For your first question; well that's pretty much unanswerable. The amount of work you do isn't proportional to the scores you get, and there's just too many factors that contribute to what study score you will ultimately get so it's hard to nail it down to 'how hard it is'. However, using your second statement as a guide, well, anything on the exam will be answerable with what is strictly on the syllabus. Problem is Biology is quite broad and they occasionally (well perhaps often) put random scenarios with which you're meant to apply the course materials. While it will ALWAYS be possible to answer these questions with solely knowing the course itself, it obviously does help to know things before hand.
For example in my year, that randomn question about pseudogenes came up and no one really knew what they were and freaked. This question was answerable without knowing anything about pseudogenes, but I somehow happened to have already read about pseudogenes previously and found the question to be a breeze. Ultimately though, I would stress that solely knowing the course and knowing how to apply it is enough for even a 50, as long as you practise the skill of applying Biological concepts to any situation (i.e. grind trial exams! hurrah).
As for the marks you can lose...well, I lost 2 and a half marks on the mid year (score taken from Statement of Marks) and I'm guessing around 6 marks on the end year exam (counted by myself on questions that I know I got wrong+1 mark of careless error) so thats about 9 marks or so. Note that these scores are before doubling up. So as you can see, Biology typically leaves a lot of room for error.