NADH drops off two high-energy electrons at Complex I and becomes NAD⁺. Those electrons begin their downhill journey through the chain.
Burning food does not make ATP directly. Instead your mitochondria do something clever: they use the energy in food to pump protons uphill across a membrane, building up a reservoir — just like a dam stores water behind a wall. Then they let those protons rush back down through a turbine to do useful work. The turbine is ATP synthase, and the work it does is making ATP, the molecule every cell spends to power almost everything it does.
On the inner mitochondrial membrane sit four respiratory complexes. Electron carriers made earlier (NADH and FADH₂) drop off high-energy electrons, which then fall step by step through the complexes to a lower energy. Three of them — Complexes I, III, and IV — use the released energy to pump H⁺ from the matrix into the intermembrane space. The build-up of protons there is an energy store called the proton-motive force, which has two parts: a concentration (pH) difference and an electrical voltage, together about $-200\ \text{mV}$ across the membrane.
This is chemiosmosis, Peter Mitchell’s idea: the gradient links the electron chain to ATP making. The electrons must end somewhere — the final acceptor is oxygen, which takes them at Complex IV to form water, $\tfrac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \rightarrow \text{H}_2\text{O}$. Meanwhile protons flow back through ATP synthase’s Fₒ ring, spinning the central stalk inside the F₁ head; each of its three sites is forced through open → loose → tight shapes that bind $\text{ADP}+\text{P}_i$ and squeeze out ATP — the binding-change mechanism, about three ATP per full turn.
Let it play and follow one batch of electrons: in at Complex I, along to oxygen, with protons pumped up at I, III, and IV, then rushing back down to spin the rotor and release green ATP. Now switch the donor to FADH₂: it enters at Complex II, skips Complex I’s pump, fewer H⁺ are pumped, and the ATP yield drops from about $2.5$ to about $1.5$. Finally flip on the Uncoupler: the chain still runs but protons leak straight back, the gradient collapses, and ATP output falls toward zero — the lost energy escapes as heat, exactly how brown fat keeps a newborn warm.
| Stage | What happens |
|---|---|
| 1. Electrons enter | NADH gives two electrons to Complex I (or FADH₂ gives them to Complex II) and is oxidized back to NAD⁺ (or FAD). |
| 2. Down the chain | Mobile ubiquinone (Q) ferries electrons in the membrane to Complex III; cytochrome c carries them along the outer face to Complex IV. Each step is to lower energy. |
| 3. Protons pumped | Complexes I, III, IV use the released energy to pump H⁺ from the matrix into the intermembrane space. Complex II pumps none. |
| 4. Oxygen finishes it | At Complex IV, $\tfrac{1}{2}\text{O}_2$ accepts the spent electrons with protons to form water. No oxygen, no flow — the chain backs up. |
| 5. Proton-motive force | H⁺ piles up outside, making the intermembrane space acidic and positive. This stored gradient is the proton-motive force. |
| 6. ATP synthase spins | Protons flow back through Fₒ, spinning the c-ring and γ stalk; F₁’s three β sites cycle open→loose→tight and release ATP — roughly 3 per turn. |
Electrons carried by NADH start at a low reduction potential ($E^{\circ\prime}\approx -0.32\ \text{V}$) and end on oxygen at a high one ($E^{\circ\prime}\approx +0.82\ \text{V}$). That large drop releases a lot of free energy, $\Delta G^{\circ\prime}\approx -220\ \text{kJ/mol}$ for the pair of electrons. The chain does not waste it as heat: it splits the drop into three smaller falls at Complexes I, III, and IV, and each fall powers a proton pump. The energy is then banked in the gradient and spent by ATP synthase. This is why the “energy staircase” beside the model has three steps, one per pump.
About 10 protons are pumped per NADH and about 6 per FADH₂, and roughly 4 protons must flow back through ATP synthase to export one ATP (about 3 to spin the rotor plus 1 to carry ATP and phosphate across the membrane). That gives the modern figures of about 2.5 ATP per NADH and 1.5 ATP per FADH₂, for roughly 26–28 ATP per glucose from this stage and about 30–32 ATP overall. Older textbooks round these to 3 and 2, giving the familiar 36–38.
ATP synthase is a molecular machine in the inner mitochondrial membrane that builds ATP, the cell’s energy currency, from ADP and inorganic phosphate. It has two parts: Fₒ, a ring of protein subunits embedded in the membrane that acts as a turbine, and F₁, a knob sitting in the matrix that does the chemistry. As protons flow downhill through Fₒ, the ring and the central stalk spin, and that rotation forces the three catalytic sites in F₁ to change shape and squeeze ADP and phosphate together into ATP.
