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From my understanding, polymers are long chain molecules containing repeating units of monomers.

For example, proteins are polymers called polypeptides with repeating units of (different) amino acids. Therefore, a fatty acid, saturated or unsaturated, would classify as a polymer as they are long chain molecules with repeating units of alkyl monomers?

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    $\begingroup$ You are wasting your energy on a semantic question of no biological importance or interest . This is not unusual for people with chemical backgrounds. For the record you will seldom read the word polymer in biochemistry books. But if you know the structure of these molecules that is sufficient — focus on what the functions of these molecules are and how it relates to structure. $\endgroup$
    – David
    Nov 1, 2018 at 18:37
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    $\begingroup$ From a chemistry perspective they would be considered oligomers rather than polymers, since they generally have quite a short and specific length. (P.S. don't let anyone tell you that chemistry is not important in biology, nor that nomenclature is not important in chemistry!) $\endgroup$
    – N. Virgo
    Nov 2, 2018 at 6:04

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In the IUPAC Gold Book, IUPAC defines a polymer as follows:

A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass

Fatty acids are relatively high in molecular mass compared to the monomer (which is essentially a carbon atom), although they're quite low in mass compared to all other biological polymers. Since we consider proteins polypeptides, as you rightly note, the monomers clearly don't have to be chemically identical, so the variations around a simple carbon that are present in various fatty acids should be fine.

The definition states nothing regarding typical numbers of repetitions or minimal numbers of repetition.

The IUPAC definition also mentions that for most (but not all) polymers, the specific number of repetition has little impact on the molecular properties of the polymer. Fatty acids do generally work more or less the same regardless of their specific length.

Sounds like fatty acids are carbon polymers to me, even if indeed we don't usually think of them like that.

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    $\begingroup$ The monomer for fatty acids is ethane, not methane. Most of the biochemical reactions that handle fatty acids work on pairs of carbon atoms. $\endgroup$
    – Jasper
    Nov 2, 2018 at 2:19
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    $\begingroup$ It is interesting that polyketides have a similar biosythetic pathway (and the enzyme complexes too) as fatty acids but the former are still labelled if not considered as polymers but not the latter. $\endgroup$
    – WYSIWYG
    Mar 10, 2019 at 10:54
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Fatty acids are usually not considered polymers.

From a biological viewpoint this is simply because there is only a very limited set of chain lengths that exists in nature (~2-20 or so) and also even numbered chains are much more common than uneven numbered ones. In contrast to this 'true' polymers can have an arbitrary length of chain length (of course there is still a limit in biology, but not a super strict one) and no specific lengths are favoured.

From a chemical viewpoint fatty acids are also not really a polymer: you can't just take a bunch of alkyl monomers and fuse them to one long fatty acid chain, because simple alkyl monomers don't have chemical groups that allow this reaction to take place (making C-C bonds is chemically not the easiest thing to do).

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    $\begingroup$ I'm not sure about your last paragraph; after all, the same could be said of polyethylene, which is certainly a polymer. (The name "polyethylene" is a hint to how it's made, but its actual structure is just a long chain of single-bonded carbon atoms with hydrogen atoms attached where appropriate.) $\endgroup$
    – ruakh
    Nov 1, 2018 at 23:45
  • $\begingroup$ @ruakh : That reaction takes places at high pressure and temperature. $\endgroup$
    – Mesentery
    Nov 2, 2018 at 4:47
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I've never heard of anyone consider fatty acid to be a polymer. I'm not a biologist, but all of my research is on polymers-specifically paints and coatings. Fatty acids are often used in the coatings industry as one of the building blocks to make an alkyd coating. In my eyes, fatty acids would be seen more as a monomer or oligomer. Like you've observed, fatty acids do have repeat units; but many would opt to use other words to describe the molecule

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