Oxidative Phosphorylation
Laura Armstrong
Teacher

Contents
Recall Questions
This topic requires prior knowledge of the link reaction and the Krebs cycle. You can test your knowledge on these below.
What molecule is produced when acetate combines with coenzyme A in the link reaction?
Acetyl-CoA.
How many ATP are produced per glucose molecule in the Krebs cycle?
2 ATP (1 per cycle, 2 cycles per glucose)
What is the role of reduced NAD and FAD in respiration?
They carry electrons and hydrogen ions to the electron transport chain for oxidative phosphorylation.
Topic Explainer Video
What Is Oxidative Phosphorylation?
- It is the final stage of aerobic respiration.
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Occurs in the inner mitochondrial membrane (cristae).
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It uses the electrons from reduced NAD (NADH) and reduced FAD (FADH₂) to generate ATP.
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Involves two key processes:
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Electron Transport Chain (ETC)
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Chemiosmosis
Step-by-Step Process of Oxidative Phosphorylation
1. Electron Transport Chain (ETC)
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NADH (and FADH) are oxidised. The hydrogen splits into a proton and an electron.
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The electrons pass to the electron transport chain and pass along electron carriers in a series of redox reactions. This transfer releases energy.
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This energy is used to pump protons into the intermembrane space of the mitochondria.
2. Chemiosmosis
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A proton gradient is established (high H⁺ in intermembrane space).
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Protons diffuse back into the matrix via the enzyme ATP synthase.
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This process provides energy for the synthesis of ATP from ADP + Pi.
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This is called oxidative phosphorylation because the production of ATP used energy from the oxidation of reduced coenzymes.
3. Formation of Water
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At the end of the ETC, electrons combine with:
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Protons (H⁺)
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Oxygen (½ O₂)
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This forms water (H₂O):
4e− + 4H+ + O₂ → 2H₂O
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We say that oxygen acts as the final electron acceptor in the electron transport chain.
ATP Yield (Theoretical)
Stage |
ATP Yield per Glucose |
---|---|
Glycolysis |
Net gain of 2 |
Krebs Cycle |
2 |
Oxidative Phosphorylation |
~28 |
Total |
~32 ATP |
Note: Actual ATP yield may vary due to mitochondrial membrane leakage and energy used for transporting ADP/Pi.
1. Electron Transport Chain (ETC) & Oxidative Phosphorylation
Oxygen is the final electron acceptor in the ETC. Without it:
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Electrons cannot be passed along the ETC because there's nowhere for them to go at the end.
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Electron carriers remain reduced (full of electrons) — NADH and FADH₂ cannot unload their electrons.
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This means NAD and FAD are not regenerated.
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No chemiosmosis occurs → no ATP is made by oxidative phosphorylation.
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Therefore, ATP production drops dramatically (from ~32 to just 2 per glucose!).
Summary
- ETC stops
- Oxidative phosphorylation stops
- No oxygen → no final electron acceptor → no ATP from ETC
2. Link Reaction
The link reaction needs NAD to accept hydrogen when pyruvate is oxidised.
Without oxygen:
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NAD is not regenerated (because NADH isn't oxidised in the ETC).
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So, pyruvate cannot be converted to acetyl-CoA.
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Link reaction stops.
Summary
No NAD = no oxidation of pyruvate = link reaction stops.
3. Krebs Cycle
The Krebs cycle also requires oxidised NAD and FAD to accept hydrogen atoms.
Without oxygen:
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NAD and FAD are not regenerated, so no further reduction of these coenzymes can occur.
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Therefore, the Krebs cycle stops almost immediately after oxygen is lost.
Summary
No NAD / FAD = no redox reactions = Krebs cycle stops.
Key Terms
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Electron Transport Chain (ETC): Series of carrier proteins that transfer electrons, releasing energy.
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Chemiosmosis: The movement of protons down a concentration gradient through ATP synthase to generate ATP.
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ATP Synthase: Enzyme that uses energy from proton flow to synthesise ATP.
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Oxidative Phosphorylation: Production of ATP using energy from the oxidation of reduced coenzymes.
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Final Electron Acceptor: Oxygen, which combines with electrons and protons to form water.
Exam Tips
Always link electron transfer to proton movement and ATP synthesis — many students forget the role of the proton gradient.
Don't just say "electrons make ATP" — instead, explain that they release energy used to pump H⁺ ions, leading to chemiosmosis.
Be precise: ATP is made by oxidative phosphorylation.
Describe the roles of carrier proteins in the synthesis of ATP. (4 marks)
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electrons transferred from carrier to carrier / series of redox reactions
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energy released as electrons passed on
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H+ /protons pumped into intermembrane space
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H+ /protons diffuse back through enzyme/ channel/ ATP synthase
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energy used to synthesise ATP from ADP and Pi / using ATP synthase
Describe the role of oxygen in oxidative phosphorylation (2 marks)
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Accepts Electrons (and protons) at the end of the electron transport chain/ acts as final electron acceptor
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Forming water
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!