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Quizzes > Science

Electron Transport Chain Quiz

Quick, free oxidative phosphorylation quiz. Instant results.

Editorial: Review CompletedCreated By: Stephanie SanchezUpdated Aug 27, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art for electron transport chain quiz featuring mitochondrial complexes proton gradient ATP on sky blue background

This electron transport chain quiz helps you practice complexes I-IV, proton gradients, and ATP production. Get instant feedback as you answer, and spot what to review before a test. For related review, try our cell membrane transport quiz and the electron configuration quiz, or build skills with a broader high school biology quiz.

Where in eukaryotic cells does the electron transport chain take place?
Cytosol
Inner mitochondrial membrane
Mitochondrial matrix
Outer mitochondrial membrane
The electron transport chain is located in the inner mitochondrial membrane of eukaryotic cells, where complexes I - IV and ATP synthase are embedded to generate a proton gradient. This gradient drives ATP synthesis during oxidative phosphorylation. The inner membrane's impermeability to protons is vital for establishing the proton?motive force.
What is the final electron acceptor in the electron transport chain?
FAD
Oxygen
NAD?
Water
Oxygen serves as the final electron acceptor at Complex IV, where it combines with electrons and protons to form water. Without oxygen, the chain would back up and NADH/FADH? could not be oxidized. This step is critical for maintaining the flow of electrons and sustaining ATP production.
Which complex of the ETC receives electrons from NADH?
Complex I
Complex III
Complex II
Complex IV
Complex I, also known as NADH dehydrogenase, oxidizes NADH and transfers electrons to ubiquinone, beginning the proton pumping process. This is the entry point for electrons derived from NADH in the mitochondrial ETC. Proper function of Complex I is essential for efficient ATP generation.
Ubiquinone is also known as what?
Iron-sulfur protein
Cytochrome c
Coenzyme Q
Flavin mononucleotide
Ubiquinone is commonly called Coenzyme Q (CoQ); it shuttles electrons and protons between Complexes I/II and III within the inner mitochondrial membrane. It exists in oxidized (ubiquinone) and reduced (ubiquinol) forms, facilitating electron transfer. CoQ is lipid-soluble, allowing it to diffuse freely in the membrane.
Complex II of the electron transport chain is also called what?
Succinate dehydrogenase
Cytochrome c oxidase
ATP synthase
NADH dehydrogenase
Complex II is succinate dehydrogenase, which participates in both the Krebs cycle and ETC by oxidizing succinate to fumarate and transferring electrons to ubiquinone. Unlike other complexes, it does not contribute to the proton gradient. Its dual role links the TCA cycle directly to oxidative phosphorylation.
Which of the following complexes does NOT pump protons across the membrane?
Complex III
Complex I
Complex II
Complex IV
Complex II (succinate dehydrogenase) transfers electrons from succinate to ubiquinone but does not translocate protons. Complexes I, III, and IV all contribute to the proton gradient essential for ATP synthesis. The absence of proton pumping by Complex II explains its lower energy yield.
What gradient drives ATP synthesis in oxidative phosphorylation?
Sodium gradient
Proton gradient
Electron gradient
Glucose gradient
A proton gradient (proton-motive force) across the inner mitochondrial membrane drives ATP synthesis by ATP synthase. Protons flow back into the matrix through the F? subunit, powering rotation and ATP formation. This chemiosmotic mechanism was proposed by Peter Mitchell.
Which substrate enters the ETC at Complex II?
NADH
Succinate
FAD
Oxygen
Succinate is oxidized by Complex II (succinate dehydrogenase) in the Krebs cycle, with electrons passed to FAD and onward to ubiquinone. This entry point bypasses Complex I and does not contribute to proton pumping. Succinate's role links the TCA cycle directly to the ETC.
Which mobile electron carrier transfers electrons between Complex III and Complex IV?
Cytochrome c
Iron-sulfur protein
Cytochrome b
Ubiquinone
Cytochrome c is a small peripheral protein that carries one electron at a time from Complex III to Complex IV in the intermembrane space. Its heme group undergoes reversible oxidation and reduction. This soluble carrier is essential for linking the Q cycle to oxygen reduction.
