• 17 parts
  • Checked October 2026

Electron Transport Chain Diagram

Four complexes, two carriers and one machine: oxidative phosphorylation on two charts.

Electron Transport Chain Diagram

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
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Write the name of each numbered part.Answer key

  1. Complex I
  2. Complex II
  3. Ubiquinone (coenzyme Q)
  4. Complex III
  5. Cytochrome c
  6. Complex IV
  7. NADH
  8. FADH₂
  9. Oxygen
  10. Water
  11. Hydrogen ions (protons)
  12. ATP synthase
  13. ADP and phosphate
  14. ATP
  15. Inner membrane
  16. Intermembrane space
  17. Matrix

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Back a stage: the citric acid cycle

All 17 parts on the diagram

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The complexes and carriers 6

  1. Complex I It moves electrons from NADH on to ubiquinone. It pumps hydrogen ions as it does.
  2. Complex II It takes electrons from FADH2 and passes them to ubiquinone. It pumps no hydrogen ions.
  3. Ubiquinone (coenzyme Q) A small carrier that moves within the membrane. It takes electrons from Complexes I and II to Complex III.
  4. Complex III It moves electrons from ubiquinone on to cytochrome c. It pumps hydrogen ions.
  5. Cytochrome c A small protein that carries one electron at a time. It goes from Complex III to Complex IV.
  6. Complex IV It moves electrons from cytochrome c on to oxygen. It pumps hydrogen ions.

What goes in and comes out 4

  1. NADH It brings electrons to Complex I. Giving them up turns it back into NAD+.
  2. FADH₂ It brings electrons to Complex II. Giving them up turns it back into FAD.
  3. Oxygen The final electron acceptor. It takes the electrons at Complex IV.
  4. Water Oxygen picks up electrons and hydrogen ions and becomes water.

Making ATP 7

  1. Hydrogen ions (protons) The chain pumps them out of the matrix. They build up here and make a gradient.
  2. ATP synthase Hydrogen ions flow back to the matrix through it. That flow drives the making of ATP.
  3. ADP and phosphate ATP synthase joins a phosphate to ADP. The product is ATP.
  4. ATP The product of oxidative phosphorylation. It is made in the matrix by ATP synthase.
  5. Inner membrane The mitochondrion's inner membrane. The chain and ATP synthase are embedded in it.
  6. Intermembrane space The space between the two membranes of a mitochondrion. Hydrogen ions collect in it.
  7. Matrix The innermost space of the mitochondrion. NADH and FADH2 are made here, and ATP is released here.

How this list was compiled: Every name and step checked on 2026-10-01 against Wikipedia: Electron transport chain and Oxidative phosphorylation, text only; StatPearls, Biochemistry, Electron Transport Chain (NCBI Bookshelf NBK526105, archived copy); and Alberts and others, Molecular Biology of the Cell, 4th edition, chapter 14 (NCBI Bookshelf NBK26904 and NBK26894, archived copies). Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

The electron transport chain sits in the inner membrane of a mitochondrion. The first chart follows the electrons from NADH and FADH₂ to oxygen; the second shows the hydrogen ions flowing back through ATP synthase.

Read the full guide

Follow the electrons, then the hydrogen ions

NADH hands its electrons to Complex I and FADH₂ hands its electrons to Complex II. Ubiquinone carries them to Complex III, cytochrome c carries them to Complex IV, and there oxygen takes them and becomes water. Complexes I, III and IV pump hydrogen ions out of the matrix as the electrons pass; Complex II does not. The ions build up in the intermembrane space, then flow back through ATP synthase, which joins ADP and phosphate into ATP. The chain and ATP synthase together are oxidative phosphorylation.

  • The shaded side of the membrane is the intermembrane space; the matrix is below.
  • Blocks with an arrow through them pump hydrogen ions.

Questions and answers

What are the parts of oxidative phosphorylation?

The electron transport chain and ATP synthase. The chain is Complexes I, II, III and IV with two carriers, ubiquinone and cytochrome c. It builds a hydrogen ion gradient across the inner membrane, and ATP synthase uses the gradient to make ATP.

What is the order of the electron transport chain?

From NADH: Complex I, ubiquinone, Complex III, cytochrome c, Complex IV, oxygen. From FADH₂ the electrons enter at Complex II and then follow the same path from ubiquinone on.

Where is the electron transport chain located?

In the mitochondrion's inner membrane. Hydrogen ions are pumped out of the matrix to the intermembrane space, and ATP is made on the matrix side.

What is the final electron acceptor?

Oxygen. At Complex IV it takes the electrons, picks up hydrogen ions and forms water.

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