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AP Biology Unit 3 Practice Questions: Cellular Energetics

10 original exam-style questions on Cellular Energetics. Answer each one to see the explanation — no account needed.

Question 1 of 10 · Oxidative Phosphorylation

A student measures the rate of oxygen consumption by isolated mitochondria under different conditions. Mitochondria are suspended in isotonic buffer and given NADH as an electron donor. The student then adds various substances sequentially and measures O₂ consumption.

O₂ consumption rate of isolated mitochondria under experimental conditions
ConditionO₂ Consumption Rate (nmol O₂/min/mg protein)
NADH only0.4
NADH + ADP + Pi18.2
NADH + ADP + Pi + Oligomycin1.1
NADH + ADP + Pi + Oligomycin + DNP (uncoupler)16.8
NADH + ADP + Pi + Cyanide0.3
Based on the data, which conclusion best explains why DNP restores high O₂ consumption in the presence of oligomycin?
  1. A. DNP activates ATP synthase through an allosteric mechanism, bypassing oligomycin inhibition
  2. B. DNP inhibits oligomycin directly, restoring ATP synthase function and ATP-coupled electron transport
  3. C. DNP dissipates the proton gradient by carrying H⁺ ions across the inner mitochondrial membrane, allowing electron transport to proceed without ATP synthase
  4. D. DNP increases NADH production in the matrix, providing more electrons to the transport chain
Show answer and explanation

Correct answer: C

Oligomycin blocks ATP synthase, causing the proton gradient to build up and stall the electron transport chain; DNP is a proton ionophore that collapses the gradient by shuttling H⁺ across the membrane independently of ATP synthase, relieving back-pressure and restoring electron flow. Option A is incorrect because DNP does not interact with or activate ATP synthase.

Question 2 of 10 · Oxidative Phosphorylation and Chemiosmosis

A cell biologist treats cells with oligomycin, a drug that blocks the proton channel of ATP synthase. Which of the following best predicts the immediate effect on the mitochondrial inner membrane?
  1. A. The proton gradient across the inner membrane increases because protons continue to be pumped by the electron transport chain but can no longer flow back through ATP synthase
  2. B. The proton gradient across the inner membrane dissipates immediately because protons flow freely through the blocked ATP synthase channel
  3. C. The proton gradient is unaffected because ATP synthase uses chemical energy from ATP hydrolysis to maintain the gradient independently of the electron transport chain
  4. D. Electron transport chain activity increases to compensate for the blocked ATP synthase by pumping more protons per electron transferred
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Correct answer: A

The electron transport chain continuously pumps protons from the matrix into the intermembrane space; with the ATP synthase channel blocked, protons can no longer flow back through it, so the electrochemical proton gradient builds up across the inner membrane. Choice B is incorrect because blocking the ATP synthase channel prevents proton flow through that route — the gradient does not dissipate, it accumulates.

Question 3 of 10 · Calvin Cycle — Carbon Fixation

A researcher uses radioactive tracers to follow the path of carbon atoms during photosynthesis. She grows algae in a chamber, supplies 14CO2^{14}CO_2, and at various time points rapidly kills the cells and analyzes which molecules contain the radioactive label.

Radioactive Label (14C^{14}C) in Molecules Over Time
Time After 14CO2^{14}CO_2 AdditionLabeled Molecule(s)
5 seconds3-phosphoglycerate (3-PGA)
30 seconds3-PGA, G3P (glyceraldehyde-3-phosphate)
60 seconds3-PGA, G3P, RuBP, sucrose, starch
Which of the following best explains why 3-PGA is the first labeled molecule detected?
  1. A. 3-PGA is produced in the light reactions when water is split by photosystem II
  2. B. 3-PGA is formed when ATP is synthesized by ATP synthase in the chloroplast
  3. C. 3-PGA is produced when glucose is broken down during the light-independent reactions
  4. D. 3-PGA is the immediate product of CO2CO_2 fixation when CO2CO_2 combines with RuBP in the Calvin cycle
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Correct answer: D

In the Calvin cycle, RuBP carboxylase (RuBisCO) catalyzes the fixation of CO2CO_2 onto RuBP, producing two 3-carbon molecules of 3-PGA; this is the first stable product containing the newly fixed carbon. 3-PGA is not produced in the light reactions (choice A) or from glucose breakdown.

Question 4 of 10 · Fermentation Pathways

During lactic acid fermentation in muscle cells, which of the following correctly describes why lactate is produced?
  1. A. Lactate is produced to generate additional ATP beyond what glycolysis provides under aerobic conditions
  2. B. Lactate is an energy-rich molecule that is exported to the liver and directly used as fuel by mitochondria
  3. C. Lactate production regenerates NADX+\ce{NAD+} from NADH\ce{NADH}, allowing glycolysis to continue producing ATP when oxygen is limited
  4. D. Lactate neutralizes the acid produced by the proton gradient in the ETC, maintaining cellular pH
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Correct answer: C

In anaerobic conditions, pyruvate is converted to lactate by lactate dehydrogenase, simultaneously oxidizing NADH\ce{NADH} back to NADX+\ce{NAD+} — this regenerates the electron acceptor needed for glycolysis to continue making ATP. Choice A is incorrect because fermentation does not produce additional ATP; it simply keeps glycolysis running by recycling NADX+\ce{NAD+}.

