Don't worry! Mock grades are not going to determine your summer grades. A lot of the time you will perform better in the summer due to the pressure of it being your final exam.
I do OCR Biology (I am doing A2) and at AS I just made sure I did loads of past papers, looked at exam reports (these are very useful to see how people structure their answers) and went through all of the content thoroughly. I would advise that instead of just reading and writing the notes, actually try and understand it as A-levels (and particularly the sciences) are about applying your knowledge, not just remembering it and re-writing it in the exam.
What are you predicted in Biology and what did you get at GCSE?
The only thing, as with any a-level is to work VERY hard and smartly, make sure your revision is actually effective, try revision cards as these are good! How much revision to you do a day for Biology?
Do you use the main endorsed text book? If so this is very good, but it doesn't really help with understanding the content. I would recommend this revision guide as it really helped me get to grips with concepts.
http://www.amazon.co.uk/AS-Level-Biology-OCR-Student-Book/dp/1847627897/ref=sr_1_3?ie=UTF8&qid=1424638052&sr=8-3&keywords=as+level+ocr+biologyBest of luck and don't panic, I am sure you will do really well!