AP Biology / Cellular Energetics
Lesson
Enzymes are picky accelerators
Shape, temperature, and pH decide whether a reaction actually happens in time.
Learning goals
- Explain activation energy and the induced fit of an active site.
- Predict how temperature, pH, or an inhibitor changes rate.
Explanation
An enzyme binds a substrate at an active site and lowers the energy barrier for a reaction. It is not used up. The site is chemically picky: a matching shape and charge work; a mismatch does not. Warmth can speed collisions up to a point; too much heat unfolds the protein and the rate collapses.
Competitive inhibitors occupy the active site. Noncompetitive inhibitors bind elsewhere and change the shape. In a lab graph, adding more substrate can sometimes outcompete a competitive inhibitor. That rescue is less available when the inhibitor changes the enzyme’s shape from the side.
Key terms
- Activation energy. The energy barrier that must be crossed for a reaction to proceed.
- Active site. The region of an enzyme that binds the substrate and performs catalysis.
Common mistakes
- Saying enzymes add energy to a reaction rather than lowering the barrier.
- Assuming every inhibitor works by blocking the front door of the active site.
Practice
Original Marlow Works items. Check the answer explanation after you try.
Explain why a cell that cannot carry out oxidative phosphorylation can still produce a small amount of ATP from glucose.
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Why does boiling usually stop an enzyme, while cooling it on ice often only slows it?
Answer. Boiling can denature the fold permanently; cooling reduces collisions without necessarily unfolding the protein.
Rate depends on both intact shape and molecular motion.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source