Explain how enzymes speed up specific chemical reactions in living things through a shape-matching mechanism, using real kitchen investigations as evidence.
An enzyme is a protein that speeds up, or catalyzes, a specific chemical reaction in a living thing, without being used up itself in the process. Enzymes work through a very specific shape match, often described as "lock and key": an enzyme's shape fits one particular target molecule, called its substrate, the way a specific key fits one specific lock. That precise shape is also why enzymes are sensitive to their conditions -- too much heat, or the wrong pH, can distort an enzyme's shape (called denaturing it), so it can no longer fit its substrate and stops working.
Your own kitchen has convincing enzyme evidence in it. Saliva contains an enzyme called amylase that begins breaking starch down into sugar the moment food enters your mouth -- which is why a bland, unsalted cracker starts tasting noticeably sweeter the longer you chew it, without swallowing, as more starch converts to sugar. Fresh pineapple contains an enzyme called bromelain that breaks down proteins, which is why fresh pineapple can keep a gelatin dessert (made of protein) from ever setting -- while canned or cooked pineapple, already heated enough to denature the enzyme, does not cause this problem.
Mastery looks like: They can explain the lock-and-key enzyme mechanism, describe a real kitchen investigation demonstrating enzyme activity, and explain what denaturing does to an enzyme.
Common stumbles: Thinking enzymes are used up or destroyed during a normal reaction; forgetting that heat affects enzymes by changing their SHAPE, not by some vaguer "killing" process.