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  By Zaheer | Updated 13 October 2026 Disclosure: I wrote the free Enzymes guide and sell the O Level Biology Lab mentioned at the end. Everything in this post is free to use without either. This is independent study material, not affiliated with Cambridge or any exam board. Temperature and pH are the two factors that appear in almost every enzyme question in IGCSE and O Level Biology. Students often learn them separately and then mix up the reasons in the exam. This post puts them side by side, so you can see what is the same, what is different, and how to write each one correctly. What they have in common Every enzyme has an optimum , the temperature or pH where its activity is highest. Move far enough away from the optimum and the enzyme is denatured . The shape of the active site changes, so it is no longer complementary to the substrate. A denatured enzyme cannot form enzyme-substrate complexes, so the rate falls. Where they differ ...

  By Zaheer | Updated 13 October 2026 Disclosure: I wrote the free Enzymes guide linked at the end. Everything in this post is free to use without it. This is independent study material, not affiliated with Cambridge or any exam board. A 5-mark "explain" question on enzymes looks scary, but it is one of the most predictable questions in IGCSE and O Level Biology. The same few ideas appear every time, and the marks go to students who put them in the right order. This post shows a four-step method, compares a weak answer with a strong one, and gives you three questions to try. Why "explain" is different from "describe" Describe asks what happens: "the rate rises, then falls". Explain asks why : you must give the reason behind each change. Students who only describe the graph get the first one or two marks and then stall. The remaining marks are for the reasons. The four-step method Start with the change. Name what is ch...

  By Zaheer | Updated 13 October 2026 Disclosure: I wrote the free Enzymes guide linked at the end. Everything in this post is free to use without it. This is independent study material, not affiliated with Cambridge or any exam board. Most students who lose marks on enzyme questions know the science. They lose them because of one loose phrase. "The enzyme dies" is a perfectly natural thing to write, and it is wrong in a way that costs marks. Here are six of the most common slips, why each one is wrong, and the wording that fixes it. Why one word matters Biology answers are marked on scientific ideas, and each idea needs the right term. "The shape changes" and "the active site changes shape, so the substrate no longer fits" describe the same event, but only the second names the idea an examiner is looking for. Fixing your vocabulary is often the quickest way to gain marks on a topic you already understand. The six traps What s...