Electron Transport Chain (ETC): Steps, Complexes and ATP Yield
Contents
In short: The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane. It passes electrons from NADH and FADH2 to oxygen, uses the released energy to pump protons across the membrane, and lets ATP synthase make ATP from the resulting gradient. This is a biochemistry topic for students, not a regulatory one.
What the ETC does
Glycolysis, the citric acid (TCA) cycle and fatty acid oxidation all produce the reduced carriers NADH and FADH2. The ETC is the last stage of aerobic respiration: it takes electrons from these carriers and finally transfers them to oxygen. The energy released on the way is used to build a proton gradient, and that gradient drives ATP synthesis. The whole coupled process is called oxidative phosphorylation.
In eukaryotes the ETC sits in the inner mitochondrial membrane. In bacteria the equivalent chain sits in the plasma membrane.
For the earlier stage, see Glycolysis: introduction, pathway, diagram and summary.
The components
| Component | What it does |
|---|---|
| Complex I (NADH dehydrogenase) | Takes electrons from NADH and passes them to ubiquinone; pumps protons into the intermembrane space |
| Complex II (succinate dehydrogenase) | Takes electrons from FADH2 (formed from succinate in the TCA cycle) and passes them to ubiquinone; does not pump protons |
| Ubiquinone (coenzyme Q) | Small lipid-soluble carrier that moves electrons within the membrane from complexes I and II to complex III |
| Complex III (cytochrome bc1 complex) | Passes electrons from ubiquinol to cytochrome c; pumps protons |
| Cytochrome c | Small protein on the outer face of the inner membrane that carries electrons from complex III to complex IV |
| Complex IV (cytochrome c oxidase) | Passes electrons to oxygen, forming water; pumps protons |
| ATP synthase (sometimes called complex V) | Uses the proton flow back into the matrix to make ATP from ADP and phosphate |
Step by step
- NADH gives up two electrons to complex I and is oxidised back to NAD+. FADH2 gives its electrons to complex II instead, so it enters the chain one step later than NADH.
- Ubiquinone carries the electrons to complex III.
- Cytochrome c carries them from complex III to complex IV.
- At complex IV the electrons, together with protons, reduce oxygen to water. Oxygen is the final electron acceptor; without it the chain stops and NADH and FADH2 cannot be re-oxidised.
- As electrons move through complexes I, III and IV, protons are pumped from the matrix into the intermembrane space. This builds an electrochemical gradient, the proton-motive force, made of a pH difference and a membrane potential.
- Protons flow back into the matrix through ATP synthase. The flow turns part of the enzyme (a rotary mechanism) and drives the synthesis of ATP.
The energy for ATP synthesis therefore comes from the proton gradient, not directly from the last electron transfer to oxygen. Respiration and ATP synthesis are “coupled” through this gradient.
How much ATP is produced?
Textbook figures for ATP yield have changed over the years.
| Carrier | Older textbook figure | Modern estimate |
|---|---|---|
| 1 NADH | 3 ATP | about 2.5 ATP |
| 1 FADH2 | 2 ATP | about 1.5 ATP |
The older whole numbers assumed that each proton pumped always gives a fixed amount of ATP. The modern, lower values reflect that ATP synthase needs roughly 3 to 4 protons per ATP (including the proton cost of moving phosphate and ADP/ATP across the membrane), and that the numbers of protons pumped per electron pair are not exact integers of ATP. They remain estimates. Using the modern values, complete oxidation of one glucose molecule gives about 30 to 32 ATP, compared with the older figure of 36 to 38. Exam questions may still expect the older numbers, so check which your syllabus uses.
Inhibitors and uncouplers
- Rotenone blocks complex I, and antimycin A blocks complex III.
- Cyanide and carbon monoxide block complex IV. This stops the chain, and cells then rely on poorly yielding glycolysis.
- Oligomycin blocks ATP synthase.
- Uncouplers (for example 2,4-dinitrophenol) let protons leak back across the membrane without passing through ATP synthase. Oxygen use continues, but the energy is released as heat instead of ATP.
Frequently asked questions
Where does the ETC take place?
In the inner mitochondrial membrane of eukaryotic cells. In bacteria it is in the cell (plasma) membrane.
Why is oxygen needed?
Oxygen is the final electron acceptor at complex IV. If it is absent, electrons back up in the chain, NAD+ and FAD cannot be regenerated, and the TCA cycle also slows.
Why does FADH2 give less ATP than NADH?
FADH2 feeds electrons in at complex II, which does not pump protons, so fewer protons are pumped per electron pair and less ATP is made.
Do all complexes pump protons?
No. Complexes I, III and IV pump protons. Complex II does not.