International A Level Biology / Cellular respiration
Oxidative phosphorylation and chemiosmosis revision notes
Edexcel International A Level Biology (YBI11), chapter 20: Cellular respiration, specification point 7.4. Free lesson.
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Oxidative phosphorylation makes most of the ATP from respiration at the inner mitochondrial membrane. It uses energy from electrons to build a proton gradient, then uses chemiosmosis to make ATP.
Where oxidative phosphorylation happens
The inner mitochondrial membrane is folded into cristae, giving it a large surface area for this stage of respiration. The intermembrane space is the narrow gap between the inner and outer membranes. The matrix is the fluid inside the inner membrane, where the link reaction and Krebs cycle make reduced NAD and reduced FAD. The electron transport chain is a row of carrier proteins in the inner membrane. ATP synthase is a channel protein and enzyme in the inner membrane. It is where ATP is made. The inner membrane and the intermembrane space are different places.
How electrons create a proton gradient
Reduced NAD and reduced FAD deliver hydrogen to the electron transport chain. This oxidises them, so they can collect more hydrogen later. The hydrogen splits into a proton, H⁺, and an electron. Electrons move from carrier to carrier, losing energy as they pass along the chain. That energy is used to pump protons from the matrix into the intermembrane space, against their gradient. As a result, far more protons build up in the intermembrane space. The difference in proton concentration across the membrane is a proton gradient. It is also called an electrochemical gradient because protons…
How chemiosmosis makes ATP
Protons diffuse back into the matrix down their gradient. The inner membrane only lets them return through ATP synthase. As protons flow through this enzyme, it joins ADP and inorganic phosphate to make ATP. Making ATP using energy from the oxidation of reduced NAD and reduced FAD is called oxidative phosphorylation. Making ATP using a proton gradient across a membrane is called chemiosmosis. Energy from the electrons pumps protons out. The proton gradient then drives their diffusion back through ATP synthase. Use the name ATP synthase, not ATPase. ATP synthase makes ATP;…
Why oxygen is the final electron acceptor
At the end of the electron transport chain, oxygen accepts electrons and combines with protons to make water: ½O₂ + 2H⁺ + 2e⁻ → H₂O. Oxygen is the final electron acceptor. Without oxygen, electrons cannot leave the chain, so the chain stops and no energy is released to pump protons. The gradient runs down, so ATP synthase stops making ATP. Without oxygen, reduced NAD cannot hand its hydrogen to the chain, so NAD is not regenerated. The link reaction and Krebs cycle stop because NAD is not made available again. Only glycolysis continues, in the cytoplasm, as anaerobic respiration. It makes…