AP Biology Unit 3 Practice Test: Cellular Energetics MCQs (Free PDF)

Here's a free AP Biology Unit 3 practice test: 20 AP-style MCQs on cellular energetics, seven data-based stimulus sets covering enzyme kinetics, action spectra, mitochondrial poisons and fermentation, and a full rationale for every answer.

Below, three mistakes I see students make on Unit 3 and how to avoid them, plus one worked example so you can see the reasoning.

Unit 3 is one of the heaviest units on the paper at 12-16% (the full 8-unit breakdown has weightings for the rest), and the MCQs test whether you can trace what happens when one step in a pathway gets blocked.

The 60-question MCQ section is stimulus-heavy across every unit, but Unit 3 is where the table density peaks. Expect:

• Enzyme activity tables (rate vs pH, rate vs [substrate] with and without an inhibitor)
• 'Block this step, predict what accumulates' tables that use poisons like cyanide, oligomycin and DNP
• Action-spectrum tables that compare chlorophyll a absorption to photosynthetic rate at different wavelengths
• Light intensity vs O₂ release tables at different CO₂ concentrations
• Temperature vs metabolic rate tables across different organisms

The practice pack pulls all of that in: 14 of the 20 questions sit on data-table stimuli, and most stimuli support two or three questions in a row. Every item is tagged to a Unit 3 topic and a named Science Practice.

Every year I see students walk into Unit 3 MCQs making the same three moves. One is called out in a recent Chief Reader Report; the other two are the ones tutors flag most often. Spot them before the exam does.

Students frequently write that enzymes 'make a reaction release more energy' or 'change the free-energy difference'. Enzymes do neither. An enzyme lowers the activation energy needed to reach the transition state; it doesn't change delta-G, the equilibrium position, or the amount of energy released overall. Endergonic reactions stay endergonic with an enzyme; they just proceed faster. The correct phrasing for any MCQ or FRQ: 'the enzyme lowers activation energy by stabilizing the transition state'.

A large fraction of students think oxygen is used throughout cellular respiration, or that glycolysis and the Krebs cycle 'use' oxygen. Neither does. Oxygen only acts at the end of the electron transport chain, where it serves as the final electron acceptor and combines with electrons and protons to form water.

Without oxygen at the end, the ETC backs up, NADH can't be reoxidized, and the whole system stalls (which is why fermentation exists, to regenerate NAD+, not to make significant ATP). Our cellular respiration walkthrough traces every step of the pathway in the level of detail the AP actually tests.

MCQs will show a diagram of aerobic respiration and ask what happens if oxygen is removed; the correct answer traces the backup to the ETC, not to glycolysis.

Students often write that protons build up in the mitochondrial matrix, or that they flow from the intermembrane space to the matrix 'against the gradient'. This is exactly reversed.

In respiration, protons are pumped OUT of the matrix INTO the intermembrane space, making the intermembrane space more acidic and the matrix less acidic. ATP synthase then lets protons flow back DOWN the gradient into the matrix, and that flow powers ATP synthesis.

On any MCQ where a poison blocks part of the chain, this direction rule tells you what will accumulate where. If oligomycin blocks ATP synthase, for example, protons keep being pumped OUT but can't return, so the gradient grows and O₂ consumption slows because the ETC eventually backs up. Getting the geometry wrong flips every downstream prediction.

Here's a question straight from the Unit 3 pack. The stimulus: isolated mitochondria were provided with pyruvate, O₂, ADP, and inorganic phosphate (Pi). At time 0, various treatments were added and the rate of O₂ consumption was measured over the next 5 minutes.

The stem: 'Which conclusion about the electron transport chain is best supported by the cyanide treatment in Table 5?'

• (A) Oxygen is consumed at a normal rate even when the electron transport chain is fully blocked.
• (B) Cyanide accelerates electron flow but blocks ATP synthesis.
• (C) Cyanide uncouples oxidation from phosphorylation, raising O₂ use.
• (D) Cyanide prevents electrons from reaching O₂, so O₂ consumption drops.

Answer: D. O₂ is the final electron acceptor at Complex IV. Cyanide blocks Complex IV, so electrons cannot reach O₂ and O₂ consumption collapses from 220 to 5 nmol·min⁻¹. A is contradicted by the near-zero rate. B is wrong because blocking Complex IV halts electron flow. C describes DNP, not cyanide.

For every process in Unit 3 (glycolysis, Krebs cycle, ETC, light reactions, Calvin cycle), memorize inputs, outputs, location, and whether ATP/NADH/NADPH is made or used. When an MCQ asks 'what would happen if the Calvin cycle was inhibited', you don't reason from scratch; you check your chart and see that the light reactions would back up because NADP+ and ADP aren't being regenerated. The 2023 AP Biology Chief Reader Report Q4 rewarded exactly this traceback logic on photosynthesis.

Enzyme MCQs always give you a graph of reaction rate against either substrate concentration, temperature or pH. Say (or think) what each axis is measuring before you look at the answer choices, then match the shape:

• Substrate concentration: saturation curve that plateaus at Vmax
• Temperature: bell curve peaking at optimum
• pH: narrower bell curve

A competitive inhibitor raises the apparent Km (curve shifts right) but Vmax stays the same; a noncompetitive inhibitor lowers Vmax (curve flattens). The 2025 Chief Reader Report Q5 flagged competitive vs noncompetitive confusion as a Unit 3 gap that year, so this really is worth drilling.

Any MCQ that combines photosynthesis and respiration (usually via a plant cell diagram) is going to test whether you know that the O2 released by photosynthesis comes from water, not CO2. Splitting water in Photosystem II is the source of every oxygen molecule you breathe. If the stem uses isotope labeling (like ¹⁸O), the label on H2O ends up in O2, and the label on CO2 ends up in glucose. Recognizing this pattern in the first 15 seconds lets you skip past distractor answers about CO2 splitting.

Three-pass method.

• First, an untimed pass: no clock, about 90 seconds per question. The goal is diagnostic, to see which stimulus types (enzyme graphs, action spectra, mitochondrial poisons) trip you up.
• Second, a timed pass one week later: 30 minutes for 20 questions, matching the AP exam pace of about 1.5 minutes per MCQ.
• Third, a review pass: check the answer key at the back of the PDF, revise the weak topics on Cognito's AP Biology notes, then retry the missed questions a week after that.

The gap between passes is what makes the retention stick.