OK, Lets get things straight. I am studying for my year 12 biology exam that's on the 11th of June. I am seriously freaking out.
The question I am about to ask is not a Homework question.

I do not understand the biology micromillimeter measurement. The one that looks like "um". I have seen in numerous amounts of times but it never seems to make much sense to me. :(
Can someone please explain it in a term that maybe I might be able to understand? :confused:
Thanks heaps, Danni

I'm sure you have looked up micromillimeter and found that it's a metric unit of length equal to one billionth of a meter. But this isn't the preferred SI unit (International System of Units). The SI unit is nanometer (nm) (= micromillimeter). The micrometer (micrometer = micron) has the symbol “um” and it is one millionth of a meter.

So a micromillimeter isn't "um".

See the Wiki SI prefix page SI prefix - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/SI_prefix)

From a biologist perspective a bacteria is about micrometer in size (E. coli is 1 X 2 um) while the diameter of a DNA double helix is about 2 nm

Just to slightly correct jem. The SI unit is the meter (m), but can be expressed with the SI prefix \mu to denote 10^{-6} meters.

As far as I'm aware, m, mm, um and nm are all equally valid (and so are the other SI prefixes).