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I've read the lipids article on Wikipedia, but I couldn't understand the answer for my question and I'll explain why.

In biochemistry we have monomers and polymers while the monomer is the basic unit molecule for the polymere. In protein for example, we have amino acid that this is the basic unit of the proteins. Amino acid is characterized as structure of 4 groups: Alpha carbon, amine group, carboxylic group, hydrogen and R chain. this is the basic of all the amino acids.

But when we are talking about lipid as a main family of molecules (like what we have we have with protein) then I can not understand what is the "amino acids" (parallel) of them, i.e. what is the basic unit of the lipids and what is the structure.

Well, I could understand that the fatty acids are the analogous for the amino acids, but the problem is that fatty acids according the article (and other schemes that I saw on google pictures), they are considered as subgroup of the lipids, and that says that not all of the lipids are made of them. So I don't have clue for the answer to my question.

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  • $\begingroup$ First sentence in the second paragraph of the wikipedia article: "Scientists may broadly define lipids as hydrophobic or amphiphilic small molecules [...]. This category is even broader than fatty acids ;) $\endgroup$ – Klaus-Dieter Warzecha Nov 28 '16 at 21:34
  • $\begingroup$ Have a look at the definition of lipids in the IUPAC Gold Book. Lipids is as exact as calling a cow a four-legged animal that gives milk ;) $\endgroup$ – Klaus-Dieter Warzecha Nov 28 '16 at 21:42
  • $\begingroup$ There are several types of lipids. Usually lipid molecules consist of long chains of carbon atoms with few polar atoms, thus making them hydrophobic (water-fearing). Triacylglycerols (triglycerides), a glycerol molecule attached to 3 fatty acids of variable length are for example mostly used as an energyy source in the body. However they are not the only 'type' of lipids that exist. Another type of lipids are for example sterols, such as cholesterol, also water-fearing but have a completely different structure than triglycerides. They're both used differently in the body. $\endgroup$ – user21398 Nov 28 '16 at 22:17
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As you note, most biomolecules — or more precisely primary metabolites — exist as polymers that can theoretically be infinitely long and consist of well-defined monomers. This is true for nucleic acids and the building block nucleotides, for proteins made up of amino acids and for carbohydrates made up of monosaccharides.

Lipids, on the other hand, do not have that kind of polymeric possibility. In essence, they are trimers or monomers by definition, depending on how you look at them. Furthermore, all lipids contain the same (with same I mean identical) connective molecule namely glycerin or propan-1,2,3-triol $\ce{CH2OH-CHOH-CH2OH}$. Glycerin has three ‘sockets’, if you wish, namely its three hydroxy groups.

The ‘plugs’ that connect to glycerin always contain an acid functionality: typically this is either a carboxy group (fatty acids) or a phosphate (e.g. phophatidyl choline). The bond between ‘plug’ and ‘socket’ is an ester bond.

Both the fatty acids and the substituted phosphates can be structurally diverse. If you wanted, you could label the different fatty acids ‘monomers’ and the overall lipid molecule a ‘trimer’. However, the most typical difference between fatty acid residues is simply their length ($\ce{C16}$, $\ce{C18}$ or $\ce{C20}$ — typically multiples of 2) and their degree of unsaturation (e.g. stearate, oleate, linoleate and linolenate have increasing degrees of unsaturation). Attempting to teach students more than a few exemplary fatty acid residues or phophate residues is rather pointless since the choice of fatty acid hardly has any effect on the properties of the overall molecule, and aside from generally knowing what constitutes cellular membranes the same can in principle be said for the phosphates. Hence, students are hardly ever bothered.

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