Mitochondria: How Your Cells Make Energy

Every cell runs on a molecule called adenosine triphosphate (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 adenosine triphosphate-synthase motor spins to stamp out adenosine triphosphate. Then hit the Cyanide toggle to poison Complex intravenous and watch the whole chain — and adenosine triphosphate output — collapse.

Diagram is illustrative — not to scale.
Intermembrane space protons pile up here → high H⁺ Matrix where ATP is made · O₂ → H₂O Inner mitochondrial membrane (electron transport chain) I II III IV V ATP synthase NADH delivers e⁻ → I FADH₂ delivers e⁻ → II O₂ final e⁻ acceptor Q shuttles in-membrane cyt c shuttles on surface
Adenosine triphosphate produced (total)
0
Adenosine triphosphate synthesis rate
0 adenosine triphosphate/s per motor
340 0
Proton-motive force (Δp)
0 mV
0O₂ used
0H₂O made

What's happening

Electrons from the food you digested enter at Complex I and Complex II…
electron (e⁻) ubiquinone Q cytochrome c proton (H⁺) adenosine triphosphate O₂ water

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, reduced nicotinamide adenine dinucleotide 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 adenosine triphosphate than reduced nicotinamide adenine dinucleotide). 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 intravenous, so electrons can't reach oxygen, proton pumping halts, the gradient collapses, and adenosine triphosphate production crashes. Flip the Cyanide toggle above to see it happen.

5. The gradient spins a motor to make adenosine triphosphate. Protons rush back into the matrix through adenosine triphosphate 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 adenosine triphosphate, roughly 3 adenosine triphosphate per rotation (on the order of a few hundred adenosine triphosphate per second per motor). The faster and higher the gradient, the faster the motor spins: watch the rotor speed up and the adenosine triphosphate/s graph rise as the gradient builds. This whole scheme is called chemiosmosis / oxidative phosphorylation, and it supplies roughly 90% of your adenosine triphosphate.

6. Uncouplers turn the gradient into heat. If the membrane is made deliberately leaky, protons pour back without passing through adenosine triphosphate synthase — so the energy is released as heat instead of adenosine triphosphate. 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 adenosine triphosphate output crashes.

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