Oxidative phosphorylation
CIE A-Level BiologyΒ· Unit 13: Energy and RespirationΒ· 20 min read
1. The Electron Transport Chain (ETC)β β β βββ± 5 min
The electron transport chain (ETC) is a series of protein electron carriers embedded in the inner mitochondrial membrane. It accepts high-energy electrons from reduced NAD () and reduced FAD (), produced in earlier stages of aerobic respiration.
Electron carrier
A membrane protein that accepts and donates electrons, transferring energy along the ETC
Example:
NADH dehydrogenase, cytochrome c, cytochrome oxidase
Electrons flow from carriers with lower electronegativity to carriers with higher electronegativity, releasing free energy at each transfer. At the end of the chain, oxygen acts as the final electron acceptor, combining with electrons and hydrogen ions to form water.
Explain why oxygen is described as the final electron acceptor in oxidative phosphorylation
- 1
Oxygen is the last molecule in the ETC chain that accepts electrons from the final protein carrier.
- 2
Electrons enter the ETC from and , then pass through successive carriers, losing energy at each step.
- 3
After accepting the electrons, oxygen combines with hydrogen ions to form water.
- 4
If oxygen does not accept electrons, the ETC stops, as no new electrons can enter the chain.
Exam tip:
CIE always requires you to link oxygen's role to accepting electrons (not just hydrogen ions) for full marks.
2. Chemiosmosis and Proton Gradient Formationβ β β β ββ± 7 min
Energy released from electron transfer along the ETC is used to actively pump hydrogen ions () from the mitochondrial matrix, across the inner mitochondrial membrane, into the intermembrane space. This creates an electrochemical gradient: a higher concentration of and more positive charge in the intermembrane space than the matrix.
Proton motive force
The potential energy stored in the combined concentration and charge gradient of protons across the inner mitochondrial membrane
The inner mitochondrial membrane is impermeable to , so protons can only diffuse back into the matrix through the transmembrane enzyme ATP synthase. This diffusion of protons down their gradient is called chemiosmosis, and the energy released drives ATP synthesis.
Describe how a proton gradient is generated and used to produce ATP
- 1
Energy released from electrons moving down the ETC is used by protein complexes to pump from the matrix into the intermembrane space.
- 2
This creates an electrochemical gradient, with stored potential energy called proton motive force.
- 3
can only cross the inner membrane through the channel in ATP synthase.
- 4
As flows down the gradient, the energy released catalyses the phosphorylation of ADP + Pi to ATP.
Exam tip:
Always specify the gradient is electrochemical (not just concentration) to gain full marks in CIE.
3. Oxidative Phosphorylation: Overview and ATP Yieldβ β β β ββ± 5 min
Oxidative phosphorylation is the entire process of ATP synthesis coupled to electron transfer from and to oxygen. It differs from substrate-level phosphorylation, which occurs in glycolysis and the Krebs cycle, where ATP is produced directly by transferring a phosphate group from a phosphorylated substrate to ADP.
Feature | Oxidative phosphorylation | Substrate-level phosphorylation |
|---|---|---|
Location | Inner mitochondrial membrane | Cytoplasm / Mitochondrial matrix |
Energy source | Proton motive force from ETC | Phosphate group from organic substrate |
ATP per reaction | 1.5 or 2.5 | 1 |
Requires oxygen | Yes (indirect) | No |
Calculate the net ATP yield from one glucose molecule per CIE 9700 conventions
- 1
Count reduced coenzymes from all stages: 10 NADH (2 glycolysis + 2 link + 6 Krebs) and 2 FADH2 (Krebs only).
- 2
Calculate ATP from oxidative phosphorylation: (10 Γ 2.5) + (2 Γ 1.5) = 25 + 3 = 28 ATP.
- 3
Add ATP from substrate-level phosphorylation: 2 (glycolysis) + 2 (Krebs) = 4 ATP.
- 4
Total net yield = 28 + 4 - 2 (used for transport) = 30 ATP, the accepted value for CIE.
