AP Biology Unit 3: Cellular Energetics MCQs – 30 Practice Questions with Answers (College Board CED)

Q1. An enzyme speeds up a reaction by a factor of a million, yet it is not consumed. How does it achieve this?

• (A) It adds energy to the products so the reaction runs downhill, raising product energy above the reactants
• (B) It lowers the activation energy by stabilising the transition state, without being changed itself
• (C) It raises the activation energy so only fast molecules react
• (D) It converts the reactants into a different enzyme

Q2. A substrate analogue binds to an enzyme’s active site but cannot be converted to product. Adding much more of the real substrate restores the reaction rate. What type of inhibition is this?

• (A) Noncompetitive inhibition, because the inhibitor binds away from the active site and changes the enzyme’s shape
• (B) Feedback inhibition, because the product shuts down its own pathway
• (C) Irreversible inhibition, because the enzyme is permanently destroyed
• (D) Competitive inhibition, because inhibitor and substrate compete for the same active site

Q3. A second inhibitor binds to a site other than the active site, changes the enzyme’s shape, and reduces the rate even when substrate is plentiful. What type of inhibition is this?

• (A) Noncompetitive inhibition, because binding at an allosteric site alters the enzyme regardless of substrate concentration
• (B) Competitive inhibition, because the inhibitor blocks the active site directly and prevents substrate binding there at all
• (C) Denaturation, because the enzyme’s primary sequence is broken
• (D) Cooperativity, because binding one substrate speeds up the next

Q4. A graph of reaction rate against substrate concentration rises steeply at first and then levels off. What explains the plateau?

• (A) The substrate has become an inhibitor
• (B) The enzyme is denaturing at high substrate levels
• (C) All active sites are occupied, so the enzyme is saturated and cannot work faster
• (D) The products are being converted back into substrate as fast as they form, cancelling the forward reaction

Q5. An enzyme from a hot spring works fastest at 75 degrees C, while a human enzyme denatures at that temperature. What does this comparison show?

• (A) All enzymes share the same optimum temperature, regardless of the organism or environment they come from
• (B) Human enzymes are not proteins
• (C) Temperature has no effect on enzyme structure
• (D) Optimum conditions reflect the environment an enzyme is adapted to, not a universal value

Q6. An experiment measures enzyme activity across a pH range and finds a sharp peak at pH 2, with almost no activity at pH 7. Where does this enzyme most likely act in the body?

• (A) In the blood, where pH is near neutral
• (B) In the stomach, where conditions are strongly acidic
• (C) In the small intestine, where conditions are mildly alkaline
• (D) In the cytoplasm, where pH is near neutral

Q7. A student proposes that an enzyme’s shape changes slightly when its substrate binds, improving the fit. What is this idea called?

• (A) Induced fit, in which substrate binding reshapes the active site for better catalysis
• (B) Competitive exclusion, in which the substrate blocks other molecules
• (C) Denaturation, in which the enzyme permanently unfolds
• (D) Allosteric activation, in which a separate molecule binds the active site and switches the enzyme on

Q8. Two test tubes contain the same enzyme and substrate. One is kept at the enzyme’s optimum temperature and the other far below it. The cold tube reacts much more slowly. Why?

• (A) The enzyme has denatured in the cold
• (B) The substrate has been used up by the cold, leaving nothing for the enzyme to act on in the tube, so no products can form
• (C) Molecules move more slowly when cold, so effective collisions between enzyme and substrate are rarer
• (D) Cold converts the enzyme into its product

Q9. A reaction releases free energy overall. What can be said about it, and what does it imply for the cell?

• (A) It is endergonic and must absorb energy from ATP before it can proceed at all
• (B) It is at equilibrium and cannot do work
• (C) It requires a constant input of light energy
• (D) It is exergonic and can be coupled to drive an endergonic process

Q10. ATP hydrolysis releases energy that a cell uses to build a protein. Which statement best describes this energy coupling?

• (A) The exergonic hydrolysis of ATP drives the endergonic synthesis of the protein
• (B) The protein synthesis reaction supplies energy to make ATP
• (C) ATP hydrolysis and protein synthesis are unrelated reactions that proceed independently in the cell
• (D) ATP is converted into glucose to power the synthesis

Q11. A cell’s ATP pool turns over rapidly: ATP is hydrolysed to ADP and then regenerated many times per minute. Why is this recycling necessary?

