Don't worry, if you're anywhere near capable it's pretty easy to get an A, we had 2 out of 50 people get As the year before I started which was a bit disturbing. It's generally because a lot of people who aren't any good at science take Biology because they think it's an easy option. I did AS last year and was one of 3 to get an A, my advice would be to revise using the OCR textbook (not the revision guide, it's crap); essentially just go over and over every single page until you have the textbook memorised. Pretty much full UMS in the bag