Respiration: (g) the process and site of oxidative phosphorylation

The idea

Oxidative phosphorylation is the final stage of aerobic respiration, occurring on the inner mitochondrial membrane (the cristae). Reduced coenzymes NADH and FADH₂ deliver electrons to the electron transport chain, a series of electron carrier proteins embedded in the membrane. As electrons pass along the chain, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space, building a proton gradient. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. The protons then flow back through ATP synthase, driving the phosphorylation of ADP to ATP. The site is the inner membrane; the process links electron transport to ATP synthesis.

What earns the marks

• Site: inner mitochondrial membrane (cristae).
• NADH and FADH₂ donate electrons to the electron transport chain.
• Electron transfer releases energy used to pump H⁺ into the intermembrane space.
• Oxygen is the final electron acceptor, forming water.
• Proton flow through ATP synthase phosphorylates ADP to ATP.

Common mistakes and how to put them right

• Saying oxidative phosphorylation occurs in the matrix.

  Put it right: Correct it to the inner mitochondrial membrane.

• Claiming oxygen directly makes ATP.

  Put it right: Correct it to oxygen acting as the final electron acceptor.

• Confusing the proton gradient direction.

  Put it right: Correct it to H⁺ pumped into the intermembrane space.

Exam tips

• Link each step to its precise location on the inner membrane.
• State the role of oxygen as the final electron acceptor explicitly.
• Use the term chemiosmosis when describing proton flow through ATP synthase.