Oxidative phosphorylation

Oxidative phosphorylation is the last stage of aerobic respiration, and it makes most of the cell's ATP. Electrons from reduced NAD and reduced FAD pass along a chain of carrier proteins in the inner mitochondrial membrane. The energy they release pumps protons (H⁺) across the membrane, and the protons flow back through the enzyme ATP synthase, which uses that flow to make ATP from ADP and phosphate. Oxygen is the final electron acceptor and forms water.

Where does oxidative phosphorylation happen?

On the inner membrane of the mitochondrion. The electron carriers and ATP synthase are proteins embedded in it. The inner membrane is folded into cristae, which gives far more surface area for these proteins, and cells that need a lot of ATP, such as muscle cells, have many mitochondria with densely folded cristae. The mitochondria guide shows the structure.

The first three stages strip hydrogen from glucose and load it onto the coenzymes NAD and FAD. Oxidative phosphorylation is where that hydrogen is cashed in for ATP. See the link reaction guide for the stage before the Krebs cycle.

The stages of oxidative phosphorylation

1. Reduced coenzymes arrive. Reduced NAD and reduced FAD give up their hydrogen atoms at the inner membrane. Each hydrogen atom splits into a proton (H⁺) and an electron.
2. Electrons pass along the electron transfer chain. The electrons move from one carrier protein to the next in a series of oxidation and reduction (redox) reactions, losing energy at each step.
3. Protons are pumped. Some carriers use that energy to pump protons from the matrix across the inner membrane into the intermembrane space.
4. A gradient builds up. Now there are many more protons in the intermembrane space than in the matrix: a concentration gradient and an electrical gradient across the membrane.
5. ATP is made. The protons can only flow back into the matrix through channels in ATP synthase. As they flow, the enzyme joins ADP and inorganic phosphate (Pi) to make ATP.
6. Oxygen accepts the electrons. At the end of the chain, oxygen combines with the electrons and with protons to form water.

Chemiosmotic theory

AQA's A-level specification describes the synthesis of ATP by oxidative phosphorylation as associated with the transfer of electrons down the electron transfer chain and the passage of protons across inner mitochondrial membranes, catalysed by ATP synthase embedded in these membranes. This is the chemiosmotic theory: the energy from the electrons isn't used to make ATP directly, but to build a proton gradient, and the gradient drives ATP synthase.

OpenStax Biology describes ATP synthase as acting like a tiny generator, turned by hydrogen ions diffusing through it, and says chemiosmosis generates about 90 percent of the ATP made when glucose is broken down aerobically. The same idea makes ATP in photosynthesis, where protons cross the thylakoid membranes of the chloroplast.

Why oxygen is needed

Oxygen is the final electron acceptor. It removes the electrons from the end of the chain, joining them with protons to make water. Without oxygen:

• Electrons can't leave the last carrier, so the whole chain stops and no more protons are pumped.
• ATP synthase stops making ATP by oxidative phosphorylation.
• Reduced NAD and reduced FAD can't be oxidised back to NAD and FAD, so the Krebs cycle and link reaction run out of coenzymes and stop too.
• Only glycolysis can continue, with anaerobic respiration regenerating NAD, which makes far less ATP.

Reduced NAD vs reduced FAD

Both deliver electrons to the chain, but not at the same point. OpenStax explains that electrons from reduced NAD enter at the first complex, while electrons from reduced FAD enter at the second and skip the first proton pump. So fewer protons are pumped for each reduced FAD, and fewer ATP molecules are made from it.

What happens with an inhibitor

Some poisons stop oxidative phosphorylation. OpenStax notes that cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. With that carrier blocked, electrons can't pass to oxygen, the chain stops, the proton gradient isn't maintained, and ATP production by oxidative phosphorylation falls sharply. Cells that rely on aerobic respiration quickly run short of ATP, which is why cyanide is so dangerous.

Types of phosphorylation

Common mistakes

• Saying oxidative phosphorylation happens in the matrix. It happens on the inner membrane; the Krebs cycle is in the matrix.
• Saying electrons go through ATP synthase. Protons go through ATP synthase; electrons go along the carriers.
• Getting the direction of pumping wrong. Protons are pumped from the matrix into the intermembrane space, then flow back into the matrix.
• Saying oxygen makes ATP. Oxygen takes the electrons off the last carrier; ATP synthase makes the ATP.
• Forgetting water. Oxygen combines with electrons and protons to form water.

Respiration in mitochondria starts in the Cell Biology course. For practice in a short timed paper, try today's A-level Biology: Year 2 paper.

Read next

Biology · Respiration and photosynthesisGlycolysis for A-level Biology: stages, products, ATPGlycolysis for A-level Biology: where it happens, the three stages from glucose to pyruvate, the net gain of ATP and reduced NAD, and what happens next.