How much in each science subject are definitions i.e. physics, chemistry biology? As in roughly what percentage of marks can be gained from a knowledge of all definitions (And for physics derivations as well) in each science subject?

I am not sure about the exact percentages but its definitely biology. And for biology, you can get asked a definition in any part of the exam. 

From looking at the past papers about 90% of the marks for Biology can be gained from knowledge of all the definitions and theory. Most questions are not direct definitions but theory questions like "describe the..." and "draw the...". Direct definitions are about 20% of the paper. As the paper is mostly theory based, calculations are quite rare unlike chemistry and physics. 

For chemistry on the other hand the marks are more mixed up. I would say about 55 - 60% can be gained from definitions and theory while the rest come from calculations. The first few lab experiment and titration questions start off with definitions and procedures, and generally has two parts at the end with calculations. This trend continues with the long questions with 60% theory and 40% calculations generally. 

For Physics about 30 - 40% of the marks can be gained from definitions, derivations and theory . You will notice that the physics course does not have all that many definitions compared to biology and chemistry and it is very calculation heavy. Although this may seem daunting at first if you have a good grasp of using formulas and you practice enough questions using past exam papers, the calculations will be second nature to you :)

These figures are just rough estimates but I hope this helps :)