First thing to remember: this is a science extension report and science is a combination of physics, chemistry and biology and everything in science is closely linked together. It doesn't matter if it goes too deep into bio or chemistry, it does not matter. You can do both or only one of them, all it has to be is hypothesis-and-experiment based writing. So I would suggest instead of thinking of whether it would be too much into bio or chem, read literature. Read proper scientific articles done by real scientists and binge as many papers as you can. Choose what you like the best and thnk about what can you contribute: can you model it, or can you find something new, or can you confirm a hypothesis. Be super specific and try to think of it this way: can I do it in my school lab and can I add to what exists. I hope this clears some of the conflict you have but feel free to ask me more questions. Happy to help (just finished my sci extension report yesterday, so... I am free i guess.)
