The Electron Transport Chain and Chemiosmosis

The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane that pass electrons from NADH and FADH₂ to oxygen, pumping protons (H⁺) across the membrane, and chemiosmosis is the use of the resulting proton gradient by ATP synthase to make ATP. In brief: this process, oxidative phosphorylation, produces about 26 to 28 of the roughly 30 to 32 ATP made from one glucose. Oxygen is the final electron acceptor, and is reduced to water. Without oxygen, the chain stops, and NADH and FADH₂ cannot be re-oxidized.

This is the stage of respiration in which most of the energy is captured. This guide describes the complexes of the chain, explains the proton gradient and ATP synthase, calculates the ATP yield, discusses inhibitors and uncouplers, works through a P/O ratio and proton-count example and corrects common mistakes.

What is oxidative phosphorylation?

Oxidative phosphorylation is the production of ATP using energy released by the oxidation of NADH and FADH₂ by oxygen. It occurs on the inner mitochondrial membrane, which is folded into cristae to give a large surface area. In prokaryotes, it occurs on the cell membrane. The process has two linked parts: the electron transport chain, which builds a proton gradient, and chemiosmosis, which uses the gradient to make ATP.

How does the electron transport chain work?

The chain consists of four large protein complexes (I to IV) and two mobile carriers (ubiquinone, or coenzyme Q, and cytochrome c). The electrons pass from carriers with low affinity for electrons to carriers with higher affinity, and they release energy in small steps.

1. Complex I (NADH dehydrogenase): receives electrons from NADH, which is oxidized back to NAD⁺, and pumps 4 H⁺ from the matrix into the intermembrane space.
2. Complex II (succinate dehydrogenase): receives electrons from FADH₂. It does not pump protons. This is why FADH₂ yields less ATP than NADH.
3. Ubiquinone (Q): carries electrons from complexes I and II to complex III.
4. Complex III: pumps 4 H⁺ and passes electrons to cytochrome c.
5. Cytochrome c: carries electrons to complex IV.
6. Complex IV (cytochrome c oxidase): pumps 2 H⁺ and transfers the electrons to oxygen, which combines with hydrogen ions to form water: ½O₂ + 2e⁻ + 2H⁺ → H₂O.

Overall, for each NADH, about 10 protons are pumped out of the matrix (4 + 4 + 2), and for each FADH₂, about 6 (4 + 2). The numbers are based on the best current estimates and vary slightly in textbooks.

What is chemiosmosis?

The pumping of protons makes the intermembrane space more acidic (lower pH) and positively charged compared with the matrix. This difference in concentration and charge across the membrane is a store of energy called the proton-motive force. The inner membrane is impermeable to protons, so they can return to the matrix only through the enzyme ATP synthase.

ATP synthase is a rotary molecular machine. The flow of protons through its base makes a part rotate, like a water wheel, and the rotation causes changes in the shape of the head in the matrix, which joins ADP and inorganic phosphate to make ATP. About 3 to 4 protons are needed for each ATP (including the transport of phosphate into the matrix). This mechanism was proposed by Peter Mitchell and is known as the chemiosmotic theory. In a similar way, chloroplasts use a proton gradient across the thylakoid membrane in photosynthesis.

Components of the chain

How much ATP is made?

The amount of ATP per NADH or FADH₂ is not a whole number, because the proton pumping and the ATP synthesis are only indirectly linked. Current estimates are about 2.5 ATP per NADH and 1.5 ATP per FADH₂. Using these numbers for one glucose:

• 10 NADH × 2.5 = 25 ATP (2 from glycolysis, 2 from the link reaction, 6 from the Krebs cycle).
• 2 FADH₂ × 1.5 = 3 ATP.
• Direct ATP from glycolysis and the Krebs cycle: 2 + 2 = 4 ATP.
• Total: 25 + 3 + 4 = 32 ATP. Older textbooks give 36 or 38, with values of 3 and 2 per NADH and FADH₂. The number is lower in practice, about 30 to 32, since the transport of cytosolic NADH reducing equivalents and phosphate across the membrane costs energy.

What are inhibitors and uncouplers?

• Inhibitors block the electron flow. Cyanide and carbon monoxide bind to complex IV and stop the chain, which is why they are deadly: cells cannot make ATP, and the proton gradient collapses. Rotenone (an insecticide) blocks complex I. Oligomycin blocks ATP synthase.
• Uncouplers make the membrane leaky to protons, so the gradient dissipates as heat and little ATP is made. The chemical 2,4-dinitrophenol was once used as a weight-loss drug and caused deaths from overheating.
• Natural uncoupling: brown fat cells, which are common in newborn babies and hibernating animals, contain the protein thermogenin (UCP1), which allows protons to flow back without making ATP. The energy is released as heat to keep the body warm.

Worked example: protons and ATP per NADH

Estimate the ATP yield from one NADH from the proton numbers.

1. Protons pumped per NADH: 4 (complex I) + 4 (complex III) + 2 (complex IV) = 10.
2. Protons needed per ATP: ATP synthase needs about 8 protons for 3 ATP, which is 2.67 H⁺ per ATP. One more proton is used for the transport of phosphate, so about 4 H⁺ per ATP overall.
3. ATP per NADH: 10 ÷ 4 = 2.5.
4. For FADH₂: 4 + 2 = 6 protons, so 6 ÷ 4 = 1.5 ATP.
5. Check with the P/O ratio: the P/O ratio is the number of ATP made per oxygen atom reduced. For NADH, 2 electrons reduce half an O₂ (one O atom), so the P/O ratio is 2.5. For FADH₂ it is 1.5.

The calculation demonstrates why NADH is more valuable than FADH₂: electrons enter at complex I and pass through three proton pumps, but electrons from FADH₂ enter at complex II, bypassing one pump. The relationship also explains why a lack of oxygen is catastrophic: all of the 10 NADH and 2 FADH₂ from each glucose depend on oxygen as the final acceptor to be reoxidized.

How was the mechanism discovered?

Otto Warburg identified the “respiratory enzyme” that uses oxygen in the 1920s and received the 1931 Nobel Prize in Physiology or Medicine. David Keilin discovered cytochromes in 1925. Peter Mitchell proposed the chemiosmotic theory in 1961, and although it was at first strongly doubted, evidence accumulated, and he received the Nobel Prize in Chemistry in 1978. Paul Boyer and John Walker studied the mechanism and structure of ATP synthase, and shared the 1997 Nobel Prize in Chemistry. Cryo-electron microscopy has shown the structure of the complexes in detail.

Which mistakes are common?

Mistake 1: Saying that oxygen is used to make ATP directly

Oxygen is the final electron acceptor. ATP is made by ATP synthase, driven by the proton gradient.

Mistake 2: Thinking that the proton gradient is made by ATP synthase

The complexes of the electron transport chain pump the protons. ATP synthase uses their return flow.

Mistake 3: Placing the chain in the matrix

It is in the inner mitochondrial membrane. Protons are pumped from the matrix to the intermembrane space.

Mistake 4: Giving whole-number ATP yields without comment

Yields per NADH and FADH₂ are estimates (about 2.5 and 1.5). Check the values used in your course.