A12 ETC

Where does the electron transport chain (ETC) occur?

In the inner mitochondrial membrane.

What is the main function of the ETC?

To transfer electrons from NADH and FADH₂ to O₂, forming water and generating a proton gradient for ATP synthesis.

What is the final electron acceptor in the ETC?

Oxygen (O₂).

What is the product when oxygen accepts electrons?

Water (H₂O).

Which complexes in the ETC are proton pumps?

Complex I, Complex III, and Complex IV.

Which complex does NOT pump protons?

Complex II.

What molecule shuttles electrons between Complex I/II and Complex III?

Ubiquinone (Coenzyme Q or CoQ).

What molecule shuttles electrons between Complex III and Complex IV?

Cytochrome c.

What is the role of NADH in the ETC?

NADH donates electrons to Complex I.

What is the role of FADH₂ in the ETC?

FADH₂ donates electrons to Complex II (succinate dehydrogenase).

What is the main energy source that drives ATP synthesis?

The proton gradient (proton-motive force).

What enzyme synthesizes ATP in oxidative phosphorylation?

ATP synthase (Complex V).

How many ATP molecules are generated from one NADH?

Approximately 2.5 ATP.

How many ATP molecules are generated from one FADH₂?

Approximately 1.5 ATP.

What is the total ATP yield from complete oxidation of one glucose molecule?

About 32 ATP.

What metal ions are important in cytochromes?

Iron (Fe²⁺/Fe³⁺).

What metal ions are important in Complex IV?

Copper (Cu⁺/Cu²⁺) and iron.

What is the purpose of the iron-sulfur centers?

To transfer single electrons between carriers.

What is reducing potential (E′°)?

A measure of a molecule’s tendency to donate or accept electrons.

What happens when electrons move from carriers with lower to higher E′° values?

Energy is released and used to pump protons.

Which inhibitor blocks electron flow in Complex I?

Rotenone.

Which inhibitor blocks Complex III?

Antimycin A.

Which inhibitor blocks Complex IV?

Cyanide (CN⁻) or carbon monoxide (CO).

What happens if Complex IV is inhibited?

Electron flow stops, and no ATP is produced.

What is the net reaction of the ETC?

2 NADH + 2 H⁺ + O₂ → 2 NAD⁺ + 2 H₂O.

Explain how NADH donates electrons to the ETC.

NADH donates two electrons to Complex I (NADH dehydrogenase), which transfers them via FMN and iron-sulfur centers to ubiquinone (CoQ), forming ubiquinol (QH₂). The process pumps four protons into the intermembrane space.

How does FADH₂ enter the electron transport chain?

FADH₂ donates electrons to Complex II (succinate dehydrogenase), which transfers them to CoQ, forming QH₂. No protons are pumped at this step.

Describe the function of ubiquinone (CoQ).

CoQ is a lipid-soluble electron carrier that transfers electrons from Complex I and II to Complex III, diffusing freely within the inner membrane.

What occurs in Complex III (cytochrome bc₁ complex)?

QH₂ is oxidized, electrons are transferred via cytochrome b and iron-sulfur centers to cytochrome c, and four protons are pumped across the membrane (Q cycle).

What is the role of cytochrome c?

Cytochrome c is a small, soluble protein that carries one electron at a time from Complex III to Complex IV.

Explain how Complex IV (cytochrome c oxidase) functions.

Complex IV receives electrons from cytochrome c, uses copper and iron centers to reduce O₂ to H₂O, and pumps two protons per pair of electrons.

How is oxidative phosphorylation regulated?

It depends on ADP availability; when ADP is low, proton flow slows and electron transport decreases (“respiratory control”).

Why does FADH₂ yield less ATP than NADH?

FADH₂ bypasses Complex I, so fewer protons are pumped and less ATP is produced.

What happens when the proton gradient collapses (e.g., by uncouplers like DNP)?

Electron transport continues, but ATP synthesis stops because the gradient is dissipated as heat.

Explain how the ETC and citric acid cycle are connected.

NADH and FADH₂ generated by the TCA cycle provide the electrons that fuel the ETC, linking catabolic oxidation to ATP production.

Describe the consequences of a defect in cytochrome b (Complex III).

Electrons cannot pass from QH₂ to cytochrome c, halting the chain. Muscle cells can survive through glycolysis, explaining mild-to-moderate symptoms.

What alternative pathways can feed electrons into the ETC?

Fatty acid oxidation via electron-transferring flavoprotein and glycerol phosphate dehydrogenase both reduce CoQ to QH₂.

Why is oxygen essential for aerobic respiration?

Oxygen’s high reduction potential allows it to accept the final electrons, keeping the chain flowing and enabling continued ATP production.