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In our science class, we're learning about the four fundamental macromolecules in every living organism: carbohydrates, proteins, lipids, and nucleic acids. Our class definition of a macromolecule essential is:

A macromolecule is a large molecule that is essential to all life, and present in all living cells. It is a polymer- a chain of monomers.

Our definition of a monomer is like so:

A monomer is the simplest building block of a macromolecule with the properties of that macromolecule. They can be strung together to produce a macromolecule (usually by dehydration synthesis).

I would have no problem with these definitions if not for my teacher mentioning once that some monomers can also be macromolecules by themselves. Because some monomers of certain macromolecules- such as the monosaccharide glucose vs. the disaccharide sucrose or the polysaccharide amylose - can act on their own as an essential and functional carbohydrate, they are macromolecules by themselves.

Is this true? For example, could glucose be a macromolecule by itself?

Thanks.

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  • $\begingroup$ Welcome to chemistry.SE! You can always visit the help center or take a tour whenever you want, though it seems you didn't need this comment. $\endgroup$ – M.A.R. Dec 30 '14 at 20:55
  • $\begingroup$ How can a lipid be considered a macromolecule? A lipid doesn't have repeating units like polymers (usually the fatty acids are different as well). $\endgroup$ – LDC3 Dec 31 '14 at 6:43
  • $\begingroup$ @LDC3 that's why I asked if some, not all, monomers could be macromolecules. I understand that a fatty acid can't be a full lipid. $\endgroup$ – Jonathan Lam Dec 31 '14 at 13:18
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    $\begingroup$ When 2 or 3 fatty acids bond with glycerol, you have a lipid. Usually, in diglycerides, one fatty acid is saturated, the other is not saturated, and the third position has a phosphate bonded to choline (or serine, etc). Since lipids don't make larger molecules, then they would be monomers. About the only reason they would be considered macromolecules is if the definition has a low starting point (like 1000 daltons or 60 atoms). $\endgroup$ – LDC3 Dec 31 '14 at 16:04
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I can't think of an example where a biological monomer would be a macromolecule.

Definitions of macromolecule vary, usually by molecular weight or number of monomers (repeat units).

Personally, I'd go with ~1000 Dalton for a minimum, but the original definition of 1000 atoms is a good start too.

In any case, no biological monomer, including glucose will function the same as a macromolecule.

Consider starch - a macromolecule of sugars. It doesn't dissolve as quickly as simple sugar and has different physical properties.

Update

To clarify my comments.. Macromolecules or polymers are made up of monomers the way words are made up of letters. So no, a glucose molecule isn't really the same as a macromolecule, just like "R" is not a word.

Yes, macromolecules can be used to make larger assemblies like microtubules, filaments, etc., much the same way that words can form sentences and paragraphs.

In the polymer literature there's even the concept of a "macromonomer" referring to a monomer that is already large in size.

In my opinion though, the basic constituents (monomers) are still amino acids, nucleic acids, sugars, etc. These are not macromolecules.

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  • $\begingroup$ Well if you look at @user137 answer, then are there some exceptions to your answer? $\endgroup$ – Jonathan Lam Jan 8 '15 at 22:46
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    $\begingroup$ It's a bit different. In this case, I'd consider these supramolecular assemblies. Yes, the protein is being repeated into a larger structure. But at least from my standpoint, it's like saying a macromolecule is like a word - made up of individual letters (monomers). Of course you can put words into sentences (supramolecular tubules, etc.) but the basic block is still the letters (DNA, sugars, amino acids). $\endgroup$ – Geoff Hutchison Jan 9 '15 at 5:06
  • $\begingroup$ Certainly as I read your question, it seemed as if you were asking "can a monomer act like a macromolecule" (i.e., can a letter act like a word). No, not really. Can we use words (macromolecules) to put together bigger things? Yes. $\endgroup$ – Geoff Hutchison Jan 9 '15 at 5:08
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If you're a little looser with your definition of monomer, then Tubulin fits right in. Alpha and Beta Tubulin proteins bind to each other to form Microtubules which are an important component of cell's cytoskeleton and play roles in moving cellular components around the cell, especially during cellular division. And tubulin's not the only protein to act as a monomer, there is also Actin, which produces Microfilaments. I'm sure there are others, but I don't have the time to dig them up right now.

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Macromolecule can be a monomer, but it's a little tricky because macromolecules are usually polymers by themeselves. I think that only common "monomeric" biomolecules which create oligomers, or even polymers are proteins - for example hemoglobin is present in erythrocytes as tetramer.

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