Approximately how many protons does Complex I pump per NADH oxidized?
4
8
2
6
Complex I translocates four protons across the inner mitochondrial membrane for each NADH oxidized. This proton pumping helps establish the proton-motive force that drives ATP synthesis. The stoichiometry of proton translocation varies slightly across species but is generally four in mitochondria.
What is the approximate P/O ratio for NADH in mitochondria?
1.5 ATP per NADH
2.5 ATP per NADH
3.0 ATP per NADH
1.0 ATP per NADH
The P/O ratio represents the number of ATP molecules synthesized per oxygen atom reduced. In mitochondria, oxidation of NADH typically yields about 2.5 ATP. This value reflects proton pumping by Complexes I, III, and IV and ATP synthase coupling.
Which subunit of ATP synthase forms the proton channel?
F? subunit
Beta subunit
Rotor ring
F? subunit
The F? subunit of ATP synthase contains the c-ring and a subunit that form the proton channel. Protons move through F?, causing rotation of the c-ring and central stalk, which drives conformational changes in the F? subunit to synthesize ATP.
What is the P/O ratio for FADH? oxidation?
1.5 ATP
3.0 ATP
2.5 ATP
1.0 ATP
FADH? donates electrons to Complex II, bypassing Complex I and resulting in fewer protons pumped overall. The P/O ratio for FADH? oxidation in mitochondria is approximately 1.5 ATP. This lower yield reflects the bypass of one proton-pumping site.
Which complexes in the ETC are involved in proton pumping?
I, II, III
II, III, IV
I, III, IV
I, II, IV
Complexes I, III, and IV in the ETC translocate protons from the matrix to the intermembrane space, generating the electrochemical gradient. Complex II does not pump protons. This gradient is harnessed by ATP synthase for ATP production.
Which inhibitor specifically blocks electron flow at Complex IV?
Cyanide
Amytal
Rotenone
Antimycin A
Cyanide binds to the heme iron in Complex IV (cytochrome c oxidase), preventing oxygen from accepting electrons and halting the ETC. This inhibition stops proton pumping and ATP synthesis, quickly leading to cellular asphyxiation.
Which molecule can carry both electrons and protons within the ETC?
NAD?
Ubiquinone (Ubiquinol)
Oxygen
Cytochrome c
Ubiquinone (Q) accepts electrons and protons to become ubiquinol (QH?), allowing it to shuttle both within the inner mitochondrial membrane. This dual capacity is crucial for linking electron flow to proton translocation at Complex III (Q cycle).
Which protein acts as an uncoupler in brown adipose tissue?
Thermogenin
Oligomycin
2,4-Dinitrophenol (DNP)
Rotenone
Thermogenin (UCP1) is an uncoupling protein in brown adipose tissue that dissipates the proton gradient to generate heat instead of ATP. It provides adaptive thermogenesis by allowing protons to re-enter the matrix without ATP synthase.
Who proposed the chemiosmotic hypothesis for ATP synthesis?
Hans Krebs
Peter Mitchell
Linus Pauling
Albert Szent-Györgyi
Peter Mitchell proposed the chemiosmotic hypothesis in 1961, suggesting that ATP synthesis is driven by a proton gradient across the membrane rather than high-energy chemical intermediates. His theory revolutionized understanding of oxidative phosphorylation. He received the Nobel Prize in 1978 for this work.
Which electron transport components contain iron-sulfur clusters?
ATP synthase
Cytochrome c
Coenzyme Q
Iron-sulfur proteins
Iron-sulfur proteins, found in Complexes I, II, and III, contain Fe-S clusters that facilitate single-electron transfers. These clusters vary in composition (e.g., [2Fe-2S], [4Fe-4S]) and are essential for efficient electron flow.
What effect does the uncoupler 2,4-dinitrophenol (DNP) have on the ETC?
Increases ATP production
Inhibits NADH dehydrogenase
Increases oxygen consumption
Decreases oxygen consumption
DNP dissipates the proton gradient by shuttling protons across the inner membrane, uncoupling electron transport from ATP synthesis. Cells must increase oxygen consumption to restore the gradient, but ATP yield drops. DNP is a potent and dangerous uncoupler.
Which of the following has the highest redox potential?
FMN
Cytochrome c
Fe-S clusters
NADH
Cytochrome c has a higher midpoint redox potential (?+0.22 V) compared to NADH (~ - 0.32 V), FMN, and iron-sulfur clusters. The increasing redox potential along the chain ensures spontaneous electron flow toward oxygen. Cytochrome c's potential is close to that of oxygen reduction.
Which metal ion is a key component of Complex IV?
Copper only
Iron only
Iron and copper
Magnesium
