Electron Transport Chain

What is the Electron Transport Chain (ETC)?

A series of protein complexes and molecules in the inner mitochondrial membrane that transfers electrons to generate ATP, the cell's energy currency.

Where does the ETC occur?

In the inner mitochondrial membrane of eukaryotic cells (or the plasma membrane of prokaryotic cells).

What is the main purpose of the ETC?

To create a proton gradient across the inner mitochondrial membrane, which drives ATP synthesis via ATP synthase.

What molecules donate electrons to the ETC?

NADH and FADH₂, which are produced during glycolysis, the citric acid cycle, and other metabolic pathways.

What are the four main protein complexes in the ETC?

Complex I (NADH dehydrogenase)

Complex II (Succinate dehydrogenase)

Complex III (Cytochrome bc₁ complex)

Complex IV (Cytochrome c oxidase)

What happens at Complex I?

NADH donates electrons to this complex. Electrons are passed to ubiquinone (CoQ), and protons (H⁺) are pumped into the intermembrane space.

What happens at Complex II?

FADH₂ donates electrons to this complex. Electrons are passed to ubiquinone (CoQ), but no protons are pumped at this step.

What is the role of ubiquinone (CoQ)?

CoQ shuttles electrons from Complexes I and II to Complex III.

What happens at Complex III?

Electrons are transferred from CoQ to cytochrome c. Protons are pumped into the intermembrane space.

What is the role of cytochrome c?

It shuttles electrons from Complex III to Complex IV.

What happens at Complex IV?

Electrons are transferred to oxygen (O₂), forming water (H₂O). Protons are pumped into the intermembrane space.

What is the final electron acceptor in the ETC?

Oxygen (O₂), which combines with electrons and protons to form water (H₂O).

What is the proton gradient?

The accumulation of protons (H⁺) in the intermembrane space creates a gradient, storing potential energy used to make ATP.

What is ATP synthase?

An enzyme that uses the proton gradient to phosphorylate ADP, producing ATP.

How many ATP molecules are produced per NADH and FADH₂?

~2.5 ATP and ~1.5 ATP, respectively.

What is oxidative phosphorylation?

The process of ATP production driven by the ETC and ATP synthase, using energy from electron transfer and the proton gradient.

What happens if oxygen is not available?

The ETC stops because oxygen is the final electron acceptor. This halts ATP production and leads to fermentation or cell death.

Summarize the ETC in one sentence.

The ETC uses electrons from NADH and FADH₂ to create a proton gradient, which drives ATP synthesis via ATP synthase, with oxygen as the final electron acceptor.