I know all the theory, but I can never score well with the essay questions in paper 2. Why?

As you've probably noticed, the mark scheme is incredibly specific about the terminology that you use. You could describe the process of, say, endocytosis, to great accuracy, but if you don't use the exact key words specified in the mark scheme, you could score zero marks.

What's annoying here is that the terminology in the mark scheme is different to the terminology used in the course textbooks and most study aids. Website Bioninja is fantastic. It works through every part of the syllabus in bullet points (which is how you should always answer essays in bio exams!) with mark scheme key words.

But bullet points aren't the best way to learn theory, so I'd say stick to textbooks/other study aids to learn theory until you understand all the biological processes inside out, and then go through past exam mark schemes and the Bioninja website during revision, making notes about the specific words they use to describe the processes.

For example, big key words for endocytosis are: -INFOLDS as opposed to 'invaginates' or 'folds in' or 'envelops' -TRAVELS as opposed to 'moves' -PLASMA MEMBRANE as opposed to 'cell membrane'

Asides from key words, you also have to make sure you're covering all you're bases in terms of content. You might've felt exam questions are worth way more marks than you think you can write for! I did, all the time. A question asking you to describe endocytosis could be worth 6 marks! Well, don't fall into the trap of only describing the process. The mark scheme also wants you to start by DEFINING the process ("bulk movement into a cell"), and state the CONSEQUENCES of what happens, not just what happens. For example, the infolding "creates a vesicle" for the smaller cell. Once it's travelled into the cytoplasm it's "digested or retained if it is a useful substance".