Chemiosmosis and Oxidative Phosphorylation
Oxidative Phosphorylation
Oxidative phosphorylation is the process by which ATP is synthesised using the energy released from the transfer of electrons along the electron transport chain (ETC) on the inner mitochondrial membrane. It is the final stage of aerobic respiration and produces the majority of ATP — approximately 26-28 ATP per molecule of glucose (out of a theoretical maximum of ~30-32 total).
The Electron Transport Chain
The ETC consists of a series of electron carrier proteins (complexes I-IV) embedded in the inner mitochondrial membrane. These carriers have progressively higher electronegativity (greater affinity for electrons), so electrons are passed from one carrier to the next in a thermodynamically favourable sequence.
Source of Electrons
Electrons enter the ETC from reduced coenzymes produced during glycolysis, the link reaction, and the Krebs cycle:
- Reduced NAD (NADH) donates electrons to Complex I (NADH dehydrogenase)
- Reduced FAD (FADH₂) donates electrons to Complex II (succinate dehydrogenase)
FADH₂ enters the chain at a lower energy level than NADH, which is why each NADH yields approximately 2.5 ATP while each FADH₂ yields approximately 1.5 ATP.
Electron Flow Through the Complexes
1. Complex I accepts electrons from NADH and passes them to ubiquinone (coenzyme Q), a mobile carrier in the membrane
2. Complex II accepts electrons from FADH₂ and also passes them to ubiquinone
3. Ubiquinone carries electrons to Complex III (cytochrome bc1 complex)
4. Cytochrome c, a small mobile protein on the intermembrane space side, transfers electrons from Complex III to Complex IV (cytochrome c oxidase)
5. At Complex IV, electrons are transferred to the final electron acceptor — oxygen (O₂). Oxygen combines with electrons and H⁺ ions to form water:
½O₂ + 2H⁺ + 2e⁻ → H₂O
This is why oxygen is essential for aerobic respiration — without it, the ETC stops, NADH and FADH₂ cannot be reoxidised, and the Krebs cycle and link reaction halt.
Chemiosmosis: The Proton Motive Force
As electrons pass through Complexes I, III, and IV, the energy released is used to actively pump hydrogen ions (H⁺ / protons) from the mitochondrial matrix into the intermembrane space. This creates:
- A concentration gradient of H⁺ (higher in the intermembrane space, lower in the matrix)
- An electrical gradient (the intermembrane space is more positively charged)
- Together, these form the proton motive force (PMF) or electrochemical gradient
The inner mitochondrial membrane is impermeable to H⁺ ions (due to its phospholipid bilayer), so protons can only return to the matrix through specific channel proteins — ATP synthase.
ATP Synthase
ATP synthase (Complex V) is a large enzyme complex spanning the inner mitochondrial membrane. It has two main components:
- F₀ — a transmembrane channel through which H⁺ ions flow down their electrochemical gradient
- F₁ — a catalytic head that protrudes into the matrix and catalyses ATP synthesis
As protons flow through the F₀ channel, they cause a rotor within ATP synthase to spin. This rotational energy drives a conformational change in the F₁ head that catalyses the phosphorylation of ADP + Pᵢ → ATP. This process is called chemiosmosis (proposed by Peter Mitchell in 1961, for which he received the Nobel Prize in 1978).
Summary of the Process
1. Reduced coenzymes (NADH, FADH₂) donate electrons to the ETC
2. Electrons pass along carriers of increasing electronegativity
3. Energy released at Complexes I, III, and IV pumps H⁺ from matrix to intermembrane space
4. The proton motive force drives H⁺ back through ATP synthase
5. The flow of H⁺ through ATP synthase drives rotational catalysis, synthesising ATP
6. Oxygen accepts electrons and H⁺ at Complex IV, forming water
The Importance of Oxygen
Oxygen is the terminal electron acceptor of the ETC. If oxygen is absent:
- Electrons cannot be passed from Complex IV — the entire chain backs up
- No protons are pumped — the proton gradient dissipates
- ATP synthase stops producing ATP via oxidative phosphorylation
- NADH and FADH₂ cannot be reoxidised — they accumulate in their reduced forms
- Without NAD⁺ and FAD, the Krebs cycle cannot continue (no oxidised coenzymes to accept hydrogen atoms)
- The link reaction also stops
- The cell is forced to rely on anaerobic respiration (glycolysis + fermentation), which produces only 2 ATP per glucose
ATP Yield from Aerobic Respiration
| Stage | ATP produced directly | Reduced coenzymes produced | ATP from oxidative phosphorylation |
|---|---|---|---|
| Glycolysis | 2 (net) | 2 NADH | ~5 |
| Link reaction (×2) | 0 | 2 NADH | ~5 |
| Krebs cycle (×2) | 2 (GTP → ATP) | 6 NADH + 2 FADH₂ | ~15 + ~3 |
| Total | 4 | ~28 | |
| Grand total | ~30-32 ATP per glucose |
The actual yield is lower than the theoretical maximum because:
- Some of the proton gradient energy is used to transport molecules across the inner membrane (e.g. ATP/ADP translocase)
- The inner membrane is not perfectly impermeable — some H⁺ leak back without passing through ATP synthase
- NADH from glycolysis (produced in the cytoplasm) requires energy to transport its electrons into the mitochondrial matrix
Uncoupling Proteins and Thermogenesis
Uncoupling proteins (UCPs) are channels in the inner mitochondrial membrane that allow H⁺ to flow back into the matrix without passing through ATP synthase. This dissipates the proton gradient as heat rather than ATP. This is important in:
- Brown adipose tissue (brown fat) — rich in mitochondria with UCP1 (thermogenin); generates heat for non-shivering thermogenesis in newborns and hibernating mammals
- 2,4-dinitrophenol (DNP) — a chemical uncoupler that carries H⁺ across the membrane, dangerously increasing metabolic rate (historically misused as a weight-loss drug)
Chemiosmosis in Photosynthesis
The same principle of chemiosmosis operates in the thylakoid membrane during the light-dependent reactions of photosynthesis:
- Light energy excites electrons in photosystems, which pass along an electron transport chain in the thylakoid membrane
- Energy from electron transfer pumps H⁺ from the stroma into the thylakoid lumen (interior)
- The proton gradient drives H⁺ back through ATP synthase embedded in the thylakoid membrane, producing ATP (this is called photophosphorylation)
- The photolysis of water inside the thylakoid also contributes H⁺ to the lumen
This parallel between mitochondrial and chloroplast chemiosmosis is a powerful example of a unifying principle in biology.
Exam Tips
- AQA requires you to describe the role of the ETC, the proton gradient, and ATP synthase — always mention all three components for full marks
- Clearly state that protons are pumped FROM the matrix TO the intermembrane space — direction matters
- When explaining why oxygen is essential, trace the consequences step by step: no O₂ → ETC stops → no reoxidation of NADH/FADH₂ → Krebs cycle stops → only glycolysis can proceed
- Remember the chemiosmotic theory applies to BOTH mitochondria and chloroplasts — a common comparison question
- Do not overstate ATP yields — use "approximately" and explain why actual yields are lower than theoretical