What sort of questions are asked in the GCSE Biology paper?

I'm sitting my Biology Higher Tier exam on Tuesday and was just wondering if anyone knew what the main topics asked are? (E.g. Pollution, global dimming, natural selection etc.) I've been revising osmosis, diffusion, circulation etc, but to those of you who've already sat the exam, did you find it hard or relatively easy? My exam board is AQA.