• 54 questions

AP Bio Cellular Respiration and Photosynthesis Practice Test

Unofficial multiple-choice practice for Unit 3 of AP Biology, Cellular Energetics: enzymes, photosynthesis and cellular respiration.

Which are the inputs of photosynthesis in a plant?

Pick an answer, then press Check answer. Then you see Correct or Not quite, with the reason. Keys: A to D pick, Enter checks.

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Unofficial practice questions, not affiliated with the College Board. Bank checked October 9, 2026.

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All 54 questions

Topic 3.1: Enzymes 8

  • After an enzyme has converted a molecule of substrate into product, what is true of the enzyme? Wikipedia, "Enzyme"; RCSB Protein Data Bank, PDB-101 Molecule of the Month, "Catalase"; course topic 3.1
  • Catalase splits hydrogen peroxide into oxygen gas and water. A student adds catalase solution to hydrogen peroxide at five different pH values and measures the oxygen released in one minute. Which is the dependent variable? RCSB Protein Data Bank, PDB-101 Molecule of the Month, "Catalase"; course topic 3.1
  • How does an enzyme increase the rate of a reaction? Wikipedia, "Enzyme" and "Activation energy"; course topic 3.1
  • In the induced-fit model of enzyme action, what happens when the substrate binds? Wikipedia, "Enzyme"; Nature Education, Scitable, "Cell Metabolism"; course topic 3.1
  • In the same catalase experiment, which extra tube is the best control for showing that the oxygen comes from the action of the enzyme? RCSB Protein Data Bank, PDB-101 Molecule of the Month, "Catalase"; course topic 3.1
  • The rate of an enzyme reaction is measured as substrate concentration rises while the amount of enzyme stays the same. Why does the rate level off at a maximum? Wikipedia, "Enzyme", section "Kinetics"; course topic 3.1
  • Which statement about what an enzyme can do is correct? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.1
  • Why does a given enzyme usually act on only one substrate, or on a few closely related ones? Wikipedia, "Enzyme"; course topic 3.1
Show all 54 questions (46 more)

Topic 3.2: Environmental impacts on enzyme function 9

  • A molecule that resembles the substrate binds an enzyme's active site and slows the reaction. Adding much more substrate brings the rate back up. What kind of inhibitor is it? Wikipedia, "Enzyme inhibitor", section "Competitive"; course topic 3.2
  • An enzyme is heated well above its optimal temperature and stops working. What has happened to it? Wikipedia, "Denaturation (biochemistry)" and "Enzyme"; course topic 3.2
  • An enzyme is slowed by a noncompetitive inhibitor. What is the predicted effect of adding a large excess of substrate? Wikipedia, "Enzyme inhibitor", section "Non-competitive"; course topic 3.2
  • An enzyme shows peak activity at pH 7 and almost none at pH 3 or pH 11. What is the best explanation? Wikipedia, "Denaturation (biochemistry)"; course topic 3.2
  • An inhibitor binds an enzyme at a place separate from the active site, and the enzyme changes shape. What is that binding place called? Wikipedia, "Allosteric regulation"; course topic 3.2
  • In many metabolic pathways the final product inhibits an enzyme that acts early in the same pathway. What does this accomplish? Wikipedia, "Enzyme inhibitor" and "Metabolic pathway"; course topic 3.2
  • Is the denaturation of a protein always permanent? Wikipedia, "Denaturation (biochemistry)", section "Reversibility and irreversibility"; course topic 3.2
  • Which bonds, easily disturbed by heat and by changes in acidity, help hold an enzyme in its folded shape? Wikipedia, "Denaturation (biochemistry)"; course topic 3.2
  • Why does the rate of an enzyme reaction rise as temperature increases toward the optimum, then fall beyond it? Wikipedia, "Enzyme"; course topic 3.2

