Falling electrons pump protons; returning protons make ATP. A proton leak sends oxygen use up and ATP down.
Respiration pulls electrons off glucose and lets them fall, step by step, to oxygen. The energy released pumps protons (H+) across the inner mitochondrial membrane. Protons flowing back through ATP synthase make most of the cell's ATP.
Key terms
- NADH — an electron carrier. NAD+ picks up electrons to become NADH, then unloads them at the chain. FADH2 works alike.
- Electron transport chain — proteins in the inner mitochondrial membrane that pass electrons to oxygen, using the energy released to pump H+.
- Chemiosmosis — making ATP with a proton gradient. H+ flows back down its gradient through ATP synthase, which joins ADP and phosphate.
- Substrate-level phosphorylation — an enzyme moves a phosphate from an intermediate onto ADP. No gradient needed. Glycolysis and the Krebs cycle use it.
- Fermentation — when the chain can't take electrons, NADH unloads onto pyruvate (or acetaldehyde, in yeast) instead, regenerating NAD+.
How it works
- Glycolysis, in the cytosol: glucose → 2 pyruvate, netting 2 ATP and 2 NADH. It needs no oxygen, but it does need NAD+.
- Each pyruvate enters the mitochondrial matrix, loses one carbon as CO2, and becomes acetyl-CoA. One NADH forms.
- The Krebs (citric acid) cycle breaks each acetyl group down to 2 CO2. The electrons load onto NADH and FADH2, and each turn makes one ATP directly.
- NADH drops its electrons at the chain's first complex. FADH2 enters at the second. As electrons move toward oxygen, their energy pumps H+ into the intermembrane space.
- Oxygen is the final electron acceptor. It takes the spent electrons plus H+ and becomes water. The CO2 you breathe out comes from glucose's carbons.
- H+ flows back into the matrix through ATP synthase. This makes about 26 of the roughly 30 ATP each glucose yields.
Why it matters
Most respiration questions are break-it questions: a poison, a mutant, a missing gas. Trace one link at a time. No oxygen means the chain stops, NADH piles up, and glycolysis stalls unless fermentation recycles NAD+. Yeast make ethanol that way; your muscles make lactate. At 2 ATP per glucose instead of about 30, a fermenting cell must burn glucose far faster. Chloroplasts make ATP by the same chemiosmosis.
Exam traps
| Trap | Actually |
|---|
| Fermentation makes extra ATP to replace what the chain made. | The fermentation steps make no ATP. They recycle NAD+ so glycolysis can keep making its 2 ATP per glucose. |
| FADH2 yields less ATP because it carries fewer electrons. | Both carry two electrons. FADH2 enters the chain past the first pump, so its electrons move fewer H+ across. |
| ATP synthase gets its energy from electrons or from oxygen. | It runs on the H+ gradient alone. An artificial gradient, with no electron transport, still makes ATP. |
| Blocking ATP synthase leaves the chain running at full speed. | The gradient climbs until pumping more H+ costs too much. Electron flow and oxygen use drop within seconds. |
Worked example
Isolated mitochondria are given pyruvate, ADP, and phosphate. Before drug D, they use 40 nmol of O2 per minute and make 190 nmol of ATP per minute. After drug D, they use 70 nmol of O2 per minute, make only 10 nmol of ATP per minute, and give off extra heat. Which action of drug D best explains these results?
- Read oxygen first. Oxygen use rose, so electrons are reaching oxygen faster. The chain is not blocked.
- Read ATP next. It collapsed while the chain sped up, so the energy of electron flow is no longer reaching ATP synthase.
- The link between them is the H+ gradient. If H+ leaks back across the inner membrane without passing through ATP synthase, the gradient never builds.
- With no gradient to push against, pumping is easy and the chain speeds up. The energy leaves as heat. The leftover 10 nmol of ATP comes from substrate-level phosphorylation in the Krebs cycle.
Drug D makes the inner mitochondrial membrane leaky to H+. It uncouples electron transport from ATP synthesis.
Oxygen use up with ATP down means the gradient is being drained, not blocked. The near miss is 'drug D blocks ATP synthase.' That also cuts ATP, but the gradient would build up, pumping would stall, and oxygen use would fall instead of rise.