3.2.8Inquiry Process
Oxidative Phosphorylation — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 114 of 160
Section 1 of 4The Electron Transport Chain
Oxidative phosphorylation uses energy from electrons to build a proton gradient that drives ATP formation.
Location
- The chain is a series of carrier proteins in the inner mitochondrial membrane, which is folded into cristae to give a large surface area.
Electron donors
- Reduced NAD and reduced FAD give up electrons to the first carriers and are oxidised back to NAD and FAD.
Redox steps
- Electrons pass from carrier to carrier, each in a lower energy state, so energy is released at each step.
Proton pumping
- The released energy is used by carriers to pump protons from the matrix into the space between the two membranes.
Final acceptor
- Oxygen at the end of the chain accepts electrons and combines with protons to form water.
Section 2 of 4Chemiosmosis
1
Protons pumped into the intermembrane space build up, so there is a high concentration of protons there compared with the matrix.
2
The inner membrane is impermeable to protons, so they can cross only through channels in ATP synthase.
3
The protons flow down their concentration gradient through ATP synthase, back into the matrix.
4
The flow of protons provides the energy that ATP synthase uses to join ADP and inorganic phosphate, forming ATP.
5
This coupling of proton movement across a membrane to ATP synthesis is called chemiosmosis.
Section 3 of 4Role of Oxygen and Yield
Role of Oxygen and Yield
Oxygen as acceptor
- Oxygen removes electrons from the end of the chain, which keeps the carriers free to accept more electrons.
Water formed
- Oxygen combines with electrons and protons to form water: ½O₂ + 2H⁺ + 2e⁻ → H₂O.
Without oxygen
- The chain stops, reduced NAD is not oxidised, and the Krebs cycle and link reaction also slow down.
ATP yield
- Oxidative phosphorylation produces by far the largest share of the ATP made from one glucose molecule.
Poisons
- Cyanide blocks the final carrier, so respiration stops and cells quickly run out of ATP.
| Stage | Location | ATP made |
|---|---|---|
| Glycolysis | Cytoplasm | Small amount |
| Krebs cycle | Matrix | Small amount |
| Oxidative phosphorylation | Inner membrane | Most of the total |
Section 4 of 4Exam Checklist & Tips
3.2.8 – Key Points for the Exam
- Electron transport chain A series of carrier proteins that pass electrons and pump protons.
- Chemiosmosis Flow of protons across a membrane through ATP synthase to make ATP.
- ATP synthase An enzyme that makes ATP using the flow of protons.
- Intermembrane space The gap between the inner and outer mitochondrial membranes.
Common Mistakes
- Saying that oxygen is used in the Krebs cycle; it is the final electron acceptor in the chain.
- Writing that protons are pumped into the matrix, when they are pumped out into the intermembrane space.
- Thinking ATP synthase pumps protons; it uses their flow to make ATP.
- Confusing substrate-level phosphorylation with oxidative phosphorylation.
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