Hey Donut!
I'm assuming that you are starting bio units 3/4?
Biology 3/4 is very manageable without 1/2, especially if you have studied at least a little bit of 1/2. I haven't done this myself, but this is what I've heard from others, and judging from VCAA's study design
https://www.vcaa.vic.edu.au/Documents/vce/biology/BiologySD-2016.pdf, it certainly should be.
Unless you want to be way ahead, you can just enjoy your holidays. Maybe brush up on bio terminology but if you do want to get really ahead anyway,
units 1/2 cover these two topics
- How do living things stay alive?
- How is the continuity of life maintained?
So for unit 1, I recommend you try to cover cell organelles, phospholipid bilayer, adaptations, homeostasis, biodiversity and population growth.
For unit 2 try to cover reproduction, stem cells, genetics (DNA, pedigree charts, mutations), ethics of genetic screening.
Some good resources to direct what you should study are:
https://www.vcaa.vic.edu.au/Documents/vce/biology/Biology_study-summary_2016-2021.docxhttps://www.vcaa.vic.edu.au/Pages/vce/studies/biology/biologyindex.aspx#H2N10023Some good learning resources can be found all over the web, but something I found very useful was:
https://micro.magnet.fsu.edu/cells/animalcell.htmlhttps://learn.genetics.utah.edu/content/basics/dnaHope this helps!

Twisty314