Complex IV (cytochrome c oxidase) contains both heme iron centers and copper centers (Cu_A and Cu_B) that work together to transfer electrons to oxygen. These metals are critical for catalytic reduction of O? to water.
The supramolecular organization of ETC complexes into supercomplexes is called what?
Protonosome
ATP synthasome
Respirasome
Oxidative phosphorylasome
Respirasomes are supercomplexes comprising multiple ETC complexes (e.g., I, III, and IV) assembled together to enhance electron transfer efficiency and minimize reactive oxygen species. This organization stabilizes interactions and optimizes function.
What is the main purpose of the Q cycle in Complex III?
Amplify proton translocation
Synthesize ATP
Pump sodium ions
Direct transfer of electrons to oxygen
The Q cycle mechanism in Complex III allows two electrons from ubiquinol to be transferred sequentially, with protons released into the intermembrane space. This amplifies the proton gradient by effectively moving four protons per two electrons, boosting the proton-motive force.
Which reaction occurs at Complex IV of the ETC?
Reduction of O? to H?O
Oxidation of succinate
Oxidation of NADH
Reduction of cytochrome c
Complex IV catalyzes the final step of the ETC, where four electrons from cytochrome c reduce O? to two molecules of H?O. This reaction also contributes to proton pumping, reinforcing the gradient for ATP synthesis.
Approximately how many protons must pass through ATP synthase to generate one molecule of ATP?
1
2
3
4
Structural studies indicate that approximately three protons must flow through the F? rotor into the matrix to drive conformational changes in the F? subunit that synthesize one ATP. Some systems also require an additional proton for phosphate import, but the core translocation is three per ATP.
How many electrons are required to reduce one molecule of O? into two H?O molecules?
2
6
4
8
The reduction of one O? molecule to two H?O molecules requires four electrons and four protons. Each O atom gains two electrons and two protons, making a total of four electrons for the full reaction. This process occurs at Complex IV.
Which factor can reverse the flow of electrons in Complex I (reverse electron transport)?
Low proton motive force
High NADH/NAD? ratio
High ADP levels
High proton motive force
Reverse electron transport through Complex I can occur when a high proton-motive force drives electrons from ubiquinol back into NAD?, forming NADH. This requires a high membrane potential and high QH?/Q ratio. Reverse transport is important in some metabolic conditions but can increase ROS production.
Which lipid stabilizes the inner mitochondrial membrane and is involved in ETC function?
Glycolipid
Cardiolipin
Phosphatidylcholine
Cholesterol
Cardiolipin is a unique dimeric phospholipid found predominantly in the inner mitochondrial membrane. It binds to several ETC complexes, stabilizing supercomplex formation and optimal electron transport. Cardiolipin deficiency impairs mitochondrial function.
Which of the following statements about the respiratory control ratio (RCR) is correct?
It measures the respiratory quotient (RQ)
It increases when ADP is limited
It decreases with increased substrate availability
It is the ratio of state 3 to state 4 respiration
The respiratory control ratio (RCR) is defined as the ratio of state 3 (ADP-stimulated) to state 4 (resting) respiration rates in isolated mitochondria. A high RCR indicates tight coupling between oxidation and phosphorylation. It is a key parameter for assessing mitochondrial integrity.
Oligomycin inhibits which component of the ETC, and what is the effect on proton gradient?
ATP synthase; gradient increases
Cytochrome c; gradient decreases
Complex IV; gradient decreases
Complex I; gradient remains same
Oligomycin binds to the F? subunit of ATP synthase, blocking proton flow through the channel. This halts ATP synthesis and causes protons to accumulate in the intermembrane space, increasing the gradient and reducing electron flow.
How many c subunits are typically found in the F? rotor ring of human ATP synthase?
8
12
10
6
Human mitochondrial ATP synthase typically contains an 8-subunit c-ring in the F? sector. Each proton translocation causes the ring to rotate by one subunit, so eight protons complete a full rotation, synthesizing approximately three ATP molecules. Structural studies confirm this stoichiometry.
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Study Outcomes

