AP Biology · Unit 3 · Topic 3.1
A reaction that would happen anyway can be far too slow to be any use. An enzyme does not change where the reaction ends up — only how hard it is to get there.
Learning objective 3.1.A Explain how enzymes catalyze reactions by lowering the activation energy, and what that does and does not change.
This is the graph the exam asks you to read. Switch the enzyme on and off and watch carefully — one thing on this diagram changes and two things do not, and knowing which is which is most of the topic.
Activation energy is measured from the reactants up to the peak — not from the bottom of the diagram, and not from the products. ΔG is measured from the reactants across to the products, and it does not care about the peak at all. That is why one of them changes when you add an enzyme and the other cannot.
Molecules do not all have the same energy. Lowering the barrier does not give any single molecule more energy — it lowers the bar, so a bigger share of the collisions already clear it.
Switch to the endergonic reaction and add the enzyme. The barrier drops and the reaction speeds up, exactly as before — and ΔG is still positive. No amount of catalysis will make an uphill reaction run downhill.
Start with the same amount of substrate in both flasks — one with enzyme, one without — and let them both run until they are finished. Predict the two end points before you press run.
Six claims about what an enzyme does. Two are true. Decide each one before you look at the reasoning.
Answer these in your own words before you open the answers.
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