Oxygen as the terminal electron acceptor
Oxygen is the terminal electron acceptor, the final place the electrons go when they reach the end of the electron transport chain, where it forms water.
Remember
Oxygen takes the electrons at the end of the chain and becomes water; taking them up keeps the chain moving. If this last step is blocked, the proton gradient (a store of energy across the membrane) collapses and ATP synthesis falls.
Oxidative phosphorylation is the fourth and last stage of energy production, and it takes place on the inner mitochondrial membrane. The reduced carriers NADH and FADH₂ (carriers that are still carrying electrons) deliver their electrons to the electron transport chain. As the electrons pass from one protein complex to the next, protons (hydrogen ions) are pumped across the membrane. This builds a proton gradient, also called the proton motive force, which is a store of energy in the difference in charge and concentration across the membrane.
ATP synthase (the enzyme that makes ATP) uses protons flowing back through it to join ADP and phosphate.
The chain needs somewhere to send the electrons, otherwise it would back up and stop. That place is oxygen, the terminal electron acceptor. At the last complex, complex IV (cytochrome c oxidase), oxygen is reduced, which means it gains electrons, and it picks up hydrogen ions to form water.
Oxygen does not drive ATP synthase itself. It keeps the chain moving by taking up the electrons at the end. Oxygen is like the plughole of a sink, and the electrons are like the water: they flow away when the plug is out, and everything backs up when the plughole is blocked.
If a toxin stops oxygen accepting electrons, the electrons have nowhere to go. NADH and FADH₂ pile up in their reduced form, proton pumping stops, the gradient runs down and ATP production falls rapidly. Cyanide poisoning is a real-life example: it inhibits complex IV.
Key facts
- Where it happens
- oxidative phosphorylation takes place on the inner mitochondrial membrane.
- Oxygen's job
- oxygen is reduced to water at complex IV, the last complex of the chain.
- What turns ATP synthase
- protons flowing back down their gradient turn it, while oxygen only keeps the chain running.
- If oxygen is blocked
- electrons have nowhere to go, the chain stalls, NADH builds up and ATP output falls.
- Real-life example
- cyanide poisoning inhibits cytochrome c oxidase (complex IV).
Exam traps
- A block at the final step of the chain does not increase the re-oxidation of NADH and FADH₂ (their giving up of electrons), so no extra ATP follows. The carriers cannot hand their electrons on, so they are re-oxidised less.
- Oxygen does not give ATP synthase its energy directly. Hydrogen ions flowing back through the enzyme down their gradient turn it. Oxygen takes up the electrons at the end, so that the pumps keep building the gradient.
- Blocking the end of the chain does not enhance proton pumping. Pumping needs electron flow, so when the electrons stop moving, pumping falls, and a surplus of reduced carriers does not help.
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