Topic 3.3: Cellular energy 6

  • A metabolic pathway runs A to B to C to D, with a different enzyme for each step. If the enzyme that converts B to C is missing, what is expected? Wikipedia, "Metabolic pathway"; course topic 3.3
  • Burning a sugar and oxidizing it step by step in a cell release the same total energy. What is the advantage of the many small steps? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.3
  • Glycolysis occurs in almost all cells, prokaryotic and eukaryotic alike. What does this wide occurrence indicate? Wikipedia, "Glycolysis"; Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.3
  • How do cells drive reactions that require an input of energy? Wikipedia, "Enzyme" and "Metabolic pathway"; course topic 3.3
  • What ultimately supplies energy to nearly all cells? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.3
  • Which pairing of pathway type and energy is correct? Wikipedia, "Metabolic pathway"; course topic 3.3

Topic 3.4: Photosynthesis 14

  • A chemical makes the thylakoid membrane freely permeable to protons. Which change is expected first? Wikipedia, "Chemiosmosis"; course topic 3.4
  • The Calvin cycle is sometimes called the "dark reactions." Why is that name misleading? Wikipedia, "Calvin cycle"; course topic 3.4
  • During the light reactions, where do protons build up? Wikipedia, "Thylakoid", section "Lumen"; course topic 3.4
  • In the light reactions, electrons pass from photosystem II to photosystem I. Where do they end up? Wikipedia, "Light-dependent reactions"; course topic 3.4
  • The oxygen gas released by a photosynthesizing plant comes from which molecule? Nature Education, Scitable, "Photosynthetic Cells"; Wikipedia, "Photosynthesis"; course topic 3.4
  • What directly powers ATP synthase in the chloroplast? Wikipedia, "Chemiosmosis"; RCSB Protein Data Bank, PDB-101 Molecule of the Month, "ATP Synthase"; course topic 3.4
  • What do the light reactions supply to the Calvin cycle? Wikipedia, "Calvin cycle"; course topic 3.4
  • What does the Calvin cycle do with the energy of ATP and NADPH? Nature Education, Scitable, "Photosynthetic Cells"; course topic 3.4
  • When chlorophyll in photosystem II absorbs light, it loses energized electrons. What replaces them? Wikipedia, "Light-dependent reactions"; Nature Education, Scitable, "Photosynthetic Cells"; course topic 3.4
  • Where in a chloroplast do the light reactions take place? Wikipedia, "Thylakoid"; Nature Education, Scitable, "Photosynthetic Cells"; course topic 3.4
  • Where in a chloroplast does the Calvin cycle take place? Wikipedia, "Calvin cycle"; course topic 3.4
  • Which are the inputs of photosynthesis in a plant? Wikipedia, "Photosynthesis"; course topic 3.4
  • Which organisms are credited with first raising the level of free oxygen in Earth's atmosphere? Wikipedia, "Great Oxidation Event"; course topic 3.4
  • Why do most leaves look green? Nature Education, Scitable, "Photosynthetic Cells"; course topic 3.4