Key takeaway: ATP synthase is a membrane-embedded rotary enzyme that uses a flow of protons to spin a rotor and make ATP from ADP and phosphate.The electron transport chain is a series of protein complexes (Complexes I, II, III, and IV) in the inner mitochondrial membrane that pass electrons from one to the next. Electrons are delivered by the carriers NADH and FADH₂, which were loaded up during glycolysis and the citric acid cycle. As the electrons hop from complex to complex they fall to lower and lower energy, and the final acceptor is oxygen, which combines with electrons and protons to form water. The energy released along the way is used by Complexes I, III, and IV to pump protons across the membrane.
Key takeaway: the electron transport chain passes high-energy electrons down a series of complexes to oxygen, releasing energy that is used to pump protons across the inner membrane.Chemiosmosis is the idea, proposed by Peter Mitchell, that the energy from electron transport is stored as a proton gradient and then cashed in to make ATP. The electron transport chain pumps protons out of the matrix into the intermembrane space, building up a higher concentration of protons (and a positive charge) on the outside. This difference is called the proton-motive force, and it has two parts: a concentration (pH) difference and an electrical voltage across the membrane. Protons then flow back into the matrix through ATP synthase, and that downhill flow powers ATP production.
Key takeaway: electron transport stores energy as a proton gradient across the membrane, and ATP synthase releases that energy by letting protons flow back through it to make ATP.Oxygen is the final electron acceptor at the end of the electron transport chain. At Complex IV, oxygen combines with electrons and protons to form water. Without oxygen to pull electrons off the end of the chain, electrons back up and the whole chain stops, like cars piling up when the exit is blocked. When the chain stops, no protons are pumped, the gradient collapses, and ATP synthase has nothing to drive it. That is why oxidative phosphorylation, which makes most of a cell’s ATP, depends on oxygen, and why cells switch to far less efficient fermentation when oxygen runs out.
Key takeaway: oxygen accepts the spent electrons at the end of the chain to form water, and without it the chain halts and oxidative phosphorylation stops.Oxidative phosphorylation makes the large majority of the ATP from a glucose molecule. Modern estimates are that each NADH that feeds the chain yields about 2.5 ATP and each FADH₂ yields about 1.5 ATP, because FADH₂ enters at Complex II and skips the proton pumping at Complex I. Adding up all the NADH and FADH₂ from glycolysis and the citric acid cycle gives roughly 26 to 28 ATP from the chain, out of a total of about 30 to 32 ATP per glucose. Older textbooks use the round numbers 3 ATP per NADH and 2 per FADH₂, giving the familiar figure of about 36 to 38 ATP.
Key takeaway: oxidative phosphorylation makes most of the cell’s ATP, roughly 2.5 ATP per NADH and 1.5 per FADH₂, for about 26 to 28 ATP per glucose.NADH hands its electrons to Complex I, the first proton pump in the chain, so its electrons drive proton pumping at Complexes I, III, and IV. FADH₂ hands its electrons to Complex II (succinate dehydrogenase), which sits later in the chain and does not pump any protons. So FADH₂ electrons skip Complex I entirely and only drive pumping at Complexes III and IV. Fewer protons pumped means a smaller contribution to the gradient and less ATP, about 1.5 ATP for FADH₂ versus about 2.5 for NADH.
Key takeaway: FADH₂ enters the chain at Complex II and bypasses the proton pumping of Complex I, so it drives fewer protons across the membrane and makes less ATP than NADH.An uncoupler is a molecule that lets protons leak back across the inner membrane without going through ATP synthase, so it uncouples electron transport from ATP production. The electron transport chain keeps running and even speeds up, but the protons short-circuit back into the matrix and their energy is released as heat instead of being captured as ATP. The chemical DNP does this artificially, and the natural protein thermogenin (UCP1) in brown fat does it on purpose to generate heat for newborns and hibernating animals.
Key takeaway: an uncoupler lets protons leak back without making ATP, so the energy of the gradient is lost as heat instead, which is how brown fat warms the body.The F₁ head of ATP synthase has three catalytic sites on its β subunits, and the spinning central stalk pushes each one through three shapes in turn, a model called the binding-change mechanism worked out by Paul Boyer. In the open state a site is empty; rotation switches it to the loose state, which binds ADP and phosphate; another turn switches it to the tight state, which squeezes them together into ATP; and a further turn returns it to the open state, releasing the ATP. The interesting part is that forming the bond needs little energy; most of the energy from the proton flow is spent prying the finished ATP off the enzyme. Each full rotation produces three ATP, one from each site.
Key takeaway: rotation drives each of the three sites through open, loose, and tight shapes that bind the building blocks, form ATP, and release it, producing three ATP per turn.