I disagree. This would be a good idea in other subjects, where you are under stress for time, something which is not the case in biology. In addition, you will find that the last few marks for a large question are often awarded for added details not explicitly specified in the question (if you look at the marking criteria you will know what i mean).
What I recommend instead is 'the scattergun method'. Answer the question directly in the first few lines, then if you have time, go out and add more detail to them.
If you're at the band six level, you'll easily finish the test with two hours to spare. Take the liberty to pad out your answers. Yes, it is INCREDIBLY ASSHOLISH to do this, but they will not deduct marks for extra information, they will only do so if you contradict yourself or say something incorrect.
(spoilers: i did three pages of writing for the last question in the exam. the examiner must want to kill me :| )