For me, this is the hardest Biology Unit of the 3 (or 4).
You just need to do past paper after past paper. There's a lot more knowledge in this topic than in U3, and the questions were worded very weirdly for me last June. I still managed to get an A, but it was my lowest Science mark by far.
Perhaps it's a good idea, then, for you to look at the June 2011 paper.
-shrugs- I do it now and get a much higher mark, so maybe it was a slip-up.
Good luck either way!