Purely memory can only get you so far; under pressure, we often forget the basics, so it's in our best interest to go one step beyond memory: application.
Content-heavy subjects, like biology, assesses you to understand the syllabus in full. Each dot point connects with another, so to minimise the work we are required to do, try to create a mind map listing out the dot points in each topic with links to another dot point. For instance, if antigenic particles trigger the immune response and we have antigens in our own organs, why doesn't our body trigger an immune response?
Understanding and asking questions is often the best way, then, to approach your studies. This will then lead you to being able to retain information clearer than just memory, because it's embedded into your knowledge now. Then we can begin to tackle questions that require specific language (jargon), particularly questions involving organ rejection.
In summary:
1) Do you know your content well enough?
2) Can you interpret the question correctly?
3) Are you able to sustain a logical response?
If you answer no to any one of these questions, you do not understand the content; you just memorise them.