Sciency Words: Macromolecule

Today’s post is part of a special series here on Planet Pailly called Sciency Words.  Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together.  Today’s term is:

MACROMOLECULE

After all the years I’ve been writing Sciency Words, I’ve noticed something.  A lot of times it might seem pretty obvious what a scientific term means, but then you dig a little deeper and find that the term is not so clearly or precisely defined as you’d expect.

Defining macromolecule should be easy.  Macro means big, molecule means molecule; ergo, a macromolecule is a big molecule.  But after I read this paper about the discovery of organic macromolecules on Mars, I had a question: just how big does a molecule need to be to get that macro- prefix?

German chemist Herman Staudinger is credited with coining the term macromolecule.  It was a highly controversial concept at the time.  Another German chemist, Nobel laureate Heinrich Wieland, wrote to Staudinger in the 1920’s saying: “My dear colleague, drop the idea of large molecules; organic molecules with a molecular weight higher than 5000 do not exist.”  But Staudinger would later become a Nobel laureate himself for proving that they do.

I take that Wieland quote to mean that the word macromolecule was defined as any molecule with a molecular weight in excess of 5000, but I’ve seen other sources claiming it was defined as any molecule containing one thousand or more atoms, and still other sources saying it’s ten thousand or more atoms.

But those were the kinds of definitions being used in the early 20th Century.  Modern usage gets far more complicated and confusing.  As Wikipedia explains, the definition of macromolecule “varies among the disciplines.”

• In biology, there are four kinds of macromolecule: lipids, proteins, nucleic acids, and carbohydrates. If it’s not one of those four things, it’s not a macromolecule, according to a biologist.
• Polymer scientists go by a definition set by the International Union of Pure and Applied Chemistry (IUPAC), which states that a macromolecule is a “molecule of high relative mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass.”
• Wikipedia also mentions a definition that involves aggregates of molecules sticking securely together due to intermolecular forces rather than covalent or ionic bonds.

It’s not unusual for one word to be defined in different ways by different fields (see my post on metallicity).  This is a big reason why some scientific terms end up being so difficult to define.

As for those organic macromolecules Curiosity found on Mars… in the context of that research, I think macromolecule simply means “very big molecule.”  Like I said on Wednesday, we don’t really know what, specifically, Curiosity found, and maybe we never will.  We just know that it must’ve had a lot of very big molecules in it.