Question 5 of 10 · Glycolysis — Electron Carriers

Which of the following best describes the role of NAD+NAD^+ in glycolysis?
  1. A. NAD+NAD^+ donates electrons to glucose, initiating the breakdown process
  2. B. NAD+NAD^+ serves as the final electron acceptor in anaerobic conditions, producing O2O_2
  3. C. NAD+NAD^+ provides the phosphate groups needed to phosphorylate glucose at the start of glycolysis
  4. D. NAD+NAD^+ acts as an electron carrier, accepting electrons (and a proton) from glucose oxidation to form NADH
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Correct answer: D

NAD+NAD^+ is the oxidized form of the electron carrier; during glycolysis, it accepts electrons (and H+H^+) from the oxidation of G3P, becoming NADH. This oxidation is coupled to substrate-level phosphorylation. NAD+NAD^+ does not donate electrons or supply phosphate groups.

Question 6 of 10 · Alcoholic Fermentation

A yeast cell is placed in an anaerobic (no oxygen) environment with excess glucose. Which process does the yeast most likely use to regenerate NAD+NAD^+ and continue glycolysis?
  1. A. Oxidative phosphorylation, because it is the most efficient ATP-generating process
  2. B. The Krebs cycle, because it regenerates NAD+NAD^+ from NADH
  3. C. Alcoholic fermentation, converting pyruvate to ethanol and CO2CO_2 while regenerating NAD+NAD^+
  4. D. Lactic acid fermentation, converting glucose directly to lactic acid
Show answer and explanation

Correct answer: C

Yeast perform alcoholic fermentation under anaerobic conditions: pyruvate is decarboxylated to acetaldehyde (releasing CO2CO_2), which is then reduced to ethanol by NADH, regenerating NAD+NAD^+ needed for continued glycolysis. Yeast do not perform lactic acid fermentation (that is typical of animal muscle cells).

Question 7 of 10 · Krebs Cycle

During the Krebs cycle, acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons). What happens to the carbons during one turn of the cycle?
  1. A. All 6 carbons are retained in oxaloacetate, which is then used directly for glucose synthesis
  2. B. All 6 carbons are released as CO2CO_2, and the cycle must import new carbon from the cytoplasm
  3. C. Two carbons are released as CO2CO_2, oxaloacetate is regenerated, and electrons are captured in NADH and FADH2FADH_2
  4. D. Two carbons are transferred to ATP, and the remaining 4 are released as CO2CO_2
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Correct answer: C

In each turn of the Krebs cycle, the 2 carbons introduced by acetyl-CoA are released as 2 CO2CO_2 molecules, oxaloacetate (4C) is regenerated to accept another acetyl-CoA, and the energy from oxidation is captured in 3 NADH, 1 FADH2FADH_2, and 1 ATP (or GTP). ATP does not directly carry carbon atoms (ruling out D).

Question 8 of 10 · Glycolysis

Which of the following correctly identifies where glycolysis occurs and what its net products are per glucose molecule?
  1. A. Mitochondrial matrix; 2 ATP, 2 NADH, 2 pyruvate
  2. B. Cytoplasm; 36 ATP, 10 NADH, 2 FADH2_2
  3. C. Cytoplasm; 2 ATP, 2 NADH, 2 pyruvate
  4. D. Thylakoid membrane; 2 ATP, 2 NADPH, 2 pyruvate
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Correct answer: C

Glycolysis occurs in the cytoplasm of all cells and yields a net of 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule. The large ATP yield in choice C represents total aerobic respiration, not glycolysis alone, and that occurs partly in the mitochondria.

Question 9 of 10 · Light-Dependent Reactions

During the light-dependent reactions of photosynthesis, which of the following correctly describes the role of water?
  1. A. Water is used as an electron acceptor in the Calvin cycle to regenerate NADP+^+
  2. B. Water is produced by ATP synthase when protons flow through the enzyme in the thylakoid membrane
  3. C. Water provides the carbon atoms that are fixed into glucose during the light reactions
  4. D. Water is split by photosystem II, providing electrons to replace those lost by chlorophyll, with O2O_2 released as a byproduct
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Correct answer: D

The photolysis of water at photosystem II replenishes the electrons lost by chlorophyll P680 when it absorbs light; the oxygen atoms from water are released as O2O_2 gas. Water is not involved in carbon fixation (that is CO2CO_2) and is not split by ATP synthase.

Question 10 of 10 · Chloroplast Structure and Function

Which of the following correctly identifies the location of the light reactions and Calvin cycle within the chloroplast?
  1. A. Light reactions in the thylakoid membrane; Calvin cycle in the stroma
  2. B. Light reactions in the stroma; Calvin cycle in the thylakoid membrane
  3. C. Light reactions in the inner envelope membrane; Calvin cycle in the intermembrane space
  4. D. Both light reactions and Calvin cycle occur in the thylakoid lumen
Show answer and explanation

Correct answer: A

The light-dependent reactions occur in the thylakoid membrane, where chlorophyll and photosystems capture light energy and produce ATP and NADPH. The Calvin cycle (light-independent reactions) occurs in the stroma using the ATP and NADPH produced by the light reactions to fix CO2CO_2.

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