  1. Understand ETC Complex Functions -

    Describe the specific roles of Complexes I - IV within the electron transport chain and how they contribute to energy conversion.

  2. Analyze Electron Flow -

    Trace the step-by-step transfer of electrons through the ETC and identify the primary electron carriers involved.

  3. Explain Proton Gradient Formation -

    Explain how proton pumping across the inner mitochondrial membrane creates an electrochemical gradient that drives ATP synthesis.

  4. Differentiate NADH and FADH₂ Contributions -

    Compare how NADH and FADH₂ feed electrons into the ETC and the impact on ATP yield.

  5. Apply Concepts in Quiz Scenarios -

    Use your knowledge to answer scored electron transport chain quiz questions accurately and efficiently.

  6. Recall Key ETC Trivia -

    Memorize essential electron transport chain trivia to reinforce fundamental concepts and boost retention.

Cheat Sheet

  1. Complexes I - IV and Electron Flow -

    The electron transport chain comprises four major complexes (I - IV) embedded in the inner mitochondrial membrane, each facilitating sequential redox reactions (e.g., NADH + H+ + CoQ → NAD+ + CoQH2 at Complex I). As electrons flow from NADH and FADH2 to O2, protons are pumped at Complexes I, III, and IV to establish the proton gradient (source: Lehninger Principles of Biochemistry). Remember the mnemonic "Naughty Fats Can Often Dilute" for NADH, FADH2, CoQ, Cyt c, and O2.

  2. Chemiosmotic Theory and Proton-Motive Force -

    Peter Mitchell's chemiosmotic theory explains that the proton gradient (ΔpH and Δψ) across the inner membrane drives ATP synthesis; the proton-motive force is quantified by Δp = Δψ - (2.303RT/F)ΔpH (source: Journal of Bioenergetics). In practice, ~10 H+ must re-enter via ATP synthase to generate ~3 ATP from one NADH oxidation. Quiz yourself on calculating Δp using typical mitochondrial values (e.g., Δψ ≈ - 150 mV, ΔpH ≈ 0.75).

  3. ATP Synthase and Rotational Catalysis -

    ATP synthase (Complex V) uses the flow of protons back into the matrix to rotate its Fo subunit and catalyze ADP + Pi → ATP in the F1 sector (source: Nature Reviews Molecular Cell Biology). One full 360° rotation synthesizes three ATP molecules - keep in mind the binding-change mechanism with "loose," "tight," and "open" conformations. A useful study trick is picturing the c-ring as a waterwheel driven by proton flow.

  4. P/O Ratios and Energy Yield -

    The P/O ratio describes ATP molecules produced per oxygen atom reduced: typically ~2.5 ATP/NADH and ~1.5 ATP/FADH2 (source: Biochemical Journal). These values stem from the number of protons each electron donor pumps (10 for NADH; 6 for FADH2) and the ~4 H+ needed per ATP synthesis. Practice ETC quiz questions by converting H+ counts to ATP yields for mixed-substrate oxidations.

  5. Inhibitors, Uncouplers, and Regulation -

    Classic ETC inhibitors include rotenone (Complex I), antimycin A (Complex III), and cyanide (Complex IV), each blocking specific electron transfers and collapsing Δp (source: Molecular Cell Biology, Alberts et al.). Uncouplers like 2,4-dinitrophenol (DNP) dissipate the proton gradient, generating heat instead of ATP. Try electron transport chain practice questions to identify inhibitor sites based on O2 consumption and pH changes.

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