The structure one of my teachers gave me which I ended getting into the Russel Group with was:
- Why you want to do your subject
- How your other subjects link to your subject
- Extra-curriculars (20% of the wordcount max. Cambridge doesn't care about extracurriculars but you should include it for the other places you're applying to)
- Final line on your subject
Why you want to do it should be what interests you about it, and your reading should demonstrate intellectual curiosity! So the best structure you can use to introduce books is "After doing X in class, I pursued my question about it by reading X. [What you found out]" Or "in this book i found out X and then compared what I found to this talk [compare them]. This shows that you're an independent learner, which Oxbridge loves.
So a structure for biology (taking with a pinch of salt that I'm a Humanities student) could be something like-
- A line on why you want to pursue natural sciences/biology
- A paragraph on one area you like, referencing books and talks etc.
- Another paragraph on another area (I know for Humanities they like these to be different, eg. 20th century and 17th century history, I have no idea for STEM.)
- Achievements/interests in your other subjects, or how the subjects link to biology (it doesn't have to be direct, something as simple as "I developed an interest in logic")
- Extra-curriculars (don't spell them out too much! Recruiters will know that, for example, having a part-time job shows you're good at managing time)
- Final line just for neatness, I would recommend something like "I would love to pursue [what you like about your subject] at a university level"
Like any of the countless pieces of advice about UCAS, take this with a big pinch of salt, but I hope one example of a personal statement structure helps.