AP Biology: Oxidative Phosphorylation Practice Questions
Connect electron flow to ATP production with original questions about mitochondrial membranes and simplified experiments. Track which process changes first when a component is altered. This is independent study practice, not an official AP resource.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
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1. Researchers separate a eukaryotic cell into fractions. Which fraction should contain both the respiratory electron transport complexes and ATP synthase used in oxidative phosphorylation?
Explanation. The inner mitochondrial membrane houses both systems. Its separation of the matrix from the intermembrane space permits a proton gradient to connect electron transport with ATP production.
2. A diagram shows electrons arriving at the end of the mitochondrial respiratory chain. Which label correctly identifies their final acceptor and the product formed with protons?
Explanation. In aerobic mitochondrial respiration, oxygen receives the terminal electrons and combines with protons to form water. ADP is a substrate for ATP synthesis, but it is not the final electron acceptor.
3. In actively respiring mitochondria, electron transport powers net proton movement across the inner membrane. Which direction builds the gradient used for ATP synthesis?
Explanation. Respiratory proton pumps move protons out of the matrix into the intermembrane space. This stores energy as an electrochemical difference across the inner membrane rather than immediately transferring a phosphate to ADP.
4. A mitochondrion has a proton gradient, available ADP and phosphate, and ATP synthase operating in its ATP-producing direction. Which movement supplies the immediate energy for ATP formation?
Explanation. ATP synthesis is coupled to proton movement down the electrochemical gradient through ATP synthase. In this operating mode, protons return to the matrix, and their movement drives phosphorylation of ADP.
5. A preparation supplies NADH to respiratory complex I. Which carrier change shows that NADH has donated electrons to the chain?
Explanation. Donating electrons oxidizes NADH to NAD+. The regenerated NAD+ can accept electrons again in other reactions. ATP formation depends on the downstream coupling machinery, rather than conversion of NADH itself into ATP.
6. A model compares one electron pair entering at complex I with one entering at complex II. All later steps are identical. Why can entry through complex II support less ATP production?
Explanation. Electrons entering through complex II bypass complex I, and complex II does not pump protons. With fewer pumping steps for that electron pair, less energy is stored in the gradient for ATP synthesis.
7. An isolated mitochondrial preparation exhausts its oxygen supply. No alternative final electron acceptor is present. After residual electron acceptors in the chain have become reduced, which process cannot continue at its previous sustained rate?
Explanation. Without oxygen to receive terminal electrons, the respiratory chain cannot sustain electron flow. A pre-existing proton gradient may briefly support ATP synthesis, so loss of oxygen does not imply instantaneous disappearance of every ATP molecule.
8. In a simplified mitochondrial experiment, a selective change lets protons cross the inner membrane without passing through ATP synthase. Electron donors and oxygen remain available, and respiratory complexes remain functional. Which outcome is most directly expected?
Explanation. This proton leak uncouples part of electron transport from ATP synthesis. Respiratory activity can continue, but returning protons that bypass ATP synthase dissipate energy without driving its ATP-producing rotation.
9. Two artificial vesicle preparations contain correctly oriented ATP synthase and accessible ADP and phosphate, but no respiratory chain. Only preparation P receives an imposed proton gradient in the direction that drives ATP synthesis. Which initial result supports chemiosmotic coupling?
Explanation. An imposed gradient can provide the immediate energy source for ATP synthase even without ongoing electron transport. The matched preparation without a gradient is the control. ATP production in P is temporary as the stored gradient is consumed.
10. A selective inhibitor closes the proton pathway through ATP synthase in tightly coupled mitochondria. Other proton leaks are negligible, and respiratory complexes initially remain functional. Why does electron transport then slow as the gradient builds?
Explanation. Blocking proton return allows a stronger electrochemical gradient to accumulate. This increases the energy cost of further proton pumping and limits coupled electron transport. The effect differs from a leak, which dissipates the gradient.
11. A teaching model specifies that four returning protons support production and delivery of one ATP equivalent. If 120 protons return through the coupled machinery with no leak or other use, what output does this model predict?
Explanation. Use the ratio stated for this model: 120 divided by 4 equals 30 ATP equivalents. The assumed cost includes delivery and is not a universal count for all ATP synthases or all cells.
12. In a simplified experiment, sample R consumes 6 units of oxygen and produces 24 units of ATP by oxidative phosphorylation. Sample S consumes 9 units of oxygen but produces 9 units of ATP. Measurements cover equal durations. Which conclusion follows directly from these data?
Explanation. R produces 24/6 = 4 ATP units per oxygen unit, whereas S produces 9/9 = 1. Reduced coupling is consistent with the lower ratio, but these measurements alone do not uniquely establish a proton leak or identify a specific molecular cause.
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FAQ
How can I distinguish electron transport from ATP synthesis?
Track two linked flows: electrons move through respiratory carriers toward oxygen, while protons move across the inner membrane and return through ATP synthase. A perturbation can affect their coupling, so oxygen consumption alone does not specify ATP output.
Should I memorize a single ATP yield for every mitochondrion?
Use the assumptions and ratios provided in a question. Electron entry points, transport costs, and proton leaks can affect yield, so an idealized calculation should not be treated as an exact total for every living cell.
What separates an ATP synthase block from a proton leak?
In a tightly coupled model, blocking the synthase prevents its proton return route and can allow the gradient to build. A leak provides a bypass that dissipates the gradient. Both can reduce ATP production, but they need not have the same effect on electron flow.