The Electron Transport Chain, Chemiosmosis and ATP Synthase

Why do you need to breathe?

You can go weeks without food and days without water, but only minutes without oxygen. That's because oxygen has one critical job in your cells: it sits at the very end of the electron transport chain and accepts the electrons that have come all the way from glucose. Without it, the whole chain backs up, ATP production collapses, and cells that depend heavily on aerobic respiration, such as brain and heart cells, are damaged quickly.

This stage, called oxidative phosphorylation, makes close to 90% of the ATP from each glucose. It also uses one of the most elegant ideas in biology: the cell first builds a gradient of protons across a membrane, like water behind a dam, and then lets them flow back through a molecular turbine that makes ATP.

Part 1: passing the electrons

The ETC is a set of four large protein complexes (I to IV) and two mobile carriers, embedded in the inner membrane. Many of these proteins contain iron or copper atoms that can be reversibly oxidized and reduced. Each carrier is a little more electronegative than the one before, so electrons move "downhill" in free energy from one to the next.

1. Complex I (NADH dehydrogenase) takes 2 electrons from NADH, regenerating NAD⁺ for glycolysis and the Krebs cycle. It passes the electrons to ubiquinone (Q).
2. Complex II (succinate dehydrogenase, the same enzyme as in Krebs step 6) takes electrons from FADH₂ and also passes them to Q. Note that FADH₂'s electrons enter after complex I.
3. Q carries electrons through the membrane to complex III.
4. Cytochrome c carries them from complex III to complex IV.
5. Complex IV (cytochrome c oxidase) hands the electrons to oxygen, the final electron acceptor. Oxygen combines with electrons and H⁺ from the matrix to form water: ½O₂ + 2e⁻ + 2H⁺ → H₂O.

The water formed here is the metabolic water in the respiration equation. The oxygen atoms you breathe in end up in water, not in CO₂ (the oxygen in exhaled CO₂ comes from the food and water molecules used in the Krebs cycle).

Part 2: pumping protons

The ETC does not make ATP directly. Instead, as electrons pass through complexes I, III and IV, the energy released is used to pump H⁺ ions from the matrix into the intermembrane space. These complexes are proton pumps.

The result is an H⁺ gradient across the inner membrane:

• a concentration gradient (more H⁺ in the intermembrane space, so it has a lower pH), and
• an electrical gradient (the intermembrane side becomes positive relative to the matrix).

Together, these store potential energy called the proton-motive force. The inner membrane is almost impermeable to H⁺, so the protons can't leak back easily.

Because NADH's electrons pass through all three pumps and FADH₂'s skip complex I, NADH causes more H⁺ to be pumped (about 10 per NADH) than FADH₂ (about 6).

Part 3: chemiosmosis and ATP synthase

In 1961 the British biochemist Peter Mitchell proposed that the energy of electron transport is stored as an H⁺ gradient and then used to make ATP. He called it chemiosmosis. Many scientists doubted him at first, but experiments supported it, and he received the 1978 Nobel Prize in Chemistry.

The only easy path back into the matrix for H⁺ is through ATP synthase:

• Its membrane part forms a channel and a rotor. As H⁺ ions flow through, down their gradient, the rotor turns, a bit like a turbine.
• The rotation turns a central stalk inside the knob that sits in the matrix. The turning stalk changes the shape of three catalytic sites in turn, so they bind ADP + Pᵢ, join them into ATP, and release it.

Paul Boyer and John Walker shared half of the 1997 Nobel Prize in Chemistry for working out how ATP synthase works (the other half went to Jens Skou for the sodium-potassium pump). ATP synthase is one of the smallest rotary motors known.

About 4 H⁺ are needed per ATP (including the H⁺ used to bring phosphate into the matrix). That gives roughly 2.5 ATP per NADH (10 ÷ 4) and 1.5 ATP per FADH₂ (6 ÷ 4). The next lesson uses these numbers to total the ATP from one glucose.

When the chain is blocked or leaky

Because everything is linked, a problem at one point affects the whole process:

• No oxygen, cyanide or carbon monoxide: cyanide and carbon monoxide bind to complex IV and stop it passing electrons to oxygen. (Carbon monoxide also binds tightly to hemoglobin, which reduces oxygen delivery.) Electrons back up, pumping stops, the H⁺ gradient runs down, and ATP production collapses. This is why Ontario's Fire Code requires carbon monoxide alarms in homes with a fuel-burning appliance, a fireplace or an attached garage: next to sleeping areas and, since January 1, 2026, on every storey.
• Rotenone (a plant compound used as a pesticide) blocks complex I.
• Oligomycin blocks the channel of ATP synthase: the gradient builds up so high that pumping, and electron flow, slow down.
• Uncouplers let H⁺ leak back across the membrane without going through ATP synthase. Electron transport and O₂ use continue, even speed up, but the energy is released as heat instead of being captured in ATP. The drug DNP is a dangerous example: it was sold as a weight-loss pill in the 1930s and caused deaths from overheating, and Health Canada warns people not to take it. A natural, controlled uncoupler, the protein thermogenin, is found in brown fat, which helps newborn babies and hibernating animals stay warm.