4. Inhibitors of Oxidative Phosphorylationβ β β β β β± 3 min
CIE commonly tests the effect of respiratory inhibitors, which target different parts of oxidative phosphorylation. Two common examples are cyanide and uncouplers like 2,4-DNP:
Cyanide: Inhibits cytochrome oxidase (the final ETC carrier), blocking electron flow. The ETC stops, no proton gradient forms, and ATP synthesis halts.
Uncouplers (e.g. 2,4-DNP): Make the inner mitochondrial membrane leaky to , dissipating the proton gradient. No ATP is made, but electron flow along the ETC continues.
Explain why uncouplers stop ATP synthesis but not electron flow
- 1
Uncouplers increase the permeability of the inner mitochondrial membrane to ions.
- 2
leaks back into the matrix from the intermembrane space, so no maintained electrochemical gradient can form.
- 3
ATP synthesis requires proton motive force from the gradient, so ATP production stops.
- 4
Electron flow along the ETC does not depend on the gradient, so electrons continue to be passed to oxygen.
5. Common Pitfalls
Wrong move:
Using oxidative phosphorylation and chemiosmosis interchangeably
Why:
They are not the same process; CIE examiners award separate marks for each
Correct move:
Use chemiosmosis for proton flow through ATP synthase, and oxidative phosphorylation for the overall ATP production process coupled to ETC electron transfer
Wrong move:
Only mentioning that oxygen accepts hydrogen ions to form water, no mention of electrons
Why:
CIE requires both to award the mark for oxygen's role in the ETC
Correct move:
Always state that oxygen accepts both electrons and hydrogen ions to form water as the final electron acceptor
Wrong move:
Claiming the ETC directly produces ATP
Why:
The ETC only creates the proton gradient; ATP is made by ATP synthase
Correct move:
Explain that the ETC releases energy to pump protons, and ATP synthase uses the gradient to make ATP via chemiosmosis
Wrong move:
Using 3 ATP per NADH and 2 per FADH2 to calculate total yield
Why:
CIE 9700 uses modern standard values, so old values lose marks
Correct move:
Use 2.5 ATP per NADH and 1.5 ATP per FADH2 to get a total net yield of 30 ATP per glucose
6. Quick Reference Cheatsheet
Component | Role | CIE ATP Yield |
|---|---|---|
NADH | Donates high energy electrons to ETC | 2.5 ATP per molecule |
FADH2 | Donates electrons to ETC (later entry) | 1.5 ATP per molecule |
Oxygen | Final electron acceptor, forms water | N/A |
ATP synthase | H+ channel, catalyses ATP synthesis | N/A |
Substrate-level phosphorylation | Direct ATP from substrate | 4 ATP per glucose |
Net ATP per glucose | Total aerobic respiration yield | 30 ATP |
7. Frequently Asked
Is oxidative phosphorylation the same as chemiosmosis?
No. Chemiosmosis is the movement of protons down their gradient through ATP synthase, which powers ATP synthesis. Oxidative phosphorylation is the entire process that uses energy from the electron transport chain to phosphorylate ADP, of which chemiosmosis is a key step.
What ATP values per NADH/FADH2 does CIE 9700 accept?
CIE 9700 uses the modern conventional values of 2.5 ATP per NADH and 1.5 ATP per FADH2, giving a net yield of 30 ATP per glucose molecule.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Explain oxidative phosphorylation process
- 2023 Β· 4
Role of proton gradient in ATP synthesis
- 2021 Β· 1
Calculate net ATP yield per glucose
Going deeper
What's Next
Oxidative phosphorylation is the final step of aerobic respiration, and produces the vast majority of ATP required for all energy-dependent cellular processes. Mastering the details of ETC function, chemiosmosis and ATP yield calculation is essential for extended response questions in CIE Paper 2 and Paper 4. This topic also provides a key comparison for learning about photophosphorylation in photosynthesis, which shares the same core mechanism of chemiosmosis. Understanding inhibitors of this process also links to common exam questions on respiratory poisons and their effects on aerobic respiration.