For self-assessment questions, with exercises and discussions, just click on the correct topic at Biology GCSE & IGCSE Question Bank. The s-cool revision site lets you revise the topic, summarizes the topic & tests you on it. Skoool gives you a lesson, including progress questions, for each Biology topic but you’ll need sound on for it. GCSE Bitesize has revision pages (that take you through the topic), a test and an activity.
An excellent powerpoint that covers it all: