Electron transport chain

What is the main purpose of the electron transport chain?

To use electrons from NADH and FADH₂ to create a proton gradient that powers ATP production.

Where is the electron transport chain located?

The inner mitochondrial membrane.

What two molecules donate electrons to the electron transport chain?

• NADH

• FADH₂

What is the final electron acceptor in the electron transport chain?

Oxygen.

What is formed when oxygen accepts electrons and protons?

Water.

Does the electron transport chain directly produce most ATP?

No. It creates the proton gradient that ATP synthase uses to produce ATP.

Where does NADH donate its electrons?

Complex I.

Where does FADH₂ donate its electrons?

Complex II.

What is the general order of electron flow from NADH?

Complex I
→ coenzyme Q
→ Complex III
→ cytochrome c
→ Complex IV
→ oxygen

What is the general order of electron flow from FADH₂?

Complex II
→ coenzyme Q
→ Complex III
→ cytochrome c
→ Complex IV
→ oxygen

What mobile electron carrier transfers electrons from Complexes I and II to Complex III?

Coenzyme Q, also called ubiquinone.

What mobile electron carrier transfers electrons from Complex III to Complex IV?

Cytochrome c.

What is the main function of Complex I?

It accepts electrons from NADH and pumps protons into the intermembrane space.

What is the main function of Complex II?

It accepts electrons from FADH₂ and transfers them to coenzyme Q.

Why is Complex II especially important to the citric acid cycle?

Complex II is also succinate dehydrogenase, the enzyme that converts succinate into fumarate.

What is the main function of Complex III?

It transfers electrons from coenzyme Q to cytochrome c and pumps protons.

What is the main function of Complex IV?

It transfers electrons to oxygen, forms water, and pumps protons.

Which electron transport chain complexes pump protons?

• Complex I

• Complex III

• Complex IV

Which electron transport chain complex does not pump protons?

Complex II.

From where to where are protons pumped?

From the mitochondrial matrix into the intermembrane space.

Where is proton concentration highest during oxidative phosphorylation?

The intermembrane space.

What is the proton-motive force?

The stored energy created by the proton concentration and charge difference across the inner mitochondrial membrane.

What enzyme uses the proton gradient to produce ATP?

ATP synthase.

In which direction do protons flow through ATP synthase?

From the intermembrane space back into the mitochondrial matrix.

What reaction is powered by proton flow through ATP synthase?

ADP + Pi → ATP

What is chemiosmosis?

The movement of protons down their gradient through ATP synthase to power ATP production

What is oxidative phosphorylation?

ATP production powered by electron transport and the proton gradient.

Approximately how much ATP is produced from one NADH?

About 2.5 ATP.

Approximately how much ATP is produced from one FADH₂?

About 1.5 ATP.

Why does NADH produce more ATP than FADH₂?

NADH enters at Complex I, while FADH₂ enters at Complex II and bypasses the proton-pumping Complex I.

What happens to the electron transport chain if oxygen is unavailable?

Electron flow stops.

What happens to NADH and FADH₂ when the electron transport chain stops?

They cannot be efficiently converted back into NAD⁺ and FAD.

Why does the citric acid cycle slow when oxygen is unavailable?

NAD⁺ and FAD are not regenerated efficiently, so the cycle lacks the oxidized electron carriers it needs.

What is the difference between an electron transport chain inhibitor and an uncoupler?

• An inhibitor blocks electron flow.

• An uncoupler allows electron flow but dissipates the proton gradient, reducing ATP production.

What happens to heat production when oxidative phosphorylation is uncoupled?

Heat production increases.

Summarize the electron transport chain and oxidative phosphorylation.

1. NADH donates electrons to Complex I.

2. FADH₂ donates electrons to Complex II.

3. Electrons travel through coenzyme Q, Complex III, cytochrome c, and Complex IV.

4. Oxygen accepts the electrons and forms water.

5. Complexes I, III, and IV pump protons into the intermembrane space.

6. Protons flow back into the matrix through ATP synthase.

7. ATP synthase uses that energy to make ATP.

Approximate yield:

• 1 NADH → 2.5 ATP

• 1 FADH₂ → 1.5 ATP