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My biology class and I have been on the topic of macromolecules for quite some time now. Chapter 2.3 of the Foundations to Biology Textbook says that lipids are not polymers, so they do not have monomers. It is also stated in our Macromolecules powerpoint.

Now here is what confused me. We did a macromolecules review assignment to prepare for our upcoming biology test. We had to match each macromolecule with its monomer, but that is when I noticed that "No monomer" wasn't an answer; glycerol and fatty acids were the only choices for the monomer of lipids.

This really had me baffled, I thought it must've been a typo on the assignment. So I decided to search it up. I searched for "What is the monomer of Lipids", and google said exactly: Lipids - polymers called diglycerides, and triglycerides; monomers are glycerol and fatty acids.

I still didn't believe what I was reading so instead I searched "What macromolecule doesn't have a monomer?", and google said exactly: Lipids are not true macromolecules because the monomers are not covalently bonded together. Simple lipids are composed of subunits made of fatty acids covalently bonded to a triose sugar – glycerol.

This really confused me because it was saying that Lipids aren't a true macromolecule. I did more research and found that Lipids are polymers, but that is not what we've been learning.

We've learned that Lipids aren't a polymer and that they have no monomer, but google is saying otherwise.

So what is it? Do they have monomers or not? Are Lipids actually considered a polymer?

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    $\begingroup$ I fear for your education for several reasons, but the most pertinent is that the term lipid has no precise structural chemical meaning, and refers to a group with similar physical properties that includes steroids such as cholesterol. Presumably the ignoramus who set this educational abomination — MCQ — was thinking of triglycerides, but wanted to hide the “tri” portion because of his absurd answer. No modern biochemist has the slightest interest in the application of chemical terminology such as polymer, but it seems to me fatty acids themselves justify that description. $\endgroup$
    – David
    Sep 26, 2022 at 21:13

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No, classic lipids are not considered polymers. Organic chemists have specific names for aliphatic chains of specific sizes, so there‘s no need for polymer terminology.

For example, fatty acids are kept at specific lengths and cells generally don’t elongate them to large sizes. (They can be connected by glycerol. And glycerol can carry only three fatty acids. But that‘s it)

Fatty acids themselves might be called a polymer (repetition of CH2) if one really wishes, but that‘s like calling propane (C3H8) „polymethylene“ or „trimethylene“, treating it like a polymer of three monomers. But no one does that. Also, fatty acids of different sizes have their own specific names (e.g. oleic acid), while polymer naming systems are usually blind to size.

Polymers are generally composed of roughly >100 monomers (Be aware that everyone sets their own threshold at which size something‘s a polymer). With fewer than that they‘re also called oligomers (oligo means few, poly means many)

lipids aggregate together forming membranes and droplets. But that‘s just lipid molecules sticking together like water molecules do. And water also is no polymer.

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    $\begingroup$ You are confusing lipids with triglycerides and pulling numbers out of a hat. If you think about the structure and synthesis of fatty acids they are clearly polymers — they result from the repeated addition of malonyl moieties resulting in multiple CH2 groups. Much more similar to chemical polymers than proteins, nucleic acids or polysaccharides. $\endgroup$
    – David
    Sep 26, 2022 at 21:27
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    $\begingroup$ Yup, that 100 is definitely a hat-derived number. But most chemists will agree with me that they „feel“ that number when talking polymer (you may dig up some definition from your favorite literature, if you wish). And yeah, you can call the aliphatic chains of fatty acids polymers of CH2. Just like you can call water dihydrogen-monoxide. But you‘re gonna be the only one who does that. $\endgroup$
    – markur
    Sep 26, 2022 at 21:38
  • $\begingroup$ @markur lipids absolutely do polymerize, lipids as David said lipids formed of many CH2 groups just like polyethylene. If lipids are not polymers, most plastics are not polymers. Also don't confuse "lipids" with triglycerides, lipids come in a huge variety, many drastically exceed 100 subunits. And you need a source for a claim like polymers need to be longer than 100 subunits, because that excludes a lot of biological polymers not just lipids. $\endgroup$
    – John
    Sep 27, 2022 at 21:28
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    $\begingroup$ Please explain why a biology beginner should know about lipids with the size of 100 carbons. $\endgroup$
    – markur
    Sep 27, 2022 at 22:59
  • $\begingroup$ Edited my answer to prevent misinterpretation. Also explaining why calling e.g. „oleic acid“ a „polymer“ is overkill. $\endgroup$
    – markur
    Sep 27, 2022 at 23:19

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