The essays, NatScis find, aren't really essays as such. Basically, they ask you to explain what a particular thing is- can't give any details, as I don't know anything about science at this level!- for which you have to read the relevant bits of the textbook, and copy from the lecture notes and so on.
More explanation than argument.
I'm not saying that they're easy or anything, just that they're a different type to an Arts-type "analyse then evaluate then argue" kind of essay.