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Chapter 7 · Part 5 of 9
  1. Chapter overview
  2. 1Why cells break down glucose
  3. 2The four stages at a glance
  4. 3Glycolysis splits glucose in two
  5. 4Pyruvate breakdown and the citric acid cycle
  6. 5The electron transport chain
  7. 6Chemiosmosis and the ATP payoff
  8. 7ATP synthase, a spinning machine
  9. 8Proteins and fats as fuel
  10. 9Making ATP without oxygen

The electron transport chain

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By the end of the citric acid cycle, all six carbons of glucose have left as CO₂, yet the cell has made only 4 ATP. Most of glucose’s energy now rides on the high-energy electrons carried by 10 NADH and 2 FADH₂. Oxidative phosphorylation turns those electrons into ATP.

Two events, one process

Oxidative phosphorylation takes place along the inner mitochondrial membrane, and it has two parts:

The name covers both parts: NADH and FADH₂ are oxidized, and ADP is phosphorylated.

Members of the chain

The chain is a team of protein complexes and small organic molecules, all set into the inner membrane. Electrons are handed from one member to the next like a baton in a relay race. Each handoff is a redox reaction: a member is reduced when it takes the electrons and oxidized when it passes them on.

Most members are protein complexes, numbered I through IV. They carry prosthetic groups, small non-protein molecules permanently attached to a protein that help it do its job. Cytochrome oxidase (complex IV), for example, has two prosthetic groups, and each holds an iron atom that can easily take on an electron and pass it along.

Two smaller members shuttle electrons between the complexes. One of them, ubiquinone (Q), isn’t a protein at all; it’s a small organic molecule.

Member What it is What it does
NADH dehydrogenase (complex I) Protein complex Takes electrons from NADH
Succinate reductase (complex II) Protein complex Takes electrons from FADH₂
Ubiquinone (Q) Small organic molecule, not a protein Carries electrons from complex I or II to complex III
Cytochrome b-c₁ (complex III) Protein complex Passes electrons to cytochrome c
Cytochrome c Small protein Carries electrons from complex III to complex IV
Cytochrome oxidase (complex IV) Protein complex Passes electrons to O₂, the end of the chain

Cytochrome oxidase is named for what it does: it oxidizes (removes electrons from) cytochrome c.

Following the electrons

  1. NADH gives two high-energy electrons, one at a time, to complex I and becomes NAD⁺. (FADH₂ gives its electrons to complex II instead.)
  2. Ubiquinone carries the electrons to complex III.
  3. Cytochrome c carries them from complex III to complex IV.
  4. Complex IV passes them to oxygen, the final electron acceptor. Oxygen combines with the electrons and H⁺ to form water.

The electrons move in this direction because each member pulls on electrons more strongly than the one before it. That pull is electronegativity, the tendency of an atom or molecule to attract electrons, and it increases along the chain. Oxygen is the most electronegative member, so it sits at the end and keeps the electrons. Because the oxygen you breathe is used here, the ETC is also called the respiratory chain.

For each NADH, the overall reaction is NADH + H⁺ + ½ O₂ → NAD⁺ + H₂O.

Intermembrane space Matrix Complex I Complex II Complex III Q Cytochrome c Complex IV ATP synthase Q H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ NADH FADH₂ O₂ ATP H₂O
Figure 7.4 Follow the red path from NADH or FADH₂ to O₂, and notice H⁺ being pumped up into the intermembrane space along the way. At the far right, H⁺ flows back down through ATP synthase, which makes ATP.

Pumping H⁺ builds a gradient

Each time electrons move to a member with a stronger pull, they drop to a lower energy and release a little energy. At particular points along the chain, some of that energy is used to pump H⁺ out of the matrix and into the intermembrane space. This is active transport: it pushes H⁺ toward the side where H⁺ is already crowded, which takes energy.

The result is an H⁺ electrochemical gradient across the inner membrane. It has two parts:

Why cyanide is deadly

Anything that blocks electron flow blocks ATP production. Cyanide inhibits cytochrome oxidase (complex IV), so electrons can’t reach oxygen. The chain shuts down, H⁺ pumping stops, and cells can’t make enough ATP to survive.

Check yourselfCyanide blocks only complex IV, at the very end of the chain. Why does H⁺ pumping stop along the whole chain?

With complex IV blocked, electrons can’t reach oxygen, so the carriers before it stay loaded and can’t accept more. The backup spreads all the way to NADH, electron flow stops, and with no electrons moving, there’s no energy to pump H⁺.

Practice questions

  1. 1 of 2

    Cyanide blocks cytochrome oxidase (complex IV). What happens in a cell poisoned by cyanide?

  2. 2 of 2

    The electron transport chain releases the energy of NADH’s electrons in many small steps rather than all at once. How does this help the cell?