The following link will take you to the AQA website (Biology section). On this page you will find past paper material for the AS and A2 Biology courses. Check you are working on the correct module if you download and work with anything!
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL1-W-MS-JUN09.PDF - Unit 1 Jun09 mark scheme
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL1-W-MS-JAN09.PDF - Unit1 Jan09 mark scheme
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL1-W-MS-JAN10.PDF - Unit 1 Jan10 mark scheme
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-MS-JUN09.PDF - Unit 2 Jun09 mark scheme
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL2-W-MS-JAN10.PDF - Unit 2 Jan10 mark scheme
http://store.aqa.org.uk/qual/gce/pdf/AQA-BIOL4-W-MS-JAN10.PDF - Unit 4 Jan10 mark scheme
Year 12 - before the January exam download Unit 1, after the January exam work with Unit 2.
Year 13 - before the January exam download Unit 4, after January exam work with Unit 5
Please note that there are also lots of old past papers in the 'legacy' materials section - you can also use these if you want to, just check against the new specification that you are not using anything irrelevant.