🎓 Prepared by students from Boğaziçi University

What is the Electron Transport Chain?

The electron transport chain (ETC) is the final stage of aerobic respiration, located in the inner mitochondrial membrane. It uses electrons carried by NADH and FADH2 to pump protons and power ATP synthase, producing the majority of a cell's ATP through oxidative phosphorylation.

Short answer

The electron transport chain passes electrons from NADH and FADH2 through protein complexes to oxygen, pumping protons that drive ATP synthase — yielding about 2.5 ATP per NADH and 1.5 ATP per FADH2.

How the Electron Transport Chain Makes ATP

1
Complex I
NADH is oxidized, donating electrons; protons are pumped into the intermembrane space.
↓
2
Complex II
FADH2 donates its electrons directly to the chain at a lower energy point (no protons pumped here).
↓
3
Complex III
Electrons pass via ubiquinone and cytochrome c, and more protons are pumped across.
↓
4
Complex IV
Electrons reduce oxygen to form water; the last set of protons is pumped.
↓
5
ATP Synthase
Protons flow back through ATP synthase, driving chemiosmosis to phosphorylate ADP into ATP.

Try it: interactive calculator

ATP produced

28ATP

= 10*2.5 + 2*1.5

NADH molecules oxidized10 moleculesFADH2 molecules oxidized2 molecules

Step-by-step worked examples

One glucose molecule's NADH (10 total) and FADH2 (2 total) reach the ETC. How much ATP is produced?

ATP from NADH = 10 × 2.5 = 25 ATP
ATP from FADH2 = 2 × 1.5 = 3 ATP
Total ATP = 25 + 3 = 28 ATP

If only 4 NADH reach the ETC (no FADH2), how much ATP results?

ATP from NADH = 4 × 2.5 = 10 ATP
ATP from FADH2 = 0
Total ATP = 10 ATP

A cell has 6 NADH and 4 FADH2 to oxidize. Find the ATP yield.

ATP from NADH = 6 × 2.5 = 15 ATP
ATP from FADH2 = 4 × 1.5 = 6 ATP
Total ATP = 15 + 6 = 21 ATP

Flashcards

Quick quiz

Q1.Where does the electron transport chain occur?

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

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

Correct answer: C. Oxygen accepts electrons and combines with protons to form water.

Q3.Approximately how much ATP does one NADH yield in the ETC?

Correct answer: C. Each NADH yields about 2.5 ATP through oxidative phosphorylation.

Q4.What process directly powers ATP synthase?

Correct answer: B. Protons flowing back through ATP synthase down their gradient drive ATP production.

📄Download this topic as a printable worksheet (PDF)Summary + 10 questions + answer key — print it, share it in class.↓

Study better with Bounlu apps

Notek

The full card deck, worked steps and AI-tutor support for “What is the Electron Transport Chain?” are in Notek — study by hand before your exam.

Get it free

PromyNever miss an exam or homework deadline with Promy: AI turns the dates in your inbox into reminders automatically.

Common mistakes

✗

The ETC produces ATP directly, like substrate-level phosphorylation. — Correct: The ETC produces ATP indirectly, by pumping protons to power ATP synthase (chemiosmosis).

✗

FADH2 yields the same ATP as NADH. — Correct: FADH2 yields less ATP (~1.5) than NADH (~2.5) because it enters the chain at a lower energy point.

✗

Oxygen is used early in respiration, during glycolysis. — Correct: Oxygen is only used at the very end of the ETC, as the final electron acceptor.

✗

The ETC happens in the mitochondrial matrix. — Correct: The ETC's complexes are embedded in the inner mitochondrial membrane, not the matrix.

FAQ

What is the electron transport chain?

The electron transport chain is the final stage of aerobic respiration, where NADH and FADH2 donate electrons to protein complexes in the inner mitochondrial membrane, driving ATP production.

What is the formula for ATP yield in the electron transport chain?

ATP ≈ NADH × 2.5 + FADH2 × 1.5, based on modern P/O ratio estimates.

What are examples of electron transport chain ATP calculations?

10 NADH and 2 FADH2 yield 10×2.5 + 2×1.5 = 28 ATP.

How do you calculate ATP produced by the electron transport chain?

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