• (A) ATP is a permanent store that never needs replacing
• (B) The cell holds only a small ATP reserve at any moment, so it must constantly regenerate ATP from ADP using energy inputs
• (C) ADP cannot be converted back into ATP under any conditions
• (D) ATP is stored in the nucleus and cannot reach the cytoplasm where most energy-requiring reactions occur, so it can only power nuclear processes

Q12. A metabolic pathway converts molecule P to molecule R in three steps, and R inhibits the first enzyme of the pathway. What is the advantage of this arrangement?

• (A) It speeds up the pathway whenever R is abundant
• (B) It destroys the first enzyme permanently
• (C) It provides feedback inhibition, preventing wasteful overproduction when the end product is plentiful
• (D) It forces the cell to make R from a different starting molecule, abandoning the original pathway entirely

Q13. A student claims that catabolic pathways release energy while anabolic pathways require energy. Which example pair supports the claim?

• (A) Cellular respiration releasing energy, and photosynthesis building sugars using energy
• (B) Photosynthesis releasing energy, and respiration building sugars
• (C) Protein synthesis releasing energy, and digestion requiring energy
• (D) ATP hydrolysis building molecules, and ATP synthesis releasing energy, so the cell gains energy by making ATP

Q14. An energy diagram shows reactants at a lower energy than products. The reaction proceeds only when coupled to ATP hydrolysis. What type of reaction is it?

• (A) Exergonic, because it releases free energy
• (B) Spontaneous in isolation, because it needs no input
• (C) At equilibrium, because reactants and products are equal and no net change occurs in either direction
• (D) Endergonic, because the products store more free energy than the reactants

Q15. A researcher blocks the ATP synthase of a cell. Which immediate consequence is most expected?

• (A) Glycolysis stops instantly because no glucose remains in the cell to break down for ATP production in the cytoplasm
• (B) ATP production from the proton gradient falls sharply, even though the gradient itself may still form
• (C) The cell begins to photosynthesise to replace the ATP
• (D) Oxygen consumption rises to compensate without limit

Q16. A plant is illuminated with green light only and its rate of photosynthesis is very low, while red or blue light gives high rates. What best explains the green-light result?

• (A) Green light destroys chlorophyll instantly
• (B) Green light carries no energy at all
• (C) Chlorophyll absorbs red and blue light strongly but reflects or transmits most green light
• (D) Green light prevents carbon dioxide from entering the leaf by closing the stomata completely

Q17. The overall equation for photosynthesis is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. How should this equation be classified for AP Biology?

• (A) As a core course equation for photosynthesis, not an item from the equations appendix
• (B) As an equation from the CED equations appendix
• (C) As a formula that applies only to cellular respiration
• (D) As a statistical formula for analysing experimental data, used to test whether results differ by chance

Q18. An experiment supplies a plant with water labelled with a heavy oxygen isotope. The heavy oxygen later appears in released oxygen gas, not in the sugar produced. What does this show?

• (A) The oxygen released in photosynthesis comes from carbon dioxide, which is split during the light reactions
• (B) The sugar is made entirely from water
• (C) Light energy splits carbon dioxide directly into sugar
• (D) The oxygen released in photosynthesis comes from the splitting of water in the light reactions

Q19. A plant is kept in light but carbon dioxide is removed from the air around it. The light reactions continue briefly, but sugar production stops. Why?

• (A) The light reactions need carbon dioxide to split water
• (B) The Calvin cycle needs carbon dioxide as its carbon source, so without it no sugar can be fixed
• (C) Chlorophyll stops absorbing light when carbon dioxide is absent
• (D) The plant switches to cellular respiration and destroys its chloroplasts, abandoning photosynthesis entirely, so no light reactions can continue

Q20. During the light reactions, electrons move through an electron transport chain and protons accumulate inside the thylakoid. What is the purpose of this proton build-up?

• (A) To break down glucose into pyruvate
• (B) To provide raw material for the Calvin cycle directly, supplying it with protons for sugar synthesis
• (C) To power ATP synthase by chemiosmosis as protons flow back across the membrane
• (D) To prevent water from being split

Q21. A student argues that a plant needs both photosystem II and photosystem I. Which evidence best supports this argument?

