Can a single monomer be a macromolecule? If so, when? 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.
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.