The Electron Transport Chain

By the end of the Krebs cycle, the carbon atoms of glucose have all left as carbon dioxide, yet only four ATP have been made per glucose. Most of the energy is still locked in the electrons carried by NADH and FADH₂. The electron transport chain and ATP synthase, together called oxidative phosphorylation, release that energy in controlled steps and use it to make around 26 to 28 more ATP. It’s one of the most elegant energy-conversion systems in nature, and it runs in every one of your cells right now.

Where it happens

The chain sits in the inner membrane of mitochondria, which is folded into shelves called cristae to increase its area. The membrane separates two spaces:

• the matrix (inside), where the Krebs cycle runs;
• the intermembrane space (between the inner and outer membranes).

In bacteria, the same machinery sits in the cell membrane. Plant chloroplasts use a very similar system in photosynthesis.

The energy source: a big redox drop

The overall process is the oxidation of NADH by oxygen:

NADH + H⁺ + ½O₂ → NAD⁺ + H₂O

Using standard reduction potentials (see oxidation and reduction):

• NAD⁺/NADH: E°′ ≈ −0.32 V (a strong electron donor)
• ½O₂/H₂O: E°′ ≈ +0.82 V (a strong electron acceptor)

The difference is ΔE°′ ≈ 1.14 V. For a two-electron transfer, the free energy change is:

ΔG°′ = −nFΔE°′ = −2 × 96,485 C mol⁻¹ × 1.14 V ≈ −220 kJ mol⁻¹

That’s enough energy, in principle, for several ATP. Releasing it all at once would waste most of it as heat. So the chain passes electrons through a series of carriers with gradually increasing reduction potentials, like water flowing down a staircase of waterwheels.

The components

Complex I: NADH dehydrogenase

• Accepts two electrons from NADH, regenerating NAD⁺ for the Krebs cycle and glycolysis.
• Passes the electrons through a flavin (FMN) and a series of iron–sulfur clusters to ubiquinone.
• Pumps 4 protons from the matrix into the intermembrane space for every two electrons.

Complex II: succinate dehydrogenase

• The same enzyme as step 6 of the Krebs cycle. It oxidises succinate to fumarate, making FADH₂, and passes the electrons to ubiquinone.
• Pumps no protons. That’s why FADH₂ yields less ATP than NADH.

Ubiquinone (coenzyme Q)

• A small, fat-soluble molecule that moves freely within the membrane.
• Collects electrons from Complexes I and II and carries them to Complex III.
• Picks up two electrons and two protons to become ubiquinol (QH₂).

Complex III: cytochrome bc₁

• Transfers electrons from ubiquinol to cytochrome c, through iron-containing haem groups and an iron–sulfur cluster.
• Pumps 4 protons per two electrons (through a mechanism called the Q cycle).

Cytochrome c

• A small, water-soluble protein with one haem group, sitting on the outer face of the inner membrane.
• Carries one electron at a time from Complex III to Complex IV, using the Fe³⁺/Fe²⁺ couple.

Complex IV: cytochrome c oxidase

• Collects four electrons, one at a time, and uses them to reduce one O₂ molecule to two water molecules: O₂ + 4H⁺ + 4e⁻ → 2H₂O
• Uses haem iron and copper centres to hold the oxygen while it’s reduced, without releasing dangerous partly reduced forms such as superoxide.
• Pumps 2 protons per two electrons.

Oxygen is the final electron acceptor. Without it, electrons back up through the chain, NADH can’t be reoxidised, and the Krebs cycle and link reaction stop.

Proton pumping: storing energy as a gradient

For each NADH, the complexes pump about 10 protons across the inner membrane (4 + 4 + 2). For each FADH₂, about 6 (Complexes III and IV only).

This builds up a proton-motive force with two parts:

• a concentration difference (pH gradient): the intermembrane space becomes more acidic than the matrix, by about 0.5 to 1 pH unit;
• an electrical potential difference: the matrix side becomes negative relative to the outside, typically by about 150 to 180 mV.

The energy is now stored as a gradient, like water held behind a dam.

Chemiosmosis: Peter Mitchell’s big idea

For years, biochemists searched for a high-energy chemical intermediate linking electron transport to ATP synthesis, and never found one. In 1961, Peter Mitchell proposed something radical: the link is the proton gradient itself. Energy from electron transport is stored as a gradient of protons across a membrane, and that gradient drives ATP synthesis. His chemiosmotic theory was controversial for years but was confirmed by many experiments, including making ATP in membrane vesicles simply by imposing an artificial pH gradient. Mitchell received the 1978 Nobel Prize in Chemistry.

ATP synthase: a rotary motor

Protons can flow back into the matrix through only one route: ATP synthase, one of the most remarkable machines in biology.

• F₀, the part in the membrane, contains a ring of subunits that rotates as protons pass through it, one by one.
• F₁, the part sticking into the matrix, has three catalytic sites arranged around a central shaft attached to the rotor.
• As the shaft turns, it changes the shape of each catalytic site in turn, cycling each one through three states: binding ADP and phosphate, forming ATP, and releasing ATP. This binding-change mechanism was proposed by Paul Boyer, and the structure was solved by John Walker; they shared the 1997 Nobel Prize in Chemistry.
• The rotation has been filmed directly under a microscope by attaching a fluorescent filament to the rotor.

In mammalian mitochondria, it takes roughly 8 protons for one full rotation, producing 3 ATP, plus about one more proton per ATP to transport ADP and phosphate into the matrix. That’s about 3.7 protons per ATP.

How much ATP?

• Per NADH: ~10 protons ÷ ~4 protons per ATP ≈ 2.5 ATP
• Per FADH₂: ~6 protons ÷ ~4 ≈ 1.5 ATP

Per glucose (10 NADH and 2 FADH₂), oxidative phosphorylation gives roughly 26 to 28 ATP. With glycolysis and the Krebs cycle, the total is about 30 to 32 ATP (see cellular respiration). The yield isn’t a whole number because the proton gradient links the two processes loosely, not through fixed chemical steps.

Poisons and uncouplers

Because the chain is so central, blocking it is deadly:

• Cyanide and carbon monoxide bind to the iron in Complex IV, preventing electrons reaching oxygen. Cells can’t use oxygen even though blood is full of it.
• Rotenone, a plant-derived pesticide, blocks Complex I.
• Oligomycin, an antibiotic, blocks the proton channel of ATP synthase.

Uncouplers let protons leak back across the membrane without passing through ATP synthase. Electron transport continues, even speeds up, but the energy is released as heat instead of making ATP.

• 2,4-Dinitrophenol (DNP) is a chemical uncoupler. It was sold as a weight-loss drug in the 1930s, and is still sold illegally, but it’s extremely dangerous: overdose causes uncontrollable overheating and death.
• Brown fat uses a natural uncoupling protein, thermogenin (UCP1), to generate heat on purpose. It’s important in newborn babies and hibernating animals.

Key takeaways

• The electron transport chain in the inner mitochondrial membrane passes electrons from NADH and FADH₂ to oxygen, releasing about 220 kJ mol⁻¹ per NADH in small steps.
• Complexes I, III and IV pump protons into the intermembrane space; Complex II doesn’t. Ubiquinone and cytochrome c carry electrons between complexes.
• The resulting proton gradient (Mitchell’s chemiosmotic theory) drives ATP synthase, a rotary motor.
• Yields: about 2.5 ATP per NADH and 1.5 per FADH₂, giving ~30–32 ATP per glucose overall.
• Cyanide blocks Complex IV; uncouplers like DNP turn the energy into heat. For the energy carrier itself, see ATP: the cell’s energy currency.