AP Biology, AP exams, 12–16%

Unit 3: Cellular Energetics

Unit 3 begins with enzymes as the controllers of reaction rate, then moves through photosynthesis and cellular respiration as coupled energy-conversion pathways. The exam is far more interested in inputs, outputs, locations, and the effect of perturbations than in memorized intermediate names. Expect graph interpretation on enzyme kinetics and environmental effects.

Position: 3 of 8 units
Published weighting: 12–16% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Enzyme structure and catalysis: Active-site specificity, activation energy, and why enzymes change reaction rate without changing equilibrium.
Environmental effects on enzyme function: Temperature, pH, substrate and enzyme concentration, and competitive versus noncompetitive inhibition.
Cellular energy and ATP: Coupled reactions, energy carriers, and the role of ATP and reduced electron carriers as usable energy currency.
Photosynthesis: Light-dependent reactions in the thylakoid membrane and carbon fixation in the stroma, with their inputs and outputs.
Cellular respiration: Glycolysis, pyruvate oxidation, the Krebs cycle, oxidative phosphorylation, and anaerobic alternatives.
Energy flow and fitness: How organisms capture, store, and expend energy, and the fitness consequences of the resulting trade-offs.

Study tips

How to practice Cellular Energetics.

Section-specific guidance that follows from how this material is usually tested.

For each pathway, memorize location, inputs, outputs, and net ATP first; the intermediate names carry far fewer points.
Practice perturbation questions out loud: block one step and predict what accumulates upstream and what disappears downstream.
Sketch and label enzyme rate curves for temperature, pH, and substrate concentration until you can produce them under time pressure.

Where this fits

Unit 3: Cellular Energetics is 3 of 8 units in the AP Biology guide, and it carries a published weighting of 12–16% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Cellular Energetics questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What effect does an enzyme have on the activation energy of the reaction it catalyzes?
1. AIt raises the activation energy
2. BIt lowers the activation energy
3. CIt has no effect on the activation energy
4. DIt lowers the free energy change of the reaction
Show answer
Answer: B. It lowers the activation energy
Enzymes stabilize the transition state, which lowers the energy barrier a reaction must cross and therefore speeds it up. The tempting distractor is the claim about free energy: an enzyme never changes the free energy difference between reactants and products, so it cannot change the equilibrium position or make an unfavorable reaction favorable.
Key insight: Enzymes change how fast equilibrium is reached, never where equilibrium lies.
Question 2Foundational
In aerobic cellular respiration, which molecule serves as the final electron acceptor at the end of the electron transport chain?
1. ACarbon dioxide
2. BNAD+
3. COxygen
4. DPyruvate
Show answer
Answer: C. Oxygen
Oxygen accepts electrons at the end of the chain and combines with protons to form water. Carbon dioxide is the tempting distractor because it is a product of respiration, but it is released during pyruvate oxidation and the Krebs cycle rather than accepting electrons. Without oxygen the chain backs up, NADH cannot be reoxidized, and the Krebs cycle stalls.
Key insight: Oxygen is the electron sink that keeps the chain moving, which is why water is a product of respiration.
Question 3Intermediate
Which two energy-carrying products of the light-dependent reactions are consumed by the Calvin cycle to reduce carbon dioxide into sugar?
1. AATP and NADH
2. BATP and FADH2
3. CNADPH and oxygen
4. DATP and NADPH
Show answer
Answer: D. ATP and NADPH
The light-dependent reactions in the thylakoid membrane produce ATP and NADPH, which are released into the stroma where the Calvin cycle uses them to reduce 3-phosphoglycerate to G3P. NADH is the tempting distractor because it is the analogous carrier, but it belongs to cellular respiration rather than photosynthesis. Oxygen is a by-product of splitting water and is not consumed by the Calvin cycle.
Key insight: Photosynthesis carries electrons on NADPH; respiration carries them on NADH.
Question 4Intermediate
During oxidative phosphorylation, what directly powers the synthesis of ATP by ATP synthase?
1. AThe flow of protons down their electrochemical gradient
2. BThe direct transfer of a phosphate group from NADH
3. CThe oxidation of glucose to pyruvate in the cytosol
4. DSubstrate-level phosphorylation in the Krebs cycle
Show answer
Answer: A. The flow of protons down their electrochemical gradient
The electron transport chain pumps protons into the intermembrane space, and those protons flow back through ATP synthase, turning it like a rotor to phosphorylate ADP. NADH delivers electrons to the chain but never hands a phosphate to ADP. Substrate-level phosphorylation does make ATP directly, but it happens in glycolysis and the Krebs cycle and does not involve ATP synthase.
Key insight: The transport chain builds the gradient; the gradient, not the electrons, drives ATP synthase.
Question 5Advanced
A compound makes the inner mitochondrial membrane freely permeable to protons while leaving every electron transport protein intact. What is the immediate effect on the mitochondrion?
1. AATP synthesis rises because protons reach ATP synthase more easily
2. BATP synthesis is unchanged because NADH is still oxidized
3. CATP synthesis falls while electron transport continues and releases heat
4. DElectron transport stops instantly because oxygen can no longer bind
Show answer
Answer: C. ATP synthesis falls while electron transport continues and releases heat
Such a compound is an uncoupler: protons leak back across the membrane without passing through ATP synthase, so the gradient collapses and ATP output drops sharply. Electron transport actually speeds up because the gradient no longer opposes proton pumping, and the energy that would have been captured in ATP is released as heat. The tempting answer is that ATP rises, but ATP synthase needs protons to move through it, not merely to move.
Key insight: Uncoupling separates electron transport from ATP synthesis, turning stored energy into heat.

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Unit 3: Cellular Energetics