Topic 3.5: Cellular respiration 17

  • A protein in the brown fat of babies lets protons return to the mitochondrial matrix without passing through ATP synthase. What is the result? Wikipedia, "Thermogenin"; course topic 3.5
  • A student claims that plants photosynthesize and so have no need of cellular respiration. Which reply is correct? Wikipedia, "Mitochondrion" and "Cellular respiration"; course topic 3.5
  • At which stages of aerobic respiration is the carbon of glucose released as carbon dioxide? Nature Education, Scitable, "Cell Energy and Cell Functions"; Wikipedia, "Citric acid cycle"; course topic 3.5
  • Bacteria have no mitochondria. Where are the proteins of their electron transport chain? Wikipedia, "Oxidative phosphorylation" and "Citric acid cycle"; course topic 3.5
  • Compared with the intermembrane space, the matrix of an actively respiring mitochondrion has Nature Education, Scitable, "Mitochondria"; course topic 3.5
  • Glucose has six carbon atoms. In what form are those carbons at the end of glycolysis? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.5
  • In which direction does the electron transport chain of a mitochondrion pump protons? Nature Education, Scitable, "Mitochondria"; course topic 3.5
  • Some prokaryotes run an electron transport chain where there is no oxygen. How? Wikipedia, "Anaerobic respiration"; course topic 3.5
  • What are cristae, and why do they matter? Wikipedia, "Crista" and "Mitochondrion"; course topic 3.5
  • What happens to pyruvate in a eukaryotic cell when oxygen is available? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.5
  • What is the final electron acceptor of the mitochondrial electron transport chain, and what forms there? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.5
  • When oxygen runs low in hard-working skeletal muscle, the cells rely on glycolysis for ATP. Which product builds up? Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.5
  • Where in a eukaryotic cell does glycolysis take place? Nature Education, Scitable, "Cell Energy and Cell Functions"; Wikipedia, "Glycolysis"; course topic 3.5
  • Which products of the citric acid (Krebs) cycle carry electrons to the electron transport chain? Wikipedia, "Citric acid cycle"; course topic 3.5
  • Which stage of aerobic respiration makes most of the ATP? Wikipedia, "Cellular respiration"; Nature Education, Scitable, "Cell Energy and Cell Functions"; course topic 3.5
  • Why does fermentation allow glycolysis to continue when no oxygen is present? Wikipedia, "Anaerobic respiration", section "As compared with fermentation"; course topic 3.5
  • Yeast cells kept without oxygen ferment sugar. Which products do they release? Wikipedia, "Fermentation"; course topic 3.5

How this list was compiled: Our own questions, written to Unit 3 (Cellular Energetics) of the College Board's AP Biology Course and Exam Description, effective fall 2025, https://apcentral.collegeboard.org/media/pdf/ap-biology-course-and-exam-description.pdf, read 2026-10-09. Each fact was read on 2026-10-09 in the public page named under its question: Scitable by Nature Education, the Molecule of the Month pages of the RCSB Protein Data Bank (PDB-101) and Wikipedia. Licence: Own work. Short quotations from the public documents named under each question.

How it works

This AP Bio cellular respiration and photosynthesis practice test has 54 questions of our own on Unit 3, Cellular Energetics. It is unofficial practice, and the College Board, which owns AP, has no part in it.

Read the full guide

What Unit 3 covers

Unit 3 has five topics in the College Board's course description, read October 9, 2026, and the bank follows them: enzymes (8 questions), environmental impacts on enzyme function (9), cellular energy (6), photosynthesis (14) and cellular respiration (17). How many questions a topic gets was our decision, since the description gives the topics no weights.

Sources you can open

Each answer links to the page it was checked on: a Scitable article from Nature Education, a Protein Data Bank feature or a Wikipedia article.

  • Follow the electrons. In photosynthesis they run from water to NADPH; in respiration, from glucose to oxygen.
  • For any change to a pathway, ask what builds up before the block and what runs short after it.

Questions and answers

What is Unit 3 in AP Bio?

Cellular Energetics: how enzymes work, how cells capture light energy in photosynthesis and how they release energy from food in cellular respiration. The course description allows about 12 to 14 class periods for it.

How much of the AP Bio exam is Unit 3?

Between 12 and 16% of the multiple-choice section, by the College Board's course page. Only Unit 7, Natural Selection, has a higher upper figure.

What should an AP Biology photosynthesis study guide cover?

The parts of the chloroplast, the light reactions that make ATP and NADPH, the proton gradient and ATP synthase, and what the Calvin cycle does with carbon dioxide. The photosynthesis topic here holds 14 questions on exactly these.

Do I have to memorize the steps of glycolysis, the Krebs cycle and the Calvin cycle?

No. The course description puts the steps, the molecules' structures and the enzymes' names, apart from ATP synthase, beyond the scope of the exam. Know the inputs, the outputs and where each stage happens.

Are these real AP Biology exam questions?

No. The College Board owns the real questions, and none appears here even in other words. Every question on this page was written for it from the Unit 3 topic list.

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