• (A) Photosystem II splits water and feeds electrons onward, while photosystem I re-energises them to make NADPH
• (B) Photosystem I alone can split water and fix carbon dioxide
• (C) Photosystem II alone produces all the ATP and NADPH the Calvin cycle needs, without any help from photosystem I at any stage
• (D) The two photosystems work in different plants, never in the same cell

Q22. A leaf disc experiment shows discs floating fastest under bright light, slower under dim light, and not at all in darkness. What variable is being tested, and what do floating discs indicate?

• (A) Temperature is tested, and floating indicates respiration
• (B) Carbon dioxide concentration is tested, and floating indicates water uptake by the leaf discs from the solution
• (C) Light wavelength is tested, and floating indicates chlorophyll breakdown
• (D) Light intensity is tested, and floating indicates oxygen production from photosynthesis filling the discs

Q23. The overall equation for cellular respiration is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. How should this equation be classified for AP Biology?

• (A) As a formula from the CED equations appendix
• (B) As a core course equation for cellular respiration, not an item from the equations appendix
• (C) As a statistical formula for analysing respiration data, used to compare oxygen use between treatments
• (D) As an equation that applies only to photosynthesis

Q24. Glycolysis produces a small net gain of ATP in the cytoplasm, even when no oxygen is present. What does this imply about glycolysis?

• (A) Glycolysis requires oxygen as a reactant
• (B) Glycolysis occurs only in mitochondria
• (C) Glycolysis is an ancient anaerobic pathway that does not itself need oxygen
• (D) Glycolysis produces glucose rather than breaking it down, building sugar from smaller molecules instead

Q25. A muscle cell working intensely without enough oxygen switches to lactic acid fermentation. What is the benefit of this switch?

• (A) Fermentation regenerates NAD+ so glycolysis can continue to make ATP
• (B) Fermentation destroys pyruvate so it cannot accumulate
• (C) Fermentation produces oxygen for the mitochondria
• (D) Fermentation produces far more ATP per glucose than aerobic respiration

Q26. A poison blocks the electron transport chain in mitochondria. Which effect is most expected?

• (A) Glycolysis speeds up without limit and produces more ATP than before, compensating fully for the blockage
• (B) The Krebs cycle stops because it needs oxygen directly
• (C) Carbon dioxide production rises sharply
• (D) ATP synthesis by oxidative phosphorylation collapses because electrons cannot flow to oxygen

Q27. During aerobic respiration, most ATP is produced at which stage, and by what mechanism?

• (A) Glycolysis, by substrate-level phosphorylation alone, producing ATP directly at each step of the pathway
• (B) Oxidative phosphorylation, by chemiosmosis using the proton gradient from the electron transport chain
• (C) The Krebs cycle, by direct splitting of water
• (D) Fermentation, by recycling NADH into ATP

Q28. A yeast culture bubbles vigorously when given glucose in the absence of oxygen, releasing carbon dioxide and ethanol. Which process is occurring?

• (A) Alcoholic fermentation, in which pyruvate is converted to ethanol and carbon dioxide while regenerating NAD+
• (B) Aerobic respiration, because oxygen is produced
• (C) Photosynthesis, because carbon dioxide is released
• (D) Lactic acid fermentation, because ethanol is a waste product of muscle cells working without oxygen during intense exercise

Q29. A researcher measures oxygen consumption and carbon dioxide production in germinating seeds. Both rise together as temperature increases toward an optimum. What is the best interpretation?

• (A) Photosynthesis is increasing, because seeds photosynthesise underground
• (B) Water uptake is increasing, because seeds absorb oxygen as water
• (C) The rate of aerobic respiration is increasing with temperature, consuming oxygen and releasing carbon dioxide faster
• (D) Fermentation is increasing, because oxygen consumption proves fermentation is taking place in the seeds, and this rise tracks temperature

Q30. A cell is treated with a chemical that makes the inner mitochondrial membrane leaky to protons. ATP production falls, but oxygen is still consumed. Why?

• (A) The electron transport chain stops completely, so no electrons flow and no protons are pumped at all
• (B) Glycolysis is blocked by the chemical
• (C) The chemical converts ADP into glucose
• (D) Protons bypass ATP synthase, so the gradient is wasted as heat instead of making ATP