Electron Transport Chain and Oxidative Phosphorylation Electron Transport Chain & Oxidative Phosphorylation

What is the main purpose of the electron transport chain (ETC)?

To transfer electrons from NADH and QH₂ to O₂, releasing energy used to pump H⁺ and create a gradient for ATP synthesis.

Where is the electron transport chain located?

Inner mitochondrial membrane.

What is the final electron acceptor in the ETC?

Oxygen (O₂), which is reduced to water (H₂O).

Which ETC complexes pump protons?

Complexes I, III, and IV.

**Complex II DOES NOT pump protons.

What does Complex I do?

Accepts electrons from NADH, transfers them through FMN and Fe-S clusters to Q, producing QH₂. Pumps H⁺.

What is the electron transfer sequence in Complex I?

NADH → FMN → Fe-S clusters → Q → QH₂.

What does Complex II do?

Transfers electrons from succinate through bound FAD and Fe-S clusters to Q, producing QH₂. Does NOT pump H⁺.

Why is Complex II also part of the citric acid cycle?

It is succinate dehydrogenase, which converts succinate → fumarate while transferring electrons to Q.

What does Complex III do?

Transfers electrons from QH₂ to cytochrome c and pumps H⁺.

What does Complex IV do?

Transfers electrons from cytochrome c to O₂, producing H₂O, and pumps H⁺.

What is the difference between Q and QH₂?

Q (ubiquinone) is oxidized. QH₂ (ubiquinol) is reduced and carries 2 electrons and 2 protons.

What is the function of cytochrome c?

A mobile electron carrier that transfers electrons one at a time from Complex III to Complex IV.

What are Fe-S clusters used for?

Transferring electrons one at a time within ETC complexes.

What is the difference between FMN and Fe-S clusters?

FMN can accept 1 or 2 electrons; Fe-S clusters transfer 1 electron at a time.

In which direction do ETC complexes pump H⁺?

From the mitochondrial matrix → intermembrane space.

In which direction do H⁺ flow through ATP synthase?

From intermembrane space → matrix, down their electrochemical gradient.

What drives ATP synthesis during oxidative phosphorylation?

The proton-motive force (electrochemical H⁺ gradient) generated by the ETC.

What are the functions of F₀ and F₁ in ATP synthase?

F₀ is the membrane-embedded proton-driven motor; F₁ contains the catalytic sites that synthesize ATP.

Which ATP synthase subunits contain the catalytic active sites?

β (beta) subunits.

Which ATP synthase subunit rotates in 120° steps?

γ (gamma) subunit, causing conformational changes in the β subunits.

What is the P:O ratio?

The number of ATP molecules synthesized per oxygen atom reduced (½ O₂).

What is the approximate P:O ratio for NADH versus QH₂?

NADH ≈ 2.5 ATP; QH₂ ≈ 1.5 ATP.

Why does NADH produce more ATP than electrons entering through Complex II?

NADH enters at Complex I, which pumps H⁺. Electrons entering through Complex II bypass that proton-pumping step.

What happens to the NADH/NAD⁺ ratio if Complex I is inhibited?

It increases because NADH cannot be efficiently oxidized to NAD⁺ by Complex I.

What happens to ATP production if the ETC stops pumping protons?

ATP synthesis decreases because the proton gradient can no longer be maintained.

If Complex II is inhibited, which electron input is directly blocked?

Electron transfer from succinate to Q through succinate dehydrogenase.

What happens if Complex IV cannot transfer electrons to oxygen?

Electron flow backs up, upstream carriers become more reduced, and proton pumping eventually stops.

What happens if the mitochondrial inner membrane becomes freely permeable to H⁺?

The proton gradient dissipates, reducing ATP synthesis even if electron transport continues.

In what direction do electrons spontaneously flow based on reduction potentials?

From a more negative reduction potential to a more positive reduction potential.

What equation relates standard free energy to reduction potential?

ΔG°′ = −nFΔE°′, where ΔE°′ = E°′acceptor − E°′donor.