Some of the hardest questions in IGCSE Biology are the 6 marker questions and the paper nearly always finishes with one which is more than often about carrying out an experiment. As there is no coursework for this course they must make sure they test you on your experimental knowledge throughout the paper.
If you can master these questions you are giving yourself a good chance of achieving a top grade.
In my new revision resource I have written 10 original example 6 marker questions followed by a mark scheme for each one. You can download it here:
Note carefully how many of the answers are looking for the same things, especially in the practical questions. The key is to think about the variables.
Remember that the aspect the scientist is altering is the independent variable. Whatever they measure or record in the experiment is the dependent variable and all other variables in the experiment must be controlled. If you explain how to vary the independent variable, how specifically to measure the dependent variable and how to control the other variables then you should do well.