Mitochondria: How Your Cells Make Energy
Every cell runs on a molecule called ATP. Mitochondria — the “powerhouses” of the cell — build almost all of it with the electron transport chain. Press Play and watch electrons hop from Complex I to Complex IV — ferried by ubiquinone (Q) and cytochrome c — while protons get pumped, the proton-motive force climbs, and the ATP-synthase motor spins to stamp out ATP. Then hit the Cyanide toggle to poison Complex IV and watch the whole chain — and ATP output — collapse.
What's happening
The Science in Plain Language
1. Electrons enter the chain. When you break down food (glucose and fats), the released energy is carried by two shuttle molecules, NADH and FADH₂. They drop their high-energy electrons onto Complex I and Complex II of the electron transport chain, embedded in the folded inner membrane (the cristae).
2. Mobile carriers ferry the electrons “downhill.” Electrons don't jump straight from complex to complex. Two tiny mobile carriers shuttle them: ubiquinone (Coenzyme Q10) is fat-soluble and diffuses within the membrane, carrying electrons from Complexes I and II over to Complex III. Then cytochrome c skims along the outer surface of the membrane, handing electrons from Complex III to Complex IV. At each hand-off the electrons drop to a lower energy level, releasing energy.
3. That energy pumps protons. Complexes I, III and IV use the released energy to pump protons (H⁺) out of the matrix into the intermembrane space (Complex II does not pump — which is why FADH₂ ultimately yields less ATP than NADH). The protons pile up like water behind a dam. The height of that “dam” is the proton-motive force, roughly 180–200 mV across a membrane only a few atoms thick.
4. Oxygen is the final electron acceptor. At Complex IV, the spent electrons combine with oxygen and protons to form water (½ O₂ + 2 H⁺ + 2 e⁻ → H₂O). This is why you breathe: without O₂ to accept the electrons, the whole chain backs up and stops. This is exactly how cyanide and carbon monoxide kill — they jam Complex IV, so electrons can't reach oxygen, proton pumping halts, the gradient collapses, and ATP production crashes. Flip the Cyanide toggle above to see it happen.
5. The gradient spins a motor to make ATP. Protons rush back into the matrix through ATP synthase (Complex V), a genuine molecular turbine. The flow spins its rotor — up to about 130 turns per second — and each turn snaps phosphate onto ADP to make ATP, roughly 3 ATP per rotation (on the order of a few hundred ATP per second per motor). The faster and higher the gradient, the faster the motor spins: watch the rotor speed up and the ATP/s graph rise as the gradient builds. This whole scheme is called chemiosmosis / oxidative phosphorylation, and it supplies roughly 90% of your ATP.
6. Uncouplers turn the gradient into heat. If the membrane is made deliberately leaky, protons pour back without passing through ATP synthase — so the energy is released as heat instead of ATP. Your body does this on purpose in brown fat (using a protein called thermogenin/UCP1) to keep newborns and hibernating animals warm. The banned diet drug DNP does the same thing dangerously, which is why it caused fatal overheating. Flip the Uncoupler toggle to see the chain run hot — burning oxygen fast while ATP output crashes.