471-21495-3 |url-access=registration |url=https://archive.org/details/fundamentalsofbi00voet_0/page/547 }}</ref>
Malonyl-CoA is then involved in a repeating series of reactions that lengthens the growing fatty acid chain by two carbons at a time. Almost all natural fatty acids, therefore, have even numbers of carbon atoms. When synthesis is complete the free fatty acids are nearly always combined with glycerol (three fatty acids to one glycerol molecule) to form [[triglyceride]]s, the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of the [[phospholipid]]s that form the [[phospholipid bilayers]] out of which all the membranes of the cell are constructed (the [[cell wall]], and the membranes that enclose all the [[organelle]]s within the cells, such as the [[Cell nucleus|nucleus]], the [[Mitochondrion|mitochondria]], [[endoplasmic reticulum]], and the [[Golgi apparatus]]).<ref name=stryer />
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====Variation between animal species====
Studies on the [[cell membrane]]s of [[mammal]]s and [[reptile]]s discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ([[docosahexaenoic acid|DHA]], [[omega-3 fatty acid|omega−3 fatty acid]]) than [[reptile]]s.<ref name=hulb1999/> Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less
==Fatty acids in dietary fats==
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====Essential fatty acids====
{{Main|Essential fatty acid}}
Fatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bond [[Omega-3 fatty acid|three carbon atoms]] away from the methyl end; the other has a double bond [[Omega-6 fatty acid|six carbon atoms]] away from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.<ref>{{cite book|last=Bolsover|first=Stephen R.|title=Cell Biology: A Short Course|url=https://books.google.com/books?id=3a6p9pA5gZ8C&pg=PA42+|date=15 February 2004|publisher=John Wiley & Sons|isbn=978-0-471-46159-3|pages=42ff|display-authors=etal}}</ref> Two essential fatty acids are [[linoleic acid]] (LA) and [[α-Linolenic acid|alpha-linolenic acid]] (ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chain [[omega-3 fatty acid]]s — [[eicosapentaenoic acid]] (EPA) and [[docosahexaenoic acid]] (DHA), which can also be obtained from fish.
===Distribution===
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