In short
An enzyme is a protein that speeds up a chemical reaction in a living thing without being used up. Each enzyme works on one kind of molecule, its substrate.
The substrate fits the active site of the enzyme, like a key in a lock, and the products leave. Enzymes work best at an optimum temperature and pH. Too much heat or the wrong pH changes the shape of the active site, which is called denaturation. The enzyme then stops working. Examples are amylase in saliva, which digests starch, and catalase, which breaks down hydrogen peroxide.
Human enzymes work best near . Pepsin in the stomach has an optimum pH of about , while amylase in saliva works best near pH .
Pick your exam to see its syllabus, where you stand in each unit and what to practise next.
See exam preparation