🎓 Prepared by students from Boğaziçi University

What is the Electron Transport Chain?

The electron transport chain (ETC) is the final stage of cellular respiration, embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 pass through protein complexes, pumping protons to build a gradient that ATP synthase uses to make most of the cell's ATP.

Short answer

The electron transport chain is a series of protein complexes that pass electrons from NADH and FADH2 to oxygen, pumping protons to drive ATP synthesis through chemiosmosis.

The Electron Transport Chain

1
Complex I
NADH is oxidized; electrons enter and protons are pumped into the intermembrane space.
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2
Complex II
FADH₂ donates electrons directly, bypassing Complex I (no protons pumped here).
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3
Complex III
Electrons pass via ubiquinone; more protons are pumped.
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4
Complex IV
Electrons combine with O₂ and H⁺ to form H₂O; more protons pumped.
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5
ATP Synthase
Protons flow back through this enzyme, driving ATP production (chemiosmosis).

Try it: interactive calculator

ATP produced (approx.)

28ATP

= 10*2.5 + 2*1.5

NADH entering the ETC10 moleculesFADH₂ entering the ETC2 molecules

Step-by-step worked examples

One glucose molecule's full oxidation delivers 10 NADH and 2 FADH2 to the ETC. Estimate the total ATP produced.

NADH: 10 × 2.5 = 25 ATP
FADH₂: 2 × 1.5 = 3 ATP
Total ≈ 25 + 3 = 28 ATP (from the ETC alone)

If only 4 NADH enter the ETC (no FADH2), how many ATP result?

4 × 2.5 = 10 ATP

A mitochondrion processes 6 FADH2 molecules only (no NADH). Find the ATP yield.

6 × 1.5 = 9 ATP

Flashcards

Quick quiz

Q1.Where does the electron transport chain occur?

Correct answer: C. The ETC protein complexes are embedded in the inner mitochondrial membrane.

Q2.What is the final electron acceptor in the ETC?

Correct answer: B. Oxygen accepts electrons and H⁺ to form water at Complex IV.

Q3.Which enzyme uses the proton gradient to produce ATP?

Correct answer: B. ATP synthase converts the proton gradient into ATP via chemiosmosis.

Q4.Why does FADH₂ yield less ATP than NADH?

Correct answer: B. FADH₂ donates electrons at Complex II, so fewer protons are pumped overall.

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Common mistakes

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The ETC directly makes ATP by transferring electrons. — Correct: The ETC pumps protons to build a gradient; ATP synthase makes the ATP using that gradient.

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The electron transport chain occurs in the mitochondrial matrix. — Correct: It occurs in the inner mitochondrial membrane, not the matrix.

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Oxygen is only needed for glycolysis. — Correct: Oxygen is the final electron acceptor in the ETC, where it forms water.

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NADH and FADH₂ yield the same amount of ATP. — Correct: NADH yields more (~2.5 ATP) than FADH₂ (~1.5 ATP) because FADH₂ enters later in the chain.

FAQ

What is the electron transport chain?

The ETC is a chain of protein complexes in the inner mitochondrial membrane that passes electrons from NADH/FADH2 to oxygen, pumping protons to drive ATP synthesis.

What is the electron transport chain's ATP formula?

Approximately ATP = (NADH × 2.5) + (FADH₂ × 1.5), reflecting the P/O ratios of each electron carrier.

What are examples of ETC ATP calculations?

10 NADH and 2 FADH2 yield about 25 + 3 = 28 ATP; 4 NADH alone yield 10 ATP.

How do you calculate ATP from the electron transport chain?

Multiply the number of NADH by 2.5 and FADH2 by 1.5, then add the results together.