We can roughly divide most phenomena of biological interest into 3 scales: the milliworld (1 metre to 1 millimetre), the microworld (1 millimetre to 1 micron), and the nanoworld (1 micron to 1 nanometre).
To get a first, rough intuition about these scales, it helps to figure out what belongs where. You already know plenty of biological phenomena that belong in the milliworld — things like cats, mice, eyeballs, fruitflies… Notice that that’s quite a range. The smaller worlds are harder to visualise because you don’t see them every day.
So whats in the microworld? Starting with the biggest stuff and going down :
- protists and single-celled animals
- cells of plants or animals (eukaryotes)
- bacteria (prokaryotes)
And whats in the nanoworld? Again, starting with the biggest stuff:
- parts of cells (like mitochondria, chloroplasts, and so on) are right on the border between the micro and nano worlds
- visible light wavelength
- cell membranes
- large molecules (like proteins, carbohydrates, hemoglobin, etc)
- DNA double helix
- small organic molecules (like simple sugars)
I am not trying to say that there is some kind of magical division between the microworld and the nanoworld, or that the rules of physics suddenly change, or anything like that. The reason that I’m talking about microworld and nanoworld is just to give you a point of reference, to help you figure out which units will be most convenient, and which biological structures have similar sizes.