Frogs and snails, And puppy-dogs’ tails

Raw cane sugar Demerara

According to the nursery rhyme* I was told when I was a child, boys are made from “Frogs and snails, And puppy dog’s tails” while girls are made from “Sugar and spice, And everything nice”. I am rewriting my biological physics lectures to talk a little bit about what we are really made of. This does include sugars, but is mainly water, protein, fats and related molecules, and mineral. We are roughly half water, with the remainder 20% protein, and 10% each of lipids and the mineral in our bones. Lipids are the molecules that make up the membranes in our cells as well as what we call fat. There are smaller amounts of lots of other stuff, like sugars, DNA, etc.

Apart from the mineral in our bones we are made of soft matter. Water, polymers and fats are all examples of soft matter. My biological physics lectures are part of a Soft Matter and Biological Physics course, so part of what I aim to do is show our students that soft matter is relevant to them, by showing them that they are made of the stuff.

Polymers are particularly important. Polymers are simply big molecules formed by sticking hundreds or more small molecules together end-to-end. This makes a long chain-like molecule: A polymer. Proteins are polymers, as are DNA and RNA. Incidentally, although DNA hogs most of the glory as it is the polymer your genes are made of, you have about ten times as much RNA in your body as DNA.

As polymers, proteins, RNA and DNA, do all the work in you cells, in some sense biology is really applied polymer science. I have to say that biologists don’t typically view it like that, but maybe final year physics students will be more open minded.

*Nursery rhyme in full:

What are little boys made of?

Frogs and snails

And puppy-dogs’ tails

That’s what little boys are made of

What are little girls made of?

Sugar and spice

And everything nice,

That’s what